Investigation of the formation and variability of dissolved inorganic carbon and dissolved organic carbon in the water of a small river (on the example of the Styr River, Ukraine)

Investigation of the formation and variability of dissolved inorganic carbon and dissolved organic carbon in the water of a small river (on the example of the Styr River, Ukraine) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Investigation of the formation and variability of dissolved inorganic carbon and dissolved organic carbon in the water of a small river (on the example of the Styr River, Ukraine) Olha Biedunkova, Pavlo Kuznietsov This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4582267/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 28 Oct, 2024 Read the published version in Environmental Monitoring and Assessment → Version 1 posted 10 You are reading this latest preprint version Abstract This paper presents the results of a study on the dynamics in the concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in water samples collected from the Styr River between 2019 and 2022. The concentrations of DIC and DOC were measured using an Elementar liqui TOC II analyzer. The study methodology involved analyzing the changes in DIC and DOC concentrations and their relationship with flow rates, temperature, seasonality, and other indicators such as hydrogen pH levels, total alkalinity (TA), and total dissolved solids (TDS). The purpose of this article is to identify patterns in the formation and changes of DIC and DOC concentrations in the Styr River. The concentrations of DIC and DOC in the samples ranged from 1.55-4.93 mM and 0.49-1.43 mM, respectively, with DOC accounting for an average of 22% of the total dissolved carbon content. The highest DOC concentrations were observed in summer, while the highest DIC concentrations were observed in winter. Based on the results, it can be concluded that water flow and temperature have an impact on DOC concentration, while flow, temperature, and pH affect DIC concentration. There was no correlation between DIC and DOC concentrations, but a strong positive relationship (r=0.9056, p<0.001) was found between DIC and TA concentrations. Therefore, the main factors influencing DIC in the Styr River are those that affect the carbonate equilibrium, such as leaching of carbonate and silicate rocks, CO2 absorption from the atmosphere, and changes in pH. Additionally, the concentration of DOC is influenced by biological activity and is higher during the warm season. These findings can be used to develop a strategy for managing water resources in the Styr River basin and to assess and predict the ecological state of the river. natural surface waters carbon cycle hydrochemical parameters managing water resources Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Highlights The study are the data for monitoring content and trends in the carbon cycle in the water of rivers. The dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the Styr River (Ukraine) were studied. The dynamics of changes in DIC and DOC, their relationship with flow velocity, temperature, seasonality and correlations with other physicochemical parameters were identified and evaluated. Factors influencing DIC and DOC concentrations in surface waters were estimated. Introduction The content of carbon components (total, dissolved, inorganic and organic carbon) is a useful tracer of carbon cycling within rivers. Soria-Reinoso et al. ( 2022 ) have shown that rivers are important sites for carbon transport and critical components of the global carbon cycle. However, there is limited knowledge about longitudinal and temporal changes in carbon species in rivers. Organic and inorganic forms of carbon content in surface waters can be distinguished (Vachon et al. 2021 ): total organic carbon (TOC) is an indicator of the content of organic compounds, while total inorganic carbon (TIC) is an indicator of the content of inorganic compounds that form the components of the carbonate system: carbon dioxide (CO 2 ), bicarbonate (HCO 3 − ), and carbonate (CO 32− ). According to Kuznietsov and Biedunkova ( 2023 ), the carbon content of TIC and TOC is an indirect indicator of the presence of carbon atoms in water, without providing information about the nature and structure of the substance. However, the majority of carbon in rivers is found in the dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) forms. According to the standard DSTU EN 1484– 2003 ( 2003 ), DOC and DIC are parameters that include active chemical matter with dispersed molecules smaller than 0.45 µm. Surface water DIC has been used as an indicator of air pollution, water quality, and the degree of dissolution of chemical species, such as carbonates. Similarly, DOC can be used as an indicator of aquatic pollution, with high levels of DOC leading to reduced biodiversity and highly degraded aquatic ecosystems (Joesoef et al. 2017 ). DIC, which is generally present at higher concentrations, is more easily measured than DOC. In terms of the carbon cycle, a large fraction is returned to the atmosphere through organic carbon decomposition within inland waters, transported to adjacent waters, and buried in freshwater. The most important sources of DIC in surface waters are carbonate and silicate weathering processes (1, 2). Additionally, the among-catchment behavior of HCO 3 − − responding to changing runoff was correlated with the average Da of each catchment. Globally, HCO 3 − behaviors and weathering characteristics are highly influenced by carbonate bedrock distributions and abundance Zhong et al. ( 2020 ). According to the study by Cai et al. ( 2015 ), the DIC and DOC contents in river waters depend on the size of the water region, climate, watercourse, and the season of sample collection. Therefore, it is important to conduct practical studies for a particular water body. CaCO 3 + CO 2 + H 2 O → 2HCO 3 − + Ca 2+ (1) CaSiO 3 + 2CO 2 + 3H 2 O → 2HCO 3 − + Ca 2+ + H 4 SiO 4 (2) The total alkalinity (TA) of natural waters is primarily composed of [HCO 3 − ] and [CO 3 2− ]. According to Guo et al. ( 2008 ), DIC-to-TA ratios can provide broad insight into the sources of carbon, aquatic pH dynamics, and regional carbonate buffering capacity. Similarly, Wakana et al. ( 2023 ) found that an increase in DOC concentration leads to an increase in the weathering rate of minerals and solubility, which in turn affects the mobility and transport of many metals and organic contaminants. Thus, the ratio of DIC to DOC can provide broad insight into the transition of inorganic forms to organic forms of carbon. The correlation between DIC and DOC is determined by the regime of organic and inorganic carbon input and is specific to each type of water (Aguilar-Torrejón et al., 2023 ). It is known that geochemical processes can be identified through the analysis of parameters such as total hardness (TH), total dissolved solids (TDS), and DIC, which provide insights into carbonate weathering and evaporite dissolution, as well as their effects on CO 2​ dynamics between the river and the atmosphere (Chen et al., 2021 ; Regnier et al., 2022 ; Xu et al., 2021 ). Additionally, the content of DIC and DOC in surface waters is not regulated according to EU and Ukrainian legislation. Although there have been numerous studies on dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in rapidly transiting large river systems, research on small rivers is limited. Therefore, there is a lack of research on DIC and DOC in water Ukraine's rivers. In this study, we examined the DIC and DOC contents of the Styr River (Ukraine). The purpose of this article is to identify patterns of formation and changes in the concentration of DIC and DOC in the water of the Styr River. Monthly sampling was conducted between 2019 and 2022 to examine the dynamics of DIC and DOC changes. The relationship between these changes and discharge, temperature, and seasonality was also investigated, as well as their correlations with other indicators such as hydrogen pH levels, total alkalinity (TA), and total dissolved solids (TDS). The dynamics of DIC and DOC in small rivers have far-reaching implications for the health and functioning of aquatic ecosystems. These carbon species serve as essential energy sources for primary producers and support the intricate food webs that sustain riverine biodiversity. Changes in DIC and DOC concentrations can disrupt these ecosystems, affecting nutrient cycling, phytoplankton productivity, and overall ecosystem stability. Furthermore, small rivers play a significant role in the global carbon cycle, acting as conduits for transporting terrestrial carbon to the ocean. Method and Theory The object of the study was the water of the river Styr. The river Styr basin is located in the north-west of Ukraine, within the Lviv, Volyn and Rivne regions. Additionally, the river is 494 km long and covers an area of 13100 km², with an average annual water flow of 49.5 m³/s at the mouth, narrowing to 10–20 m in the upper reaches and widening to 30–50 m in the middle and lower reaches Kuznietsov and Biedunkova ( 2024 ). The bottom of the riverbed is composed of sandstones, and shales of Cretaceous and Paleogene age, and in the lower reaches - Miocene clay deposits. However, according to the surface water typology, the Styr River is a lowland, sandy loamy river (Kuznietsov et al. 2023 ). Sampling for chemical monitoring was carried out at the Styr River (hydrological post) in the Rivne Nuclear Power Plant intake zone according to 2019–2022. The study results were statistically processed using the BioEstar software package (Version 5.3, MLM). This involved determining the range of data series (min-max), arithmetic mean (M), standard deviation (± SD), and coefficient of variation (CV) of the corresponding sample. Pearson's correlation coefficient (r) was used to assess the relationship between the control outcome variables, along with the significance of influence (p) according to Kahaer et al. (2019). In particular, the river Styr water flow (D) during monitoring at RNPP hydrological post varied in the range (min-max) from 10 to 63m 3 /s, arithmetic mean (M) 27m 3 /s, standard deviation (± SD) ± 18m 3 /s (Fig. 1 ). In addition, the water temperature of the Styr River (Т) during monitoring at RNPP hydrological post varied in the range (min-max) from 0.3 to 24.6 о С, M = 12.6 о С, ±SD = ± 8.7 о С (Fig. 2 ). This study involved measuring the concentration of DIC, DOC, hydrogen pH levels, total alkalinity (TA), total dissolved solids (TDS) in the certified laboratory of RNPP according to a standardized method (Table 1 ). Table 1 Characteristics of measurement methods of this study* Х C I δ, % (Δ, unit) ME DIC, DOC, mM 0.3–100 from 0.3 to 10, δ=±10; more than 10, δ=± 5 Elementar liquiTOC II, thermocatalytic oxidation method at 680°C with NDIR, measurement time DOC: 3 min, DIC: 4 min pH, unit 1–10 (Δ = ± 0.2) Ionomer I-160 with glass electrodes TA, mM 0.4–20 - Titration burettes TDS, mg/dm 3 50–10000 δ = ± 5 Laboratory scale Note: * X – the components, C I – the measurement range, δ – the relative (δ) or absolute (Δ) measurement errors, MPM - method of measurement, ME -measuring equipment. Sampling was conducted following the guidelines of DSTU ISO 5667-2: 2003 ( 2003 ). The concentration of DIC, and DOC was measured using DSTU EN 1484– 2003 ( 2003 ) guidelines. The measurement method for DIC and DOC is based on the detection of CO 2 using a nondispersive infrared sensor (NDIR), which is produced during sample preparation through acidification (DIC) or oxidation (DOC) of the sample. Based on the infrared absorption grading graph of CO2 (Fig. 3 ), the suitable concentration of DIC and DOC can be determined. To measure the concentrations of DIC and DOC, the samples were filtered through a 0.45 µm membrane filter (MF-Millipore) after preliminary preparation. The pH of the water was measured using the potentiometric method, TDS was measured using the gravimeter method, and TA was measured by titration (Table 1 ). Results And Discussion The concentrations of DIC and DOC in the Styr River varied significantly depending on the season, as shown in Fig. 4 , 5 . During spring and winter, when there is a higher river flow (Fig. 1 ) and the lowest water temperature (Fig. 2 ) in the Styr River, the DOC concentration was lower (Fig. 5 ). In contrast, the DIC concentration was lowest in summer and the first months of autumn (Fig. 5 ), when river flows were minimal (Fig. 1 ) and the highest water temperature (Fig. 2 ) of the Styr River was observed. The Styr River exhibits maximum DOC concentration during the summer months, which Kuznietsov et al. ( 2024 ) attribute to an increase in water temperature and intensification of biological processes, as well as the development and growth of plants and hybrids. Conversely, the maximum DIC concentration in the Styr River are observed during the winter months, which Drake et al. ( 2021 ) explain as a result of decreased photosynthesis processes leading to the formation of CO 2 and the accumulation of HCO 3 − . Note that an increase in flow rate, and therefore an increase in carbonate leaching by reaction (2), led to an increase in DIC concentration. Conversely, a decrease in flow rate and an increase in photosynthesis during summer resulted in an increase in DIC concentration (Meng et al. 2023 ). Therefore, concentrations of DOC and DIC in the Styr River is primarily influenced by carbon leaching processes and biological processes that regulate photosynthesis, development, and mortality of aquatic organisms. Seasonal variations in river flow and water temperature appear to be the primary drivers of the opposing trends observed in DOC and DIC concentrations in the Styr River. Increased flow during spring and winter likely leads to dilution of DOC and enhanced carbonate mineral leaching, resulting in higher DIC concentrations. Conversely, summer's low flow and high temperatures appear to favor increased biological activity, leading to higher DOC production and lower DIC through CO 2 consumption via photosynthesis. Our findings on the contrasting seasonal patterns of DOC and DIC in the Styr River align with observations in other small river systems (e.g., Zhong et al. ( 2023 ); Chaplot and Mutema ( 2021 ). Further research is needed to explore the relative contributions of specific biological processes (e.g., phytoplankton blooms, macrophyte decomposition) to DOC dynamics in the Styr River. Furthermore, the Styr river's DOC concentration is more prone to temporal changes, with a coefficient of variation of 37%, which is higher than the DIC concentration's coefficient of variation of 16%. DIC concentration is the dominant form of carbon, accounting for up to 78% of the total soluble carbon content. The Styr River exhibits a higher coefficient of variation for DOC (37%) compared to DIC (16%), suggesting that DOC concentrations are more susceptible to temporal fluctuations. This could be attributed to factors like seasonal variations in biological activity and rainfall patterns, which can significantly impact the production and delivery of organic matter to the river (Kuznietsov and Biedunkova ( 2024 ). The average DOC concentration was 0.81 mM, significantly higher than the global average of 0.479 mM reported in Reddy et al. ( 2019 ), Liu et al. ( 2021 ). However, study Chaplot and Mutema ( 2021 ) have shown that DOC concentrations in small rivers tend to be lower than in large rivers. The Styr River water has high DOC concentrations due to the steep landscape morphology, which limits water contact with soils and reduces dilution. Furthermore, the average DIC concentration in the Styr River water is 3.65 mM, which is significantly higher than the global average concentration of 0.858 mM reported by Zhang et al. ( 2020 ); Smith and Swart ( 2022 ). The dominance of carbonate rocks in the Styr basin's lithology likely explains the elevated DIC concentration observed in the Styr River (Kuznietsov and Biedunkova ( 2023 ). This concentration is expected to be higher than the global average and constitute the major form of total dissolved carbon. However, small rivers generally have shorter flow paths and exhibit faster water turnover rates compared to larger rivers (Elenius et al. ( 2024 ). These characteristics result in less time for DIC to accumulate from the weathering of carbonate minerals. This suggests that the influence of carbonate geology on DIC and DOC concentrations in small rivers may vary depending on specific hydrological conditions and basin characteristics. Furthermore, the lower DIC concentrations in rivers contribute to the overall freshwater carbon cycle influencing the transfer of carbon between terrestrial and aquatic ecosystems. Understanding the distribution and dynamics of DIC in small rivers is crucial for comprehending the global carbon cycle and its interactions with freshwater systems. By recognizing the distinct characteristics of small rivers, we can gain a more holistic perspective on their role in regulating carbon fluxes and supporting aquatic life. The data shows a strong positive correlation between DIC concentration and water discharge (r = 0.8556, p < 0.001), as well as a strong negative correlation between DOC concentration and water flow in the Styr river (r = -0.7865, p < 0.001) (Fig. 6 a,b). Additionally, the positive relationships depicted in Fig. 6 a suggest that the leaching of DIC forms by reaction (1, 2) is not offset by dilution dilution when the river flow rate increase. However, there is a strong negative correlation between the decrease of DОC forms and flow rate (Fig. 6 b), indicating that dilution significantly contributes to the formation of DОC concentration. The decrease in flow rate is observed during the period of DIC form accumulation, while the increase in flow rate occurs in spring at low temperatures when photosynthetic intensity is insignificant and does not lead to an increase in DIC concentration. During periods of high flows, it is presumed that more CO 2 and CO 3 − accumulate in the water layer of the river and are not released into the atmosphere due to rapid transport and a lower surface area to volume ratio caused by the greater water depth. However, comparing the observed DIC and DOC to Q relationship most studies show dilution effects (Zhong et al. ( 2021 ). Moreover, it is plausible that the flux of carbon release exceeds the flux of CO 2 consumption attributed to silicate weathering (1, 2), a notion corroborated by research Zhong et al. ( 2023 ). Mitchell et al. ( 2009 ) found that an increase in the pH of natural waters leads to a shift in carbonate equilibrium towards the formation of calcium carbonate precipitates. The different correlations between water flow and DOC vs. DIC concentrations in the Styr River (Fig. 6 a, b) highlight the different responses of these carbon compounds to hydrological changes. While increased flow dilutes DOC, it appears to enhance the leaching of DIC from carbonate rocks (1, 2). This suggests that the influence of water flow on dissolved carbon dynamics in rivers depends on the specific carbon fraction and the geological context. In addition, Kuznetsov and Bedunkova (2023) note that the pH of the Styr River is subject to seasonal variations. For instance, during winter, the pH decreases to 7.5-8.0 units, while in summer, it increases to 7.8–8.5 units, depending on the water temperature. A strong negative correlation was observed between DIC and pH (r = -0.7568, p < 0.001) (Fig. 6 c). This can be explained by the dependence of water pH and the content of carbonate system components on CO 2 absorption and release. No correlation was found between DOC and pH (Fig. 6 d). The Styr River experiences its highest water temperatures during the summer and autumn months. A strong negative correlation (r = -0.7166, p < 0.001) exists between DIC and water temperature, while a strong positive correlation (r = -0.6874, p < 0.001) exists between DOC and temperature (Fig. 6 e, f). The correlation between DIC and TDS (Fig. 6 g) confirms that dilution has an insignificant effect (r = 0.4876, p < 0.001) on DIC concentration in the Styr River water. Moreover, the lack of correlation (r = -0.2154, p < 0.001) between DOC and TDS (Fig. 6 h) confirms the absence of a dilution effect on DIC concentration in the water of the Styr river. The strong negative correlation between DIC and pH (Fig. 6 c) supports the hypothesis that CO 2 outgassing is enhanced during periods of high flow and low DIC concentration. This aligns with the observation that increased water depth during high flow events reduces the surface area to volume ratio, potentially limiting CO2 exchange with the atmosphere. Additionally, the lack of correlation between DOC and pH suggests that biological activity is not the primary driver of seasonal pH fluctuations. This finding establishes that the primary temperature processes intensifying biological productivity in the warm season are dominant. While most studies report a dilution effect on DIC with increasing flow (Zhong et al. ( 2021 ), the Styr River exhibits a positive correlation between DIC and water discharge (Fig. 6 a). This suggests that the dominance of carbonate rock weathering in the Styr basin outweighs the dilution effect of flow, potentially due to the relatively short flow paths and rapid water turnover characteristic of small rivers (Elenius et al. ( 2024 ). Further research into the specific geological formations and weathering rates within the Styr basin could help elucidate the unique DIC dynamics observed in this river. The Styr River's dominant form of carbon is DIC, and the temporal variation of the DIC-to-DOC ratio follows the variation of DIC concentration (Fig. 7 ). According to Smith and Swart ( 2022 ), the global average value of the DIC-to-DOC ratio is 4.25. In practice, the actual value of the DIC-to-DOC ratio for Styr river water ranged from 1.85 to 9.45, with an average of 4.53 and a standard deviation of ± 1.65mM (Fig. 7 b), which is comparable to the global average DIC-to-DOC ratio. Thus, the study found a weak negative correlation between DIC and DOC concentrations (r=-0.5089, p < 0.001), indicating a weak link between the processes of DIC and DOC concentration formation (Fig. 8 a). This suggests that while specific processes governing DIC and DOC may differ, and the seasonal variation in the DIC-to-DOC ratio suggests a potential link between this carbon cycle. Increased flow during spring and winter might enhance both DIC leaching and DOC dilution, leading to a lower ratio. Conversely, lower flow and higher temperatures in summer could favor biological activity, increasing DOC production and potentially impacting DIC through CO2 consumption, resulting in a higher ratio. As per Sharp and Byrne ( 2020 ), total alkalinity (TA) is defined as TA = [HCO 3 − ] + 2[CO 3 2− ]. In comparison, DIC is expressed as the sum of all types of inorganic carbon formed by the components of the carbonate system ([CO 2 ], [HCO 3 − ], [CO 3 2− ]). Thus, the ratio of DIC-to-TA changes linearly (Fig. 8 b) and has a strong positive relationship (r = 0.9056, p < 0.001), which may indicate the sustainability of the carbonate system components without anthropogenic inputs of inorganic carbon ([CO 2 ], [HCO 3 − ], [CO 3 2− ]). Despite the seasonal variation in DIC and DOC concentrations, the strong positive correlation between DIC and TA suggests a relatively stable carbonate system in the Styr River, possibly with minimal external inputs of inorganic carbon throughout the year. The DIC-to-TA ratio, as reported by Giani et al. ( 2023 ), can provide insight into the sources of carbon input in surface water, which is mainly composed of [HCO 3 − ] and [CO 3 2− ]. During high flow periods, DIC-to-TA ratios below and close to 1.0 were observed for the Styr River water (Fig. 9 a), while ratios above 1.0 were observed during low flow periods. On average, the ratio of DIC-to-TA was M = 1.22, ± SD = 0.21 (Fig. 9 b). The DIC-to-TA ratio values close to 1, as noted by Cai et al. ( 2004 ), indicate the formation of DIC concentration due to weathering of carbonate and silicate minerals, as shown in equations (1, 2). However, values greater than 1 may suggest the dominant reaction of CO 2 uptake through photosynthesis, resulting in the formation of DOC. This is supported by the increase in DОC concentration during the warm season and minimal water flows in the Styr River. In the waters of the Styr River, an increase in total hardness (TH, the sum of Ca 2+ and Mg 2+ ) is accompanied by an increase in Dissolved Inorganic Carbon (DIC) as shown in Fig. 9 a. Specifically, the positive moderate correlation between TH and DIC (r = 0.5526, p < 0.0001) indicates that higher concentrations of Ca 2+ and Mg 2+ contribute to elevated levels of DIC. Figure 9 b shows the relationship between DIC/TH ratio and TDS. The positive moderate correlation (r = 0.5623, p < 0.0001) suggests that higher TDS concentrations are associated with lower DIC/TH ratios. Elevated concentrations of Ca 2+ , Mg 2+ , and TDS, combined with reduced DIC/TH ratios, indicate an increase in the contribution of evaporite dissolution alongside carbonate weathering (Chen et al., 2021 ). Moreover, the intensity of carbonate weathering can be enhanced during rainfall due to increased flow and faster dissolution of carbonate rocks (Regnier et al., 2022 ). River inputs with DIC/TH < 2 lead to an increase in partial pressure of CO 2 ​ (pCO2​), suggesting that weathering fluxes with 1 < DIC/TH < 2 act as sinks for atmospheric CO 2 ​ on short-term timescales but become sources over long-term scales (Xu et al., 2021 ). Therefore, the data suggest that in the Styr River, the balance between carbonate weathering and evaporite dissolution plays a crucial role in regulating CO 2 ​ dynamics. Short-term increases in DIC/TH ratios act as temporary CO 2 ​ sinks, while prolonged conditions with low DIC/TH ratios contribute to long-term CO 2 ​ release. The concentrations of DIC and DOC in small rivers are dynamically influenced by a complex interplay of natural and anthropogenic factors (Fig. 11). Understanding these factors is crucial for comprehending the biogeochemical processes and ecological implications of carbon cycling in small riverine ecosystems (Sharma, 2024 ). These factors include the leaching of carbonate and silicate rocks and the absorption of CO 2 from the atmosphere. The underlying geological formations of a river basin significantly impact DIC and DOC dynamics (Xu et al. ( 2024 ). Carbonate rocks, such as limestone and dolomite, are a primary source of DIC, releasing calcium and magnesium ions that react with H 2 CO 3 to HCO 3 - and CO 3 2- ions. In contrast, silicate rocks, such as granite and gneiss, contribute less to DIC but can influence DOC levels through weathering processes that release organic matter (Klaes, 2023). This study demonstrates that water flow patterns and discharge rates play a critical role in transporting and diluting DIC and DOC. During high flow periods, increased water discharge can lead to dilution of both DIC and DOC, resulting in lower concentrations. Conversely, low flow periods can cause concentrations to rise as water residence time increases and interactions with the surrounding environment intensify. Biological processes, including photosynthesis and respiration, dynamically influence DIC and DOC concentrations. Photosynthesis removes CO 2 from the water, reducing DIC and potentially contributing to DOC production through the release of organic compounds. Conversely, respiration releases CO 2 , increasing DIC and potentially consuming DOC. Temperature has a direct influence on biological activity and the solubility of dissolved carbon species. The results of this study also show that warmer temperatures generally enhance biological processes, leading to increased DOC production and potential changes in DIC concentrations, which is common to all small rivers. Additionally, higher temperatures can decrease the solubility of CO 2 , potentially affecting the balance between dissolved and gaseous forms of carbon. Modifications to land cover, such as deforestation and urbanization, can alter the natural inputs of organic matter and inorganic carbon to rivers (Kuznietsov and Biedunkova ( 2024 ). Moreover, this study can be an initial monitoring step to identify detrimental anthropogenic factors, as it is known that deforestation can reduce DOC inputs from vegetation (Kozicka et al. ( 2023 ), while urbanisation can increase DOC and DIC concentrations due to runoff from impervious surfaces and wastewater treatment plants (Malin et al. ( 2024 ). Agricultural activities, such as the use of fertilisers, can introduce significant amounts of inorganic carbon and organic matter into rivers (Rashmi, I. et al. ( 2020 ). The deposition of atmospheric pollutants, such as sulphur dioxide and nitrogen oxides, can directly influence DIC and DOC concentrations (Bade ( 2009 ). These pollutants can acidify water bodies, reducing the availability of carbonate ions and potentially altering biological processes that affect carbon cycling (Kuznietsov ( 2024 ). It is important to consider that the Styr River is located in the zone of anthropogenic load of the RNPP (Kuznetsov and Tichomirov ( 2017 ); Kuznietsov et al. ( 2022 ), which increases the need for more thorough monitoring of water quality indicators. Consequently, the dynamics of DIC and DOC in small rivers may show unique characteristics compared to large rivers due to their specific hydrological conditions and basin properties. Small rivers often have shorter water flow paths and a faster water turnover than large rivers. This rapid water movement can limit the accumulation of DIC from weathering of carbonate rocks, as less time is allowed for these interactions. The steep topography of small river basins may limit the contact between water and terrestrial soils, potentially reducing the dilution of DOC from terrestrial sources. This limited interaction may result in higher DOC concentrations in small rivers compared to larger rivers with wider floodplains. The authors agree that the specific influence of factors on DIC and DOC dynamics may differ in other small river basins due to differences in geology, land use and climate. Nevertheless, understanding these basin-specific variations is important for developing effective management strategies for small river ecosystems. Conclusions The global carbon cycle involves dynamic ecological processes, which necessitates the study of carbon formation in various environments, including small rivers. This study analyzes the seasonal dynamics of DIC and DOC concentrations in the Styr River, Ukraine. Seasonal variations in DIC and DOC levels, alongside their correlations with water temperature, flow seasonality, and other indicators (TA, TDS, pH), were examined. A strong negative correlation between DIC and water flow suggests that rock leaching in the river's catchment area predominantly influences DIC concentration. DOC levels increased during warm seasons with low flows and high temperatures, indicating that biological processes play a dominant role in DOC formation. The high DIC-to-TA ratio during low flows may indicate significant CO 2 uptake reactions, leading to a shift in carbonate equilibrium and a reduction in DIC concentration. DIC constitutes 78% of the river's carbon content, with average concentrations (3.65 mM) exceeding global averages. Increased river flow enhances carbonate and silicate mineral leaching, raising DIC concentrations. Conversely, DOC concentration decreases with higher river flow, suggesting dilution as a key factor. Water temperature negatively correlates with DIC but positively with DOC, underscoring the influence of biological activity on DOC formation during warmer periods. The DIC-to-DOC ratio reflects DIC's seasonal pattern, with higher ratios during low flows potentially indicating CO 2 uptake processes. Geochemical analyses reveal that the interactions between TH, TDS, and DIC in the Styr River indicate active carbonate weathering and evaporite dissolution processes, which influence CO 2 ​ exchange dynamics with the atmosphere. This study will help to understand the processes of formation and alteration of DIC and DOC concentrations in small rivers and to monitor possible negative anthropogenic influences through changes in their concentrations. Declarations Author contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Olha Biedunkova and Pavlo Kuznietsov. The frst draft of the manuscript was written by Olha Biedunkova and Pavlo Kuznietsov, and all authors commented on previous versions of the manuscript. All authors read and approved the fnal manuscript. Data availability Data will be made available on request. Ethics approval and consent to participate Not applicable. Consent for publication Not applicable. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding Not applicable References Aguilar-Torrejón JA, Balderas-Hernández P, Roa-Morales G (2023) Relationship, importance, and development of analytical techniques: COD, BOD, and, TOC in water—An overview through time. SN Appl. Sci. 5:118. https://doi.org/10.1007/s42452-023-05318-7 Bade D (2009) Freshwater Carbon and Biogeochemical Cycles. The Princeton Guide to Ecology, edited by Simon A. Levin, Stephen R. Carpenter, H. 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Zhong J, Wang L, Caracausi A, Galy A, Li SL, Wang W, Zhang M, Liu CQ, Liu GM, Xu S (2023) Assessing the Deep Carbon Release in an Active Volcanic Field Using Hydrochemistry, δ13CDIC and Δ14CDIC Journal of Geophysical Research: Biogeosciences, 128, e2023JG007435. https://doi.org/10.1029/2023JG007435. Additional Declarations No competing interests reported. Supplementary Files GA.png Graphical abstract Cite Share Download PDF Status: Published Journal Publication published 28 Oct, 2024 Read the published version in Environmental Monitoring and Assessment → Version 1 posted Editorial decision: Revision requested 27 Sep, 2024 Reviews received at journal 26 Sep, 2024 Reviews received at journal 22 Sep, 2024 Reviewers agreed at journal 13 Sep, 2024 Reviewers agreed at journal 13 Sep, 2024 Reviewers agreed at journal 27 Jun, 2024 Reviewers invited by journal 25 Jun, 2024 Editor assigned by journal 20 Jun, 2024 Submission checks completed at journal 20 Jun, 2024 First submitted to journal 14 Jun, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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13:04:47","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4582267/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4582267/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10661-024-13309-3","type":"published","date":"2024-10-28T16:20:06+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":60080937,"identity":"ce37ddf2-0639-4847-bfce-776d78907d86","added_by":"auto","created_at":"2024-07-11 13:48:42","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":69116,"visible":true,"origin":"","legend":"\u003cp\u003eAlterations in water course of the Styr River from 2019-2022\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/3d490128ae753aed98b326d4.png"},{"id":60080138,"identity":"7f6fb438-c47d-4e63-9548-861c5782316d","added_by":"auto","created_at":"2024-07-11 13:40:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":18902,"visible":true,"origin":"","legend":"\u003cp\u003eAlterations of water temperate of the Styr River from 2019-2022\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/789c8b06cf5b1116fc93f576.png"},{"id":60081482,"identity":"05ed7877-4029-4e32-b127-c3f288ac5782","added_by":"auto","created_at":"2024-07-11 13:56:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":344925,"visible":true,"origin":"","legend":"\u003cp\u003eThe the measuring DIC and DOC of the Elementar liqui TOC II device (a – device, b – autosampler, с – measurement process)\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/adf5e6ba177a862b77710bb6.png"},{"id":60080939,"identity":"1c0dd29c-eae0-475a-8005-fca892e14c6c","added_by":"auto","created_at":"2024-07-11 13:48:42","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":101530,"visible":true,"origin":"","legend":"\u003cp\u003eDynamics of DOC concentration in the water of the Styr River\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/78ee0e4b28b4b220e0b36516.png"},{"id":60080942,"identity":"50ae51ef-9fef-4288-ae2c-7fef5549f530","added_by":"auto","created_at":"2024-07-11 13:48:42","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":44073,"visible":true,"origin":"","legend":"\u003cp\u003eDynamics of DIC concentration in the water of the Styr River\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/376e19563ee7032570ab1b2e.png"},{"id":60080940,"identity":"b7b92440-b405-4008-849d-e44a43a42cb4","added_by":"auto","created_at":"2024-07-11 13:48:42","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":176699,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelations of DIC and DOC concentrations between flow (a, b), pH (c, d), temperature (e, f), TDS (g, h) in the Styr river water\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/fc8feb6b8e9c771be9c341d2.png"},{"id":60080145,"identity":"fc576230-2f98-4bf4-85c8-bd7811063b6e","added_by":"auto","created_at":"2024-07-11 13:40:42","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":65414,"visible":true,"origin":"","legend":"\u003cp\u003eDynamics of the DIC-to-DOC ratio in the water of the Styr River\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/57ad24c7b1c93b7095c9a056.png"},{"id":60080144,"identity":"24d286af-d97e-411f-8070-237e2f624443","added_by":"auto","created_at":"2024-07-11 13:40:42","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":61098,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelations between the concentrations of DIC, DOC and TA in the water of the Styr River\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/7d10020149d8c91a2f89ad96.png"},{"id":60080941,"identity":"7853752a-cdb7-4dbd-8789-a90f3136d73f","added_by":"auto","created_at":"2024-07-11 13:48:42","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":70426,"visible":true,"origin":"","legend":"\u003cp\u003eDynamics of DIC-to-TA concentration in the water of the Styr River\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/03dba60fd68f03ec5f752db8.png"},{"id":60080147,"identity":"356ccb98-4c90-4493-99ef-8659caeee13f","added_by":"auto","created_at":"2024-07-11 13:40:42","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":123374,"visible":true,"origin":"","legend":"\u003cp\u003eDynamics of DIC, TH, and TDS concentration in the water of the Styr River\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/e32e15f3db9bce8b88ef12ed.png"},{"id":60080149,"identity":"0ea90f8c-de4c-4bca-ab12-4617d334ffe8","added_by":"auto","created_at":"2024-07-11 13:40:42","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":205994,"visible":true,"origin":"","legend":"\u003cp\u003eFactors influencing the formation of DIC and DOC concentrations in surface water\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/c56a8308fb8411fc137e5b28.png"},{"id":68207089,"identity":"544ad189-f284-4b22-bc41-8205c710234e","added_by":"auto","created_at":"2024-11-04 16:34:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1748670,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/f6175efe-384a-4a1c-b16a-d0b0660e2cbc.pdf"},{"id":60080140,"identity":"bdeb9fa7-50a4-4aad-b345-f58fced494a3","added_by":"auto","created_at":"2024-07-11 13:40:42","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":716098,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGraphical abstract\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"GA.png","url":"https://assets-eu.researchsquare.com/files/rs-4582267/v1/e07c2390c0d8048cf30b5b91.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Investigation of the formation and variability of dissolved inorganic carbon and dissolved organic carbon in the water of a small river (on the example of the Styr River, Ukraine)","fulltext":[{"header":"Highlights","content":"\u003cul\u003e\n \u003cli\u003eThe study are the data for monitoring content and trends in the carbon cycle in the water of rivers.\u003c/li\u003e\n \u003cli\u003eThe dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the Styr River (Ukraine) were studied.\u003c/li\u003e\n \u003cli\u003eThe dynamics of changes in DIC and DOC, their relationship with flow velocity, temperature, seasonality and correlations with other physicochemical parameters were identified and evaluated.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eFactors influencing DIC and DOC concentrations in surface waters were estimated.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"Introduction","content":"\u003cp\u003eThe content of carbon components (total, dissolved, inorganic and organic carbon) is a useful tracer of carbon cycling within rivers. Soria-Reinoso et al. (\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) have shown that rivers are important sites for carbon transport and critical components of the global carbon cycle. However, there is limited knowledge about longitudinal and temporal changes in carbon species in rivers. Organic and inorganic forms of carbon content in surface waters can be distinguished (Vachon et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2021\u003c/span\u003e): total organic carbon (TOC) is an indicator of the content of organic compounds, while total inorganic carbon (TIC) is an indicator of the content of inorganic compounds that form the components of the carbonate system: carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e), bicarbonate (HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e), and carbonate (CO\u003csub\u003e32\u0026minus;\u003c/sub\u003e). According to Kuznietsov and Biedunkova (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), the carbon content of TIC and TOC is an indirect indicator of the presence of carbon atoms in water, without providing information about the nature and structure of the substance.\u003c/p\u003e \u003cp\u003eHowever, the majority of carbon in rivers is found in the dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) forms. According to the standard DSTU EN 1484\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2003\u003c/span\u003e (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2003\u003c/span\u003e), DOC and DIC are parameters that include active chemical matter with dispersed molecules smaller than 0.45 \u0026micro;m. Surface water DIC has been used as an indicator of air pollution, water quality, and the degree of dissolution of chemical species, such as carbonates. Similarly, DOC can be used as an indicator of aquatic pollution, with high levels of DOC leading to reduced biodiversity and highly degraded aquatic ecosystems (Joesoef et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). DIC, which is generally present at higher concentrations, is more easily measured than DOC. In terms of the carbon cycle, a large fraction is returned to the atmosphere through organic carbon decomposition within inland waters, transported to adjacent waters, and buried in freshwater. The most important sources of DIC in surface waters are carbonate and silicate weathering processes (1, 2). Additionally, the among-catchment behavior of HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026minus; responding to changing runoff was correlated with the average Da of each catchment. Globally, HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e behaviors and weathering characteristics are highly influenced by carbonate bedrock distributions and abundance Zhong et al. (\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). According to the study by Cai et al. (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), the DIC and DOC contents in river waters depend on the size of the water region, climate, watercourse, and the season of sample collection. Therefore, it is important to conduct practical studies for a particular water body.\u003c/p\u003e \u003cp\u003eCaCO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO \u0026rarr; 2HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e + Ca\u003csup\u003e2+\u003c/sup\u003e (1)\u003c/p\u003e \u003cp\u003eCaSiO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;3H\u003csub\u003e2\u003c/sub\u003eO \u0026rarr; 2HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e + Ca\u003csup\u003e2+\u003c/sup\u003e + H\u003csub\u003e4\u003c/sub\u003eSiO\u003csub\u003e4\u003c/sub\u003e (2)\u003c/p\u003e \u003cp\u003eThe total alkalinity (TA) of natural waters is primarily composed of [HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e] and [CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e]. According to Guo et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), DIC-to-TA ratios can provide broad insight into the sources of carbon, aquatic pH dynamics, and regional carbonate buffering capacity. Similarly, Wakana et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) found that an increase in DOC concentration leads to an increase in the weathering rate of minerals and solubility, which in turn affects the mobility and transport of many metals and organic contaminants. Thus, the ratio of DIC to DOC can provide broad insight into the transition of inorganic forms to organic forms of carbon. The correlation between DIC and DOC is determined by the regime of organic and inorganic carbon input and is specific to each type of water (Aguilar-Torrej\u0026oacute;n et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). It is known that geochemical processes can be identified through the analysis of parameters such as total hardness (TH), total dissolved solids (TDS), and DIC, which provide insights into carbonate weathering and evaporite dissolution, as well as their effects on CO\u003csub\u003e2​\u003c/sub\u003e dynamics between the river and the atmosphere (Chen et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Regnier et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Xu et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Additionally, the content of DIC and DOC in surface waters is not regulated according to EU and Ukrainian legislation. Although there have been numerous studies on dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in rapidly transiting large river systems, research on small rivers is limited. Therefore, there is a lack of research on DIC and DOC in water Ukraine's rivers. In this study, we examined the DIC and DOC contents of the Styr River (Ukraine). The purpose of this article is to identify patterns of formation and changes in the concentration of DIC and DOC in the water of the Styr River. Monthly sampling was conducted between 2019 and 2022 to examine the dynamics of DIC and DOC changes. The relationship between these changes and discharge, temperature, and seasonality was also investigated, as well as their correlations with other indicators such as hydrogen pH levels, total alkalinity (TA), and total dissolved solids (TDS).\u003c/p\u003e \u003cp\u003eThe dynamics of DIC and DOC in small rivers have far-reaching implications for the health and functioning of aquatic ecosystems. These carbon species serve as essential energy sources for primary producers and support the intricate food webs that sustain riverine biodiversity. Changes in DIC and DOC concentrations can disrupt these ecosystems, affecting nutrient cycling, phytoplankton productivity, and overall ecosystem stability. Furthermore, small rivers play a significant role in the global carbon cycle, acting as conduits for transporting terrestrial carbon to the ocean.\u003c/p\u003e"},{"header":"Method and Theory","content":"\u003cp\u003eThe object of the study was the water of the river Styr. The river Styr basin is located in the north-west of Ukraine, within the Lviv, Volyn and Rivne regions. Additionally, the river is 494 km long and covers an area of 13100 km\u0026sup2;, with an average annual water flow of 49.5 m\u0026sup3;/s at the mouth, narrowing to 10\u0026ndash;20 m in the upper reaches and widening to 30\u0026ndash;50 m in the middle and lower reaches Kuznietsov and Biedunkova (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The bottom of the riverbed is composed of sandstones, and shales of Cretaceous and Paleogene age, and in the lower reaches - Miocene clay deposits. However, according to the surface water typology, the Styr River is a lowland, sandy loamy river (Kuznietsov et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Sampling for chemical monitoring was carried out at the Styr River (hydrological post) in the Rivne Nuclear Power Plant intake zone according to 2019\u0026ndash;2022. The study results were statistically processed using the BioEstar software package (Version 5.3, MLM). This involved determining the range of data series (min-max), arithmetic mean (M), standard deviation (\u0026plusmn;\u0026thinsp;SD), and coefficient of variation (CV) of the corresponding sample. Pearson's correlation coefficient (r) was used to assess the relationship between the control outcome variables, along with the significance of influence (p) according to Kahaer et al. (2019). In particular, the river Styr water flow (D) during monitoring at RNPP hydrological post varied in the range (min-max) from 10 to 63m\u003csup\u003e3\u003c/sup\u003e/s, arithmetic mean (M) 27m\u003csup\u003e3\u003c/sup\u003e/s, standard deviation (\u0026plusmn;\u0026thinsp;SD)\u0026thinsp;\u0026plusmn;\u0026thinsp;18m\u003csup\u003e3\u003c/sup\u003e/s (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). In addition, the water temperature of the Styr River (Т) during monitoring at RNPP hydrological post varied in the range (min-max) from 0.3 to 24.6 \u003csup\u003eо\u003c/sup\u003eС, M\u0026thinsp;=\u0026thinsp;12.6 \u003csup\u003eо\u003c/sup\u003eС, \u0026plusmn;SD\u0026thinsp;=\u0026thinsp;\u0026plusmn;\u0026thinsp;8.7 \u003csup\u003eо\u003c/sup\u003eС (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis study involved measuring the concentration of DIC, DOC, hydrogen pH levels, total alkalinity (TA), total dissolved solids (TDS) in the certified laboratory of RNPP according to a standardized method (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCharacteristics of measurement methods of this study*\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eХ\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC\u003csub\u003eI\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eδ, % (Δ, unit)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eME\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDIC, DOC, mM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.3\u0026ndash;100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003efrom 0.3 to 10, δ=\u0026plusmn;10; more than 10, δ=\u0026plusmn; 5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eElementar liquiTOC II, thermocatalytic oxidation method at 680\u0026deg;C with NDIR, measurement time DOC: 3 min, DIC: 4 min\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003epH, unit\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1\u0026ndash;10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e(Δ\u0026thinsp;=\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIonomer I-160 with glass electrodes\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTA, mM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.4\u0026ndash;20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTitration burettes\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTDS, mg/dm\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50\u0026ndash;10000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eδ = \u0026plusmn; 5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLaboratory scale\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eNote: * X \u0026ndash; the components, C\u003csub\u003eI\u003c/sub\u003e \u0026ndash; the measurement range, δ \u0026ndash; the relative (δ) or absolute (Δ) measurement errors, MPM - method of measurement, ME -measuring equipment.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eSampling was conducted following the guidelines of DSTU ISO 5667-2:\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2003\u003c/span\u003e (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). The concentration of DIC, and DOC was measured using DSTU EN 1484\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2003\u003c/span\u003e (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) guidelines. The measurement method for DIC and DOC is based on the detection of CO\u003csub\u003e2\u003c/sub\u003e using a nondispersive infrared sensor (NDIR), which is produced during sample preparation through acidification (DIC) or oxidation (DOC) of the sample. Based on the infrared absorption grading graph of CO2 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), the suitable concentration of DIC and DOC can be determined. To measure the concentrations of DIC and DOC, the samples were filtered through a 0.45 \u0026micro;m membrane filter (MF-Millipore) after preliminary preparation. The pH of the water was measured using the potentiometric method, TDS was measured using the gravimeter method, and TA was measured by titration (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e "},{"header":"Results And Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003cp\u003eThe concentrations of DIC and DOC in the Styr River varied significantly depending on the season, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. During spring and winter, when there is a higher river flow (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and the lowest water temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) in the Styr River, the DOC concentration was lower (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). In contrast, the DIC concentration was lowest in summer and the first months of autumn (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), when river flows were minimal (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and the highest water temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) of the Styr River was observed. The Styr River exhibits maximum DOC concentration during the summer months, which Kuznietsov et al. (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) attribute to an increase in water temperature and intensification of biological processes, as well as the development and growth of plants and hybrids. Conversely, the maximum DIC concentration in the Styr River are observed during the winter months, which Drake et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) explain as a result of decreased photosynthesis processes leading to the formation of CO\u003csub\u003e2\u003c/sub\u003e and the accumulation of HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. Note that an increase in flow rate, and therefore an increase in carbonate leaching by reaction (2), led to an increase in DIC concentration. Conversely, a decrease in flow rate and an increase in photosynthesis during summer resulted in an increase in DIC concentration (Meng et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Therefore, concentrations of DOC and DIC in the Styr River is primarily influenced by carbon leaching processes and biological processes that regulate photosynthesis, development, and mortality of aquatic organisms. Seasonal variations in river flow and water temperature appear to be the primary drivers of the opposing trends observed in DOC and DIC concentrations in the Styr River. Increased flow during spring and winter likely leads to dilution of DOC and enhanced carbonate mineral leaching, resulting in higher DIC concentrations. Conversely, summer's low flow and high temperatures appear to favor increased biological activity, leading to higher DOC production and lower DIC through CO\u003csub\u003e2\u003c/sub\u003e consumption via photosynthesis. Our findings on the contrasting seasonal patterns of DOC and DIC in the Styr River align with observations in other small river systems (e.g., Zhong et al. (\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2023\u003c/span\u003e); Chaplot and Mutema (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Further research is needed to explore the relative contributions of specific biological processes (e.g., phytoplankton blooms, macrophyte decomposition) to DOC dynamics in the Styr River.\u003c/p\u003e\u003cp\u003eFurthermore, the Styr river's DOC concentration is more prone to temporal changes, with a coefficient of variation of 37%, which is higher than the DIC concentration's coefficient of variation of 16%. DIC concentration is the dominant form of carbon, accounting for up to 78% of the total soluble carbon content. The Styr River exhibits a higher coefficient of variation for DOC (37%) compared to DIC (16%), suggesting that DOC concentrations are more susceptible to temporal fluctuations. This could be attributed to factors like seasonal variations in biological activity and rainfall patterns, which can significantly impact the production and delivery of organic matter to the river (Kuznietsov and Biedunkova (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The average DOC concentration was 0.81 mM, significantly higher than the global average of 0.479 mM reported in Reddy et al. (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), Liu et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). However, study Chaplot and Mutema (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) have shown that DOC concentrations in small rivers tend to be lower than in large rivers. The Styr River water has high DOC concentrations due to the steep landscape morphology, which limits water contact with soils and reduces dilution. Furthermore, the average DIC concentration in the Styr River water is 3.65 mM, which is significantly higher than the global average concentration of 0.858 mM reported by Zhang et al. (\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2020\u003c/span\u003e); Smith and Swart (\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The dominance of carbonate rocks in the Styr basin's lithology likely explains the elevated DIC concentration observed in the Styr River (Kuznietsov and Biedunkova (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). This concentration is expected to be higher than the global average and constitute the major form of total dissolved carbon. However, small rivers generally have shorter flow paths and exhibit faster water turnover rates compared to larger rivers (Elenius et al. (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). These characteristics result in less time for DIC to accumulate from the weathering of carbonate minerals. This suggests that the influence of carbonate geology on DIC and DOC concentrations in small rivers may vary depending on specific hydrological conditions and basin characteristics. Furthermore, the lower DIC concentrations in rivers contribute to the overall freshwater carbon cycle influencing the transfer of carbon between terrestrial and aquatic ecosystems. Understanding the distribution and dynamics of DIC in small rivers is crucial for comprehending the global carbon cycle and its interactions with freshwater systems. By recognizing the distinct characteristics of small rivers, we can gain a more holistic perspective on their role in regulating carbon fluxes and supporting aquatic life.\u003c/p\u003e \u003cp\u003eThe data shows a strong positive correlation between DIC concentration and water discharge (r\u0026thinsp;=\u0026thinsp;0.8556, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), as well as a strong negative correlation between DOC concentration and water flow in the Styr river (r = -0.7865, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea,b). Additionally, the positive relationships depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea suggest that the leaching of DIC forms by reaction (1, 2) is not offset by dilution dilution when the river flow rate increase. However, there is a strong negative correlation between the decrease of DОC forms and flow rate (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb), indicating that dilution significantly contributes to the formation of DОC concentration. The decrease in flow rate is observed during the period of DIC form accumulation, while the increase in flow rate occurs in spring at low temperatures when photosynthetic intensity is insignificant and does not lead to an increase in DIC concentration. During periods of high flows, it is presumed that more CO\u003csub\u003e2\u003c/sub\u003e and CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e accumulate in the water layer of the river and are not released into the atmosphere due to rapid transport and a lower surface area to volume ratio caused by the greater water depth. However, comparing the observed DIC and DOC to Q relationship most studies show dilution effects (Zhong et al. (\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Moreover, it is plausible that the flux of carbon release exceeds the flux of CO\u003csub\u003e2\u003c/sub\u003e consumption attributed to silicate weathering (1, 2), a notion corroborated by research Zhong et al. (\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Mitchell et al. (\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) found that an increase in the pH of natural waters leads to a shift in carbonate equilibrium towards the formation of calcium carbonate precipitates. The different correlations between water flow and DOC vs. DIC concentrations in the Styr River (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, b) highlight the different responses of these carbon compounds to hydrological changes. While increased flow dilutes DOC, it appears to enhance the leaching of DIC from carbonate rocks (1, 2). This suggests that the influence of water flow on dissolved carbon dynamics in rivers depends on the specific carbon fraction and the geological context.\u003c/p\u003e \u003cp\u003eIn addition, Kuznetsov and Bedunkova (2023) note that the pH of the Styr River is subject to seasonal variations. For instance, during winter, the pH decreases to 7.5-8.0 units, while in summer, it increases to 7.8\u0026ndash;8.5 units, depending on the water temperature. A strong negative correlation was observed between DIC and pH (r = -0.7568, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec). This can be explained by the dependence of water pH and the content of carbonate system components on CO\u003csub\u003e2\u003c/sub\u003e absorption and release. No correlation was found between DOC and pH (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed). The Styr River experiences its highest water temperatures during the summer and autumn months. A strong negative correlation (r = -0.7166, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) exists between DIC and water temperature, while a strong positive correlation (r = -0.6874, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) exists between DOC and temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee, f). The correlation between DIC and TDS (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eg) confirms that dilution has an insignificant effect (r\u0026thinsp;=\u0026thinsp;0.4876, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) on DIC concentration in the Styr River water. Moreover, the lack of correlation (r = -0.2154, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) between DOC and TDS (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eh) confirms the absence of a dilution effect on DIC concentration in the water of the Styr river. The strong negative correlation between DIC and pH (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec) supports the hypothesis that CO\u003csub\u003e2\u003c/sub\u003e outgassing is enhanced during periods of high flow and low DIC concentration. This aligns with the observation that increased water depth during high flow events reduces the surface area to volume ratio, potentially limiting CO2 exchange with the atmosphere. Additionally, the lack of correlation between DOC and pH suggests that biological activity is not the primary driver of seasonal pH fluctuations. This finding establishes that the primary temperature processes intensifying biological productivity in the warm season are dominant. While most studies report a dilution effect on DIC with increasing flow (Zhong et al. (\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), the Styr River exhibits a positive correlation between DIC and water discharge (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea). This suggests that the dominance of carbonate rock weathering in the Styr basin outweighs the dilution effect of flow, potentially due to the relatively short flow paths and rapid water turnover characteristic of small rivers (Elenius et al. (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Further research into the specific geological formations and weathering rates within the Styr basin could help elucidate the unique DIC dynamics observed in this river.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Styr River's dominant form of carbon is DIC, and the temporal variation of the DIC-to-DOC ratio follows the variation of DIC concentration (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). According to Smith and Swart (\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), the global average value of the DIC-to-DOC ratio is 4.25. In practice, the actual value of the DIC-to-DOC ratio for Styr river water ranged from 1.85 to 9.45, with an average of 4.53 and a standard deviation of \u0026plusmn;\u0026thinsp;1.65mM (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), which is comparable to the global average DIC-to-DOC ratio. Thus, the study found a weak negative correlation between DIC and DOC concentrations (r=-0.5089, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), indicating a weak link between the processes of DIC and DOC concentration formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea). This suggests that while specific processes governing DIC and DOC may differ, and the seasonal variation in the DIC-to-DOC ratio suggests a potential link between this carbon cycle. Increased flow during spring and winter might enhance both DIC leaching and DOC dilution, leading to a lower ratio. Conversely, lower flow and higher temperatures in summer could favor biological activity, increasing DOC production and potentially impacting DIC through CO2 consumption, resulting in a higher ratio.\u003c/p\u003e \u003cp\u003eAs per Sharp and Byrne (\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), total alkalinity (TA) is defined as TA = [HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e]\u0026thinsp;+\u0026thinsp;2[CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e]. In comparison, DIC is expressed as the sum of all types of inorganic carbon formed by the components of the carbonate system ([CO\u003csub\u003e2\u003c/sub\u003e], [HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e], [CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e]). Thus, the ratio of DIC-to-TA changes linearly (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb) and has a strong positive relationship (r\u0026thinsp;=\u0026thinsp;0.9056, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), which may indicate the sustainability of the carbonate system components without anthropogenic inputs of inorganic carbon ([CO\u003csub\u003e2\u003c/sub\u003e], [HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e], [CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e]). Despite the seasonal variation in DIC and DOC concentrations, the strong positive correlation between DIC and TA suggests a relatively stable carbonate system in the Styr River, possibly with minimal external inputs of inorganic carbon throughout the year.\u003c/p\u003e \u003cp\u003eThe DIC-to-TA ratio, as reported by Giani et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), can provide insight into the sources of carbon input in surface water, which is mainly composed of [HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e] and [CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e]. During high flow periods, DIC-to-TA ratios below and close to 1.0 were observed for the Styr River water (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea), while ratios above 1.0 were observed during low flow periods. On average, the ratio of DIC-to-TA was M\u0026thinsp;=\u0026thinsp;1.22, \u0026plusmn; SD\u0026thinsp;=\u0026thinsp;0.21 (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb). The DIC-to-TA ratio values close to 1, as noted by Cai et al. (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2004\u003c/span\u003e), indicate the formation of DIC concentration due to weathering of carbonate and silicate minerals, as shown in equations (1, 2). However, values greater than 1 may suggest the dominant reaction of CO\u003csub\u003e2\u003c/sub\u003e uptake through photosynthesis, resulting in the formation of DOC. This is supported by the increase in DОC concentration during the warm season and minimal water flows in the Styr River.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the waters of the Styr River, an increase in total hardness (TH, the sum of Ca\u003csup\u003e2+\u003c/sup\u003e and Mg\u003csup\u003e2+\u003c/sup\u003e) is accompanied by an increase in Dissolved Inorganic Carbon (DIC) as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea. Specifically, the positive moderate correlation between TH and DIC (r\u0026thinsp;=\u0026thinsp;0.5526, p\u0026thinsp;\u0026lt;\u0026thinsp;0.0001) indicates that higher concentrations of Ca\u003csup\u003e2+\u003c/sup\u003e and Mg\u003csup\u003e2+\u003c/sup\u003e contribute to elevated levels of DIC. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb shows the relationship between DIC/TH ratio and TDS. The positive moderate correlation (r\u0026thinsp;=\u0026thinsp;0.5623, p\u0026thinsp;\u0026lt;\u0026thinsp;0.0001) suggests that higher TDS concentrations are associated with lower DIC/TH ratios. Elevated concentrations of Ca\u003csup\u003e2+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e, and TDS, combined with reduced DIC/TH ratios, indicate an increase in the contribution of evaporite dissolution alongside carbonate weathering (Chen et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Moreover, the intensity of carbonate weathering can be enhanced during rainfall due to increased flow and faster dissolution of carbonate rocks (Regnier et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). River inputs with DIC/TH\u0026thinsp;\u0026lt;\u0026thinsp;2 lead to an increase in partial pressure of CO\u003csub\u003e2\u003c/sub\u003e​ (pCO2​), suggesting that weathering fluxes with 1\u0026thinsp;\u0026lt;\u0026thinsp;DIC/TH\u0026thinsp;\u0026lt;\u0026thinsp;2 act as sinks for atmospheric CO\u003csub\u003e2\u003c/sub\u003e​ on short-term timescales but become sources over long-term scales (Xu et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Therefore, the data suggest that in the Styr River, the balance between carbonate weathering and evaporite dissolution plays a crucial role in regulating CO\u003csub\u003e2\u003c/sub\u003e​ dynamics. Short-term increases in DIC/TH ratios act as temporary CO\u003csub\u003e2\u003c/sub\u003e​ sinks, while prolonged conditions with low DIC/TH ratios contribute to long-term CO\u003csub\u003e2\u003c/sub\u003e​ release.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe concentrations of DIC and DOC in small rivers are dynamically influenced by a complex interplay of natural and anthropogenic factors (Fig.\u0026nbsp;11). Understanding these factors is crucial for comprehending the biogeochemical processes and ecological implications of carbon cycling in small riverine ecosystems (Sharma, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). These factors include the leaching of carbonate and silicate rocks and the absorption of CO\u003csub\u003e2\u003c/sub\u003e from the atmosphere. The underlying geological formations of a river basin significantly impact DIC and DOC dynamics (Xu et al. (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Carbonate rocks, such as limestone and dolomite, are a primary source of DIC, releasing calcium and magnesium ions that react with H\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e to HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e ions. In contrast, silicate rocks, such as granite and gneiss, contribute less to DIC but can influence DOC levels through weathering processes that release organic matter (Klaes, 2023). This study demonstrates that water flow patterns and discharge rates play a critical role in transporting and diluting DIC and DOC. During high flow periods, increased water discharge can lead to dilution of both DIC and DOC, resulting in lower concentrations. Conversely, low flow periods can cause concentrations to rise as water residence time increases and interactions with the surrounding environment intensify. Biological processes, including photosynthesis and respiration, dynamically influence DIC and DOC concentrations. Photosynthesis removes CO\u003csub\u003e2\u003c/sub\u003e from the water, reducing DIC and potentially contributing to DOC production through the release of organic compounds. Conversely, respiration releases CO\u003csub\u003e2\u003c/sub\u003e, increasing DIC and potentially consuming DOC. Temperature has a direct influence on biological activity and the solubility of dissolved carbon species. The results of this study also show that warmer temperatures generally enhance biological processes, leading to increased DOC production and potential changes in DIC concentrations, which is common to all small rivers. Additionally, higher temperatures can decrease the solubility of CO\u003csub\u003e2\u003c/sub\u003e, potentially affecting the balance between dissolved and gaseous forms of carbon. Modifications to land cover, such as deforestation and urbanization, can alter the natural inputs of organic matter and inorganic carbon to rivers (Kuznietsov and Biedunkova (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Moreover, this study can be an initial monitoring step to identify detrimental anthropogenic factors, as it is known that deforestation can reduce DOC inputs from vegetation (Kozicka et al. (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), while urbanisation can increase DOC and DIC concentrations due to runoff from impervious surfaces and wastewater treatment plants (Malin et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Agricultural activities, such as the use of fertilisers, can introduce significant amounts of inorganic carbon and organic matter into rivers (Rashmi, I. et al. (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The deposition of atmospheric pollutants, such as sulphur dioxide and nitrogen oxides, can directly influence DIC and DOC concentrations (Bade (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). These pollutants can acidify water bodies, reducing the availability of carbonate ions and potentially altering biological processes that affect carbon cycling (Kuznietsov (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). It is important to consider that the Styr River is located in the zone of anthropogenic load of the RNPP (Kuznetsov and Tichomirov (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2017\u003c/span\u003e); Kuznietsov et al. (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), which increases the need for more thorough monitoring of water quality indicators.\u003c/p\u003e \u003cp\u003eConsequently, the dynamics of DIC and DOC in small rivers may show unique characteristics compared to large rivers due to their specific hydrological conditions and basin properties. Small rivers often have shorter water flow paths and a faster water turnover than large rivers. This rapid water movement can limit the accumulation of DIC from weathering of carbonate rocks, as less time is allowed for these interactions. The steep topography of small river basins may limit the contact between water and terrestrial soils, potentially reducing the dilution of DOC from terrestrial sources. This limited interaction may result in higher DOC concentrations in small rivers compared to larger rivers with wider floodplains. The authors agree that the specific influence of factors on DIC and DOC dynamics may differ in other small river basins due to differences in geology, land use and climate. Nevertheless, understanding these basin-specific variations is important for developing effective management strategies for small river ecosystems.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThe global carbon cycle involves dynamic ecological processes, which necessitates the study of carbon formation in various environments, including small rivers. This study analyzes the seasonal dynamics of DIC and DOC concentrations in the Styr River, Ukraine. Seasonal variations in DIC and DOC levels, alongside their correlations with water temperature, flow seasonality, and other indicators (TA, TDS, pH), were examined. A strong negative correlation between DIC and water flow suggests that rock leaching in the river's catchment area predominantly influences DIC concentration. DOC levels increased during warm seasons with low flows and high temperatures, indicating that biological processes play a dominant role in DOC formation. The high DIC-to-TA ratio during low flows may indicate significant CO\u003csub\u003e2\u003c/sub\u003e uptake reactions, leading to a shift in carbonate equilibrium and a reduction in DIC concentration. DIC constitutes 78% of the river's carbon content, with average concentrations (3.65 mM) exceeding global averages. Increased river flow enhances carbonate and silicate mineral leaching, raising DIC concentrations. Conversely, DOC concentration decreases with higher river flow, suggesting dilution as a key factor. Water temperature negatively correlates with DIC but positively with DOC, underscoring the influence of biological activity on DOC formation during warmer periods. The DIC-to-DOC ratio reflects DIC's seasonal pattern, with higher ratios during low flows potentially indicating CO\u003csub\u003e2\u003c/sub\u003e uptake processes. Geochemical analyses reveal that the interactions between TH, TDS, and DIC in the Styr River indicate active carbonate weathering and evaporite dissolution processes, which influence CO\u003csub\u003e2\u003c/sub\u003e​ exchange dynamics with the atmosphere. This study will help to understand the processes of formation and alteration of DIC and DOC concentrations in small rivers and to monitor possible negative anthropogenic influences through changes in their concentrations.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Olha Biedunkova and Pavlo Kuznietsov. The frst draft of the manuscript was written by Olha Biedunkova and Pavlo Kuznietsov, and all authors commented on previous versions of the manuscript. All authors read and approved the fnal manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on request. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u0026nbsp;\u003c/strong\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u0026nbsp;\u003c/strong\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e Not applicable\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAguilar-Torrej\u0026oacute;n JA, Balderas-Hern\u0026aacute;ndez P, Roa-Morales G (2023) Relationship, importance, and development of analytical techniques: COD, BOD, and, TOC in water\u0026mdash;An overview through time. 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Sustainability 15:16316. https://doi.org/10.3390/su152316316\u003c/li\u003e\n\u003cli\u003eKuznietsov P, Tykhomyrov A, Biedunkova O, Zaitsev S (2022) Improvement of methods for controlling power oil of cooling tower recycling water supply units at Rivne nuclear power plant. Scientific Horizons, 25(12), 69-79. https://doi.org/10.48077/scihor.25(12).2022.69-79.\u003c/li\u003e\n\u003cli\u003eKuznietsov PМ, Biedunkova OО, Yaroshchuk OV, Рryshchepa AM (2024) Optimization of the Anti-Scale Corrective Treatment of Water by Organic Phosphonate. Science and Innovation 20(2):79\u0026ndash;90. https://doi.org/10.15407/scine20.02.079\u003c/li\u003e\n\u003cli\u003eLiu F, Wang D, Zhang B, Huang J (2021) Concentration and biodegradability of dissolved organic carbon derived from soils: A global perspective. Science of The Total Environment 754:142378. https://doi.org/10.1016/j.scitotenv.2020.142378\u003c/li\u003e\n\u003cli\u003eMalin, J.T., Kaushal, S.S., Mayer, P.M. et al. 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Global Biogeochemical Cycles, 34, e2020GB006541. https://doi.org/10.1029/2020GB006541.\u003c/li\u003e\n\u003cli\u003eZhong J, Li SL, Zhu X, Liu J, Xu S, Xu S, Liu CQ (2021) Dynamics and fluxes of dissolved carbon under short-term climate variabilities in headwaters of the Changjiang River, draining the Qinghai-Tibet Plateau, Journal of Hydrology, Volume 596, 2021, 126128, ISSN 0022-1694, https://doi.org/10.1016/j.jhydrol.2021.126128.\u003c/li\u003e\n\u003cli\u003eZhong J, Wang L, Caracausi A, Galy A, Li SL, Wang W, Zhang M, Liu CQ, Liu GM, Xu S (2023) Assessing the Deep Carbon Release in an Active Volcanic Field Using Hydrochemistry, \u0026delta;13CDIC and \u0026Delta;14CDIC Journal of Geophysical Research: Biogeosciences, 128, e2023JG007435. https://doi.org/10.1029/2023JG007435.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-monitoring-and-assessment","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"emas","sideBox":"Learn more about [Environmental Monitoring and Assessment](http://link.springer.com/journal/10661)","snPcode":"10661","submissionUrl":"https://submission.nature.com/new-submission/10661/3","title":"Environmental Monitoring and Assessment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"natural surface waters, carbon cycle, hydrochemical parameters, managing water resources","lastPublishedDoi":"10.21203/rs.3.rs-4582267/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4582267/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"This paper presents the results of a study on the dynamics in the concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in water samples collected from the Styr River between 2019 and 2022. The concentrations of DIC and DOC were measured using an Elementar liqui TOC II analyzer. The study methodology involved analyzing the changes in DIC and DOC concentrations and their relationship with flow rates, temperature, seasonality, and other indicators such as hydrogen pH levels, total alkalinity (TA), and total dissolved solids (TDS). The purpose of this article is to identify patterns in the formation and changes of DIC and DOC concentrations in the Styr River. The concentrations of DIC and DOC in the samples ranged from 1.55-4.93 mM and 0.49-1.43 mM, respectively, with DOC accounting for an average of 22% of the total dissolved carbon content. The highest DOC concentrations were observed in summer, while the highest DIC concentrations were observed in winter. Based on the results, it can be concluded that water flow and temperature have an impact on DOC concentration, while flow, temperature, and pH affect DIC concentration. There was no correlation between DIC and DOC concentrations, but a strong positive relationship (r=0.9056, p\u003c0.001) was found between DIC and TA concentrations. Therefore, the main factors influencing DIC in the Styr River are those that affect the carbonate equilibrium, such as leaching of carbonate and silicate rocks, CO2 absorption from the atmosphere, and changes in pH. Additionally, the concentration of DOC is influenced by biological activity and is higher during the warm season. These findings can be used to develop a strategy for managing water resources in the Styr River basin and to assess and predict the ecological state of the river.","manuscriptTitle":"Investigation of the formation and variability of dissolved inorganic carbon and dissolved organic carbon in the water of a small river (on the example of the Styr River, Ukraine)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-11 13:40:37","doi":"10.21203/rs.3.rs-4582267/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-09-27T05:20:04+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-26T13:56:19+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-23T03:22:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"252493654212704697715504422309492241710","date":"2024-09-14T02:53:29+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"225292330009209267072667433447117530273","date":"2024-09-13T13:50:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"40023334902719372861578733656227822688","date":"2024-06-27T12:38:44+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-25T12:20:47+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-06-21T01:27:59+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-06-21T01:27:00+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Monitoring and Assessment","date":"2024-06-14T13:03:29+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-monitoring-and-assessment","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"emas","sideBox":"Learn more about [Environmental Monitoring and Assessment](http://link.springer.com/journal/10661)","snPcode":"10661","submissionUrl":"https://submission.nature.com/new-submission/10661/3","title":"Environmental Monitoring and Assessment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"230c64d3-739d-43fd-95d6-54ba52f1118f","owner":[],"postedDate":"July 11th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-11-04T16:25:22+00:00","versionOfRecord":{"articleIdentity":"rs-4582267","link":"https://doi.org/10.1007/s10661-024-13309-3","journal":{"identity":"environmental-monitoring-and-assessment","isVorOnly":false,"title":"Environmental Monitoring and Assessment"},"publishedOn":"2024-10-28 16:20:06","publishedOnDateReadable":"October 28th, 2024"},"versionCreatedAt":"2024-07-11 13:40:37","video":"","vorDoi":"10.1007/s10661-024-13309-3","vorDoiUrl":"https://doi.org/10.1007/s10661-024-13309-3","workflowStages":[]},"version":"v1","identity":"rs-4582267","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4582267","identity":"rs-4582267","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}