Rapid shallowing can trigger dramatic ecological changes and challenges within and beyond arid, nutrient-rich lakes

Rapid shallowing can trigger dramatic ecological changes and challenges within and beyond arid, nutrient-rich lakes | 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 Article Rapid shallowing can trigger dramatic ecological changes and challenges within and beyond arid, nutrient-rich lakes Caroline Hung, Charles Diamond, Timothy Lyons This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5688203/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract The Salton Sea, California's largest lake, faces severe environmental degradation from agricultural runoff and diminishing water supply. Here we show the lake's shifted biogeochemical regime from these anthropogenic pressures. Field, historical and satellite data over the past 20 years reveal a breakdown of the lake's thermal stratification. As the lake level drops, the thermal barrier between oxygen-rich surface waters and anoxic bottom waters weakens, leading to a more mixed water column. Unlike its historical pattern of extreme anoxic events during late summer overturn, the lake now maintains poorly oxygenated, low-sulfidic waters, which alters seasonal algal production patterns. These conditions create a consistently unfavorable environment for wildlife and have regional implications for air quality. The lake's water quality evolution, categorized into three stages (stratified oxygenated-deoxygenated layers, expanded low-oxygen zones, and potential full oxygenation in shallow, nutrient-rich waters), highlights the utmost importance of targeted nutrient input reduction for effective management of shallowing arid lakes worldwide. Earth and environmental sciences/Environmental social sciences/Climate-change impacts/Environmental health Earth and environmental sciences/Biogeochemistry/Element cycles Earth and environmental sciences/Environmental sciences/Environmental chemistry/Pollution remediation Figures Figure 1 Figure 2 Figure 3 II. Introduction Tipping points in ecological and climate systems represent critical thresholds where minor disruptions can cause significant and often irreversible shifts between stable states, fundamentally reshaping system dynamics [ 1 , 2 , 3 ]. In the context of anthropogenic influences, these thresholds [ 4 ] frequently coincide with policy lags, where slow political and societal responses fail to match the rapid pace of ecological change [ 5 ].Current evidence of ecosystem shifts from climate change is apparent in actualized biological responses, such as shifts in forest types with ongoing drought and warming [ 6 ], delayed phenological patterns [ 7 ], and coral reef degradation [ 8 ]. These observations employ a combination of modeling, theoretical, and empirical approaches. However, documenting comprehensive biogeochemical responses at the ecosystem level before, during, and after tipping points can be challenging [ 9 ], including those within lake systems. Ecosystem-scale processes, critical for Earth-system functioning, are often based on biogeochemical flows, such as carbon cycle feedbacks [ 10 ] or phosphorus and nitrogen cycling [ 11 ]. Changes in elemental cycling often originate from localized zones, such as agricultural regions, but can lead to severe perturbations that affect global nutrient flows [ 12 , 13 , 14 ]. To capture regime shifts in biogeochemical cycling within an ecosystem, it is crucial to identify measurable indicators of ecosystem interactions. Lakes serve as valuable systems for this type of study, revealing sensitivities to anthropogenic modifications through responsive ecological transitions to nutrient loading and water-level changes [ 15 , 16 ]. They provide insights into early warning signals that foreshadow regime shifts, and observing lakes oscillate between states adds nuance to our understanding of ecological dynamics. Recent satellite observations have revealed that over half of the world's largest lakes are experiencing significant storage declines, with one-quarter of the human population residing in a basin of a drying lake [ 17 ]. Global human activities [ 18 ] such as overconsumption, agriculture, urbanization, land diversification, and climate-induced drought have exacerbated the water crisis, and the cascading impacts, while complex, can be seen on the ecosystem level [ 19 , 20 ]. The heavily nutrient polluted Salton Sea, a highly saline endorheic system and California’s largest lake, is well suited to studies of large-scale biogeochemical regime shifts [ 21 , 22 ]. Currently with a maximum depth of 10 meters and decreasing by approximately 0.3 meters per year [ 23 ], the lake covers an area of about 800 km² in the Coachella Valley region of the Sonoran Desert and is highly sensitive to non-point source agricultural pollution [ 24 ]. Factors such as untreated agricultural runoff, climate-induced drought, and water transfers for agricultural and urban usage have significantly altered its water chemistry and volume. After the Salton Basin was flooded in 1905 due to an irrigation canal breach, the lake would have dried up if not for former President Coolidge’s Executive Order in 1924, which designated it as an agricultural drainage collection site. In 1968, the California Legislature adopted a statute to further establish the Salton Sea primarily for the collection of agricultural drainage [ 25 ]. Under this policy, freshwater from the Colorado River is first channeled via aqueducts to irrigate agricultural fields in the Coachella and Imperial Valley regions before draining into the lake. In the arid climate of California’s desert, large amounts of water are used to flush excess salt residue from crops, resulting in tributaries and agricultural canals having significantly higher concentrations of dissolved solutes, nutrients, and pesticides compared to the original Colorado River water [ 26 , 27 ]. This untreated input has sustained the lake for the past century but has also led to hypereutrophication due to the influx of high nutrient levels (phosphate: >>0.5 mg/L; nitrate: >>1.5 mg/L [ 28 ]). Consequently, the lake has suffered from harmful algal blooms, anoxia, massive fish kills that have led to the local extinction of fish [ 29 ], and foul odors from sulfide volatilization [ 30 , 31 ]. While hydrological models often focus on major drivers of lake elevation decline, such as reduced Colorado River inflows due to State water transfer agreements [ 32 , 33 ], they may overlook crucial patterns and changes in biogeochemical cycling. These ecosystem interactions, including water depth, inflow volume, nutrient dynamics, and temperature changes, are essential for understanding lake stratification and associated ecological responses, which can have significant public health and economic impacts [ 34 ]. Current research largely focuses on how climate-driven temperature rises affect thermal stratification in lakes [ 35 , 36 ]. However, there is a gap in integrated research addressing how lake shallowing—another major consequence of climate-induced drought—impacts water column biogeochemical cycling and thus lake ecology. Our study addresses a critical knowledge gap by examining the Salton Sea's biogeochemical regime shift in response to rapid shallowing. We focus on how increased mixing frequency has altered thermal stratification and redox processes over a 20-year period. Our analysis captures the progression of annual and seasonal variations in water column redox, driven by the complex interplay of microbial metabolisms and elemental cycling processes along redox gradients. These processes include primary producer photosynthesis, organic matter decay, and sulfide production by anaerobic sulfate reducers at depth. To document recent, incremental, and fundamental changes, we synthesize multiple data sources: our in-situ measurements, previous studies [ 37 , 30 ], public sulfide emissions data [ 38 ], and satellite observations of redox-sensitive phenomena [ 39 ]. This comprehensive approach, integrating physical, chemical, and biological factors, provides a holistic view of the Salton Sea's evolving biogeochemistry under climate change and anthropogenic pressures. III. Results Shift in Thermal Stratification and Its Impact on Chemo-stratification Figure 1 illustrates changes in temperature (°C) and dissolved oxygen (mg/L) during representative summer and winter days in 1998 [ 37 ], 2006 [ 30 ], and 2020. We focus on July and August for summer analysis, with July representing typical baseline conditions and August showcasing mixing events driven by monsoon winds and seasonal temperature shifts [ 40 ]. Summer stratification patterns have evolved significantly: July thermoclines transitioned from three distinct layers in 1998 and 2006 to barely two in 2020, while August profiles show a reduction from three layers in 2006 to minimal layering in 2020. In terms of dissolved oxygen profiles in July, there is a shrinking layer of bottom anoxic water over time. In July 1998 and 2006, anoxia extended 4–5 meters up from the bottom (in regions with depths of 10m and 7m, respectively, or greater). By 2020, this bottom anoxic layer had shrunk to just 2 meters (starting at 9m depth). During August mixing events, patterns also changed: in 2006, waters below surface 2m depth consistently became anoxic, while in 2020, oxygen levels typically stayed at summer baseline levels (i.e., in July), with complete oxygen depletion occurring only rarely. Supplementary Fig. 1 compares water column oxygen data from two periods (2005–2007 and 2020–2022), confirming this shift in lake conditions. The earlier period (2005–2007) shows frequent occurrences of completely oxygen-depleted water columns just below surface waters, while recent data (2020–2022) reveal a trend toward low dissolved oxygen levels (i.e., < 3 mg/L) in the water column but without full anoxia. This longer-term dataset strengthens our understanding of how seasonal oxygen patterns have changed over time. Winter conditions, represented by December profiles, show similar temperature patterns between 2006 and 2020 with no distinct layering but lower dissolved oxygen levels in 2020 (~ 5 mg/L) compared to 2006 (~ 7 mg/L). Shift in Sulfur Cycling with the Expansion of Low-Oxygenated Zones The sulfur cycle plays a crucial part of water column biogeochemistry because of its relationship to nutrient and metal cycling, oxygen levels, and microbial community structure [ 42 , 43 ]. Hydrogen sulfide, a reduced sulfur species, is produced by anaerobic microbial sulfate reduction (MSR) and accumulates in completely anoxic waters. Excessive sulfide in the water column can lead to toxicity to aquatic life and the production of foul odors through sulfide volatilization [ 44 ]. The high sulfate concentrations (~ 190mM in the Salton Sea, more than 6 times the levels of seawater), large organic loading, and periods of anoxia have historically led to ample accumulation of sulfide in the deep water during summer months. We examined water column sulfide concentrations and volatilization patterns over the past two decades. Data for sulfide concentrations in the water column from 2005–2007 [ 30 ] and 2020–2022 (this study) were compiled (Fig. 2 ; bottom right). During the period 2005–2007, bottom water sulfide concentrations reached approximately 0.6 mM during summer anoxia. In contrast, the maximum recorded sulfide concentration in 2020–2022 dropped to about 0.06 mM, a tenth of the value recorded a decade earlier. Data on sulfide emission from the lake surface for the period 2014 to 2018 (Fig. 2 ; top right) were obtained from the South Coast Air Quality District [ 38 ] at the Nearshore Monitor (33.5183, -116.075356), located approx. 1.6 km from the northern shoreline of the lake. Although these data, which became available starting in 2014, are from different years than the sulfide concentration data presented earlier, the sulfide emission patterns show a trend like that observed for the bottom water sulfide concentrations. In the summers of 2014–2016, sulfide emissions showed a single large peak, suggesting a strong late summer influence on sulfide release at the water-air interface. However, in 2017–2018, multiple smaller peaks were detected as early as spring and spanning across the summer, indicating a shift in the timing, frequency, and magnitude of sulfide volatilization. In other words, there was a shift from a single large release to many smaller releases at different times of the year, not just the late summer. However, these emissions data are constrained by availability and the location of the monitoring station, which limits a comprehensive understanding of the sulfide volatilization mechanisms related to water column mixing. Shifting Biogeochemical Cycles During hot summer months, accelerated bacterial respiration of decaying algae leads to oxygen depletion and anoxia in bottom waters, promoting MSR and hydrogen sulfide accumulation. When these sulfidic bottom waters rise with subsurface currents and mix with oxic surface waters, sulfide irruptions coincide with oxidation and gypsum precipitation events often referred to as "greentides". This term originates from the high-reflectance gypsum crystals that emit green light in the visible spectrum, specifically at the 469 nm and 645 nm bands, oversaturating the surface waters [ 39 ]. These gypsum crystals typically settle after 2–3 days [ 31 ]. The NASA MODIS satellite can observe these distinctive features, with archives available from 2000 to the present day (NASA). Figure 3 shows that since 2007, the spatiotemporal relationships of greentides have changed significantly, specifically as expressed in shifts in their seasonal distributions and intensities. Originally, gypsum blooms had their largest peaks for surface area coverage during the summer months. Now, peaks are smaller but more frequent throughout the spring to fall, with significant transitions starting around 2007. This shift, specifically to lower intensities but more frequent occurrences, aligns with the expansion of low-oxygenated conditions throughout the water column (rather than anoxia confined to bottom waters; Fig. 1 ), as well as the lower sulfide accumulation in bottom waters and more frequent sulfide release throughout the year as lake mixes more often (Fig. 2 ). While the exact mechanisms behind greentides remain unclear, they are closely linked to key factors in biogeochemical systems, such as oxygen availability, MSR rates and locations (sediment v. water column), water upwelling and mixing, wind conditions, and the physicochemical controls on gypsum precipitation. IV. Discussion Impacts on Ecology Throughout Earth's history, shifts in the redox state of the atmosphere, oceans, and biosphere have driven biogeochemical cycling [ 44 , 45 ]. Since the industrial revolution, anthropogenic modifications have significantly amplified the sensitivities and dynamics of those processes [ 46 ]. The Salton Sea captures many of the causes and consequences of these changes, serving as a valuable microcosm for observing biogeochemical shifts on human timescales. The lake's physical and chemical dynamics are primarily controlled by its mixing patterns. Historically, it defied its common classification as a polymictic (well-mixed) lake [ 30 ]. While mixing completely in winter, summer brought strong thermal stratification and substantial hydrogen sulfide accumulation in bottom waters—a characteristic typically found in meromictic lakes where persistent density layers create isolated zones for sulfide formation and accumulation. This summer stratification created a clear division between oxygen-rich surface waters and anoxic bottom layers, driven by the temperature differential between sun-heated surface water and cooler bottom water. Multiple factors influence the properties of this layering: nutrient availability, lake level decline, temperature differential, wind stress, and water depth. Their interplay determines how seasonal biogeochemical cycles are maintained or disrupted in eutrophic lakes [47, 48]. Recent changes in these dynamics in the Salton Sea are evident in phenomena like gypsum precipitation events, which, governed in part by thermodynamic principles [ 49 ], signal broader shifts in catchment-scale processes since 2007 (Fig. 3 ; [ 50 , 51 ]). Today, the Salton Sea has transformed significantly. Its historical pattern of seasonal stratification has given way to true persistently polymictic conditions with frequent mixing throughout the year. This shift has fundamentally altered the lake's chemical character, resulting in more persistent low-level sulfide and expanded oxygen minimum zones due to increased summer mixing (Fig. 1 , 2 ; Supplementary Fig. 1). The ecosystem has transitioned from one with cyclical periods of thriving wildlife in surface waters interrupted by occasional catastrophic events to one with consistently unfavorable conditions, especially during summer months. These ecological challenges are exacerbated by the dramatic coeval increases in salinity [52]. This transformation poses significant challenges for restoration efforts. The persistent poor conditions could severely impede efforts to restore fish populations and other wildlife habitats, potentially rendering local extinction for fish year-round. This new reality contradicts the State Waterboard's designated use for the Salton Sea in habitat preservation and recreation [ 41 , 53 ], alongside agricultural drainage collection, necessitating a reevaluation of restoration strategies and management goals. Impacts on Local Communities The Salton Sea, situated in California's agricultural heartland of Coachella and Imperial Valleys and near the Los Angeles Metropolitan area, significantly impacts regional air quality and public health. Local agricultural activities and water diversion policies heavily influence the lake's water supply and quality. As the lake continues to shallow (Fig. 1 ), its internal nutrient cycling is projected to intensify. Phosphorus, typically retained in sediments when overlaying bottom waters are oxic, are released when the overlaying waters become anoxic [54], such as during the summer in the Salton Sea. Changing seasonal patterns of water column mixing and redox are certain to impact nutrient cycling and thus the spatiotemporal patterns of algal and bacterial production in the surface waters, including the frequency and magnitude of bloom events. We are just beginning to see and characterize these effects, including shifts in feedbacks related to primary production controls on water column oxygen distributions. Further, while single-pulse sulfide release events may become less common [ 55 ], changing patterns of biological production, and blooms specifically, could have dire consequences. For example, the link to public health is evident by rising hospitalization rates in the region on days of algal/bacterial blooms [ 56 ], and this phenomenon is particularly alarming in an area where childhood asthma prevalence is already double that of state and national averages [ 57 ]. Further complicating the situation is recent interest in the geothermal field of the lake’s southern shore as a potential lithium source [58, 59]. While this development could bring economic opportunities, the influx of workers settling into communities already susceptible to harm from the polluted lake risks exacerbating existing environmental and health challenges. Global Implications for Arid Lakes in Rapidly Changing Anthropogenic Environments This 20-year observation of biogeochemical cycling in the Salton Sea offers valuable insights for managing global inland arid lakes facing rapid shallowing and low water supply, particularly because many of those are similarly struggling with eutrophication [ 60 ]. Our findings highlight the profound influence of nutrient delivery on lake biogeochemistry and suggest a potential global trend in water quality deterioration prior to complete lake desiccation. Specifically, our study reveals a sequence of redox shifts that should be common in shallowing eutrophic lakes around the world: Initially, lakes are strongly stratified with distinct oxic and anoxic layers, As shallowing progresses, lakes experience expanded low-oxygenated zones throughout the water column (the current stage of the Salton Sea), Only when lakes become shallow will highly eutrophic settings develop full and persistent oxygenation at all depths, with a new suite of potential challenges via remobilization of sequestered metals and pesticides, for example. This sequence contradicts the common expectation that reducing lake level and corresponding increasing efficiency in mixing would consistently increase oxygenation [ 61 ]. Our observations also challenge the predictions made by Rueda and Schladow (2009) [ 62 ] that as the Salton Sea becomes shallower and its surface area decreases, it should experience stronger stratification, based on energy partitioning in horizontal scaling models. Our findings indicate that vertical depth is more crucial in determining stratification stability. Contrary to the common understanding that smaller lakes have weaker mixing due to reduced surface shear from diurnal winds, we found that shallower depths lead to more frequent mixing because of weakened vertical temperature differentials. The Salton Sea's most pressing, yet often overlooked, management priority is the reduction of nutrient inputs to improve water quality. This approach is important for managing similar eutrophic, shallowing systems worldwide [63, 64]. However, effective lake cleanup is complicated by broader societal trends: climate change, drought, population growth, and agriculture and urbanization that often lead to water transfer policies that prioritize economic benefits or human consumption over sustainable ecosystem preservation. Further, the Federal Clean Water Act does not regulate non-point source pollution from agriculture, leaving much of the assessment and mitigation to individual states. Martin et al. (2020)[ 5 ] highlights critical knowledge gaps in our understanding of ecological tipping points, particularly the concept of social lag, which cautions the delay between ecological deterioration and society's recognition and response to those changes. This lag impedes our ability to identify and prevent critical transitions in ecosystems [ 65 ]. To address this, we need more focus on thresholds and transitions between ecological states in polluted environments. This approach emphasizes the practical application of scientific findings into timely policy and management to address real human tragedies associated with ecosystem degradation. V. Methods Water quality monitoring data shown in Fig. 1 and Supplementary Fig. 1 were collected at the deepest portion of the southern basin (33.26265, -115.739), along with transects at shallower depths. Temperature, pH, and dissolved oxygen were measured in-situ using a calibrated YSI EXO2 multi-parameter sonde probe (YSI Incorporated, Yellow Springs, OH, USA) to determine the temporal variation of lake stratification. Schroeder et al. (2002) [ 37 ] and Reese et al. (2008) [ 30 ] used comparable methods at the same location, albeit during different time periods in 1998 and 2005–2007, respectively. Water column samples for sulfide concentration determination (Fig. 2 ) were collected from 2020 to 2022 in the same location using a battery-powered peristaltic pump with in-situ filtering capabilities at 0.4 micron. Samples were preserved with powdered zinc acetate and analyzed spectrophotometrically using the Cline method [ 66 ]. This approach is consistent with the methods reported by Reese et al. (2008) [ 30 ] for the period 2005–2007. SCAQMD sulfide emissions data was collected at the Nearshore Monitor (33.5183, -116.075356) on an hourly basis [ 38 ]. Due to the frequent occurrence of 0 ppb readings (indicating no sulfide detection), the monthly maximum was chosen as a representative measure. The calculation of gypsum surface areas in the Salton Sea was conducted by Ma et al. (2020) [ 39 ] using MODIS/Terra [ 67 ] Level-1A satellite data from 2000 to 2018. The process involved converting the data to Level-1B format, applying partial atmospheric correction, and calculating the Rayleigh-corrected reflectance. A Gypsum Bloom Index (GI) was developed based on the spectral characteristics of gypsum-infested waters, particularly the difference between the baseline spectral curve and the peak value at 555 nm. To determine gypsum-covered areas, a threshold GI value of 0.01986 was established using the maximum gradient method. Pixels exceeding this threshold were classified as gypsum bloom-infested waters, and their total area was calculated to quantify the gypsum surface area. The accuracy of this method was validated through cross-comparison with Landsat TM/ETM+/OLI images, using 58 same-day matched image pairs. Declarations Author contributions Conceptualization: C.H., C.D., T.L. Methodology: C.H., C.D., T.L. Investigation: C.H., C.D., T.L. Writing: C.H., T.L. The authors declare no competing interests. Acknowledgements The authors are grateful to the Sonny Bono Salton Sea National Wildlife Refuge, who made fieldwork possible. The Coachella Valley Mountains Conservancy grant 38500000P010017 and the Lewis and Clark Fund for Exploration and Field Research in Astrobiology supported this project. Field supplies were partially funded by the EDGE Institute at the University of California, Riverside. References Lenton, T. M. et al. Tipping elements in the Earth's climate system. Proc. Natl. Acad. Sci. U. S. A. 105, 1786–1793 (2008) Scheffer, M. et al. Early-warning signals for critical transitions. Nature 461, 53–59 (2009) Dakos, V. et al. Ecosystem tipping points in an evolving world. Nat Ecol Evol 3, 355–362 (2019) Spake, R. et al. Detecting Thresholds of Ecological Change in the Anthropocene. Annu. Rev. Environ. Resour. 47, 797–821 (2022) Martin, R., Schlüter, M. & Blenckner, T. The importance of transient social dynamics for restoring ecosystems beyond ecological tipping points. Proc. 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Latinx and Indigenous Mexican Caregivers' Perspectives of the Salton Sea Environment on Children's Asthma, Respiratory Health, and Co-Presenting Health Conditions. Int. J. Environ. Res. Public Health 20, (2023) Dobson, P. et al. Characterizing the Geothermal Lithium Resource at the Salton Sea. Lawrence Berkeley National Laboratory Report (2023) Slattery, M., Kendall, A., Helal, N. & Whittaker, M. L. What do frontline communities want to know about lithium extraction? Identifying research areas to support environmental justice in Lithium Valley, California. Energy Research & Social Science 99, 103043 (2023) Wurtsbaugh, W. A. & Sima, S. Contrasting management and fates of two sister lakes: Great Salt Lake (USA) and Lake Urmia (Iran). Water 14, 3005 (2022) Lawson, R. & Anderson, M. A. Stratification and mixing in Lake Elsinore, California: an assessment of axial flow pumps for improving water quality in a shallow eutrophic lake. Water Res. 41, 4457–4467 (2007) Rueda, F. & Schladow, G. Mixing and stratification in lakes of varying horizontal length scales: Scaling arguments and energy partitioning. Limnol. Oceanogr. 54, 2003–2017 (2009) Scheffer, M. et al. Alternative equilibria in shallow lakes. Trends Ecol. Evol. 8, 275–279 (1993) Scheffer, M. & van Nes, E. H. Shallow lakes theory revisited: various alternative regimes driven by climate, nutrients, depth and lake size. Hydrobiologia 584, 455–466 (2007) Bentley, R. A. et al. Social tipping points and Earth systems dynamics. Front. Environ. Sci. Eng. China 2, (2014) Cline, J. D. SPECTROPHOTOMETRIC DETERMINATION OF HYDROGEN SULFIDE IN NATURAL WATERS1. Limnology and Oceanography 14.3, 454–458 (1969) NASA. NASA Worldview. NASA Earth Observing System Data and Information System https://worldview.earthdata.nasa.gov/ (2023) Additional Declarations There is NO Competing Interest. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5688203","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":395902226,"identity":"cc879c53-d2d8-4369-91aa-862bce49d784","order_by":0,"name":"Caroline Hung","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAvUlEQVRIiWNgGAWjYDACCRBhwMbDD+EyE6ulgk9GsoE0LWfkbAwOEKuFf3aP2cOvbWY8xjeSn31gqLBObCBoyZ0z5saybWk8ZjfSjGcwnEknrMVAIsdMWrLtGFBLDjMDY9thorX85zGeAdLyj0gtkh/OsPEAGUAtDURokbiRVibNUMHGI3HmmTFDwrF0Y4Ja+Gckb5P8YcBmz9+e/JjhQ421LEEtIMDMA2MlEKMcBBh/EKtyFIyCUTAKRiYAADWPNjsdwFohAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-5060-9389","institution":"University of California, Riverside","correspondingAuthor":true,"prefix":"","firstName":"Caroline","middleName":"","lastName":"Hung","suffix":""},{"id":395902227,"identity":"ea2cc884-50c8-46e2-a512-bbc293a62ddb","order_by":1,"name":"Charles Diamond","email":"","orcid":"","institution":"University of California, Riverside","correspondingAuthor":false,"prefix":"","firstName":"Charles","middleName":"","lastName":"Diamond","suffix":""},{"id":395902228,"identity":"d8b48616-0614-43d8-bf8b-b50681942f2a","order_by":2,"name":"Timothy Lyons","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Timothy","middleName":"","lastName":"Lyons","suffix":""}],"badges":[],"createdAt":"2024-12-21 06:55:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5688203/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5688203/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86327768,"identity":"4bba8c59-21fe-4b2a-a04d-94bc62ccef5d","added_by":"auto","created_at":"2025-07-09 11:22:24","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1192023,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eRepresentative water column temperature (°C) and dissolved oxygen (mg/L) profiles sampled at the deepest point in the southern Salton Sea (see inset) in the months of July, August, and December. Green, blue, and red lines show the year of collection for 1998 (only available for July [37]), 2006 [30], and 2020 (our data), respectively. For the dissolved oxygen graphs, the faded red box depicts the 5 mg/L water quality objective standard to protect beneficial uses designated for the Salton Sea [41]. Dotted red line shows the rare extreme water column state for August 2020.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Fig1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5688203/v1/427b734757dcc05c5568e97b.jpg"},{"id":86327781,"identity":"017d0764-e98c-4aeb-b76d-4a98e7895355","added_by":"auto","created_at":"2025-07-09 11:22:24","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1207336,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eThe left panel illustrates the connection between sulfide concentrations in the water column, sulfide gas volatilization, and the frequency of water column mixing as a function of thermal stratification and lake depth under two contrasting regimes. The bottom right panel presents monthly and quarterly water column sulfide concentrations sampled from the surface (grey circles), middle (open circles), and bottom (black circles) depths from 2005-2007 [30] and 2020-2022. The top right panel displays maximum monthly sulfide emissions in parts per billion (ppb) from the South Coast Air Quality Management District [38] during 2014-2016 and 2017-2018. The water column and emissions data for sulfide highlight the contrast between single-pulse high emissions and multiple lower emissions throughout the year.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Fig2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5688203/v1/e0539b5ef1a812658600349a.jpg"},{"id":86327780,"identity":"3d02aba2-0cdd-4d2c-92c3-e87196ad71c0","added_by":"auto","created_at":"2025-07-09 11:22:24","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":468081,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eMaximum monthly gypsum bloom area (km²), adapted from Ma et al. (2020)[39], is shown with filled circles. A three-period moving average trendline indicates a regime shift occurring between 2006 and 2007.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Fig3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5688203/v1/cf8f6c4332b694431af2a233.jpg"},{"id":86329132,"identity":"0e71b69f-7043-461e-9fab-de91355b5080","added_by":"auto","created_at":"2025-07-09 11:39:00","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4504404,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5688203/v1/6aeba2f5-e91d-48af-8674-baef9e4c8181.pdf"},{"id":86327764,"identity":"0829e73f-cc6f-414f-865b-f1aaaad17dc0","added_by":"auto","created_at":"2025-07-09 11:22:23","extension":"jpg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":901260,"visible":true,"origin":"","legend":"Supplementary Figure 1","description":"","filename":"SupplFig1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5688203/v1/2a30ba2968bae785dc990d8c.jpg"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Rapid shallowing can trigger dramatic ecological changes and challenges within and beyond arid, nutrient-rich lakes","fulltext":[{"header":"II. Introduction","content":"\u003cp\u003eTipping points in ecological and climate systems represent critical thresholds where minor disruptions can cause significant and often irreversible shifts between stable states, fundamentally reshaping system dynamics [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. In the context of anthropogenic influences, these thresholds [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] frequently coincide with policy lags, where slow political and societal responses fail to match the rapid pace of ecological change [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].Current evidence of ecosystem shifts from climate change is apparent in actualized biological responses, such as shifts in forest types with ongoing drought and warming [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], delayed phenological patterns [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], and coral reef degradation [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. These observations employ a combination of modeling, theoretical, and empirical approaches. However, documenting comprehensive biogeochemical responses at the ecosystem level before, during, and after tipping points can be challenging [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], including those within lake systems.\u003c/p\u003e \u003cp\u003eEcosystem-scale processes, critical for Earth-system functioning, are often based on biogeochemical flows, such as carbon cycle feedbacks [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] or phosphorus and nitrogen cycling [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Changes in elemental cycling often originate from localized zones, such as agricultural regions, but can lead to severe perturbations that affect global nutrient flows [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. To capture regime shifts in biogeochemical cycling within an ecosystem, it is crucial to identify measurable indicators of ecosystem interactions. Lakes serve as valuable systems for this type of study, revealing sensitivities to anthropogenic modifications through responsive ecological transitions to nutrient loading and water-level changes [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. They provide insights into early warning signals that foreshadow regime shifts, and observing lakes oscillate between states adds nuance to our understanding of ecological dynamics. Recent satellite observations have revealed that over half of the world's largest lakes are experiencing significant storage declines, with one-quarter of the human population residing in a basin of a drying lake [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Global human activities [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] such as overconsumption, agriculture, urbanization, land diversification, and climate-induced drought have exacerbated the water crisis, and the cascading impacts, while complex, can be seen on the ecosystem level [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe heavily nutrient polluted Salton Sea, a highly saline endorheic system and California\u0026rsquo;s largest lake, is well suited to studies of large-scale biogeochemical regime shifts [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Currently with a maximum depth of 10 meters and decreasing by approximately 0.3 meters per year [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], the lake covers an area of about 800 km\u0026sup2; in the Coachella Valley region of the Sonoran Desert and is highly sensitive to non-point source agricultural pollution [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Factors such as untreated agricultural runoff, climate-induced drought, and water transfers for agricultural and urban usage have significantly altered its water chemistry and volume. After the Salton Basin was flooded in 1905 due to an irrigation canal breach, the lake would have dried up if not for former President Coolidge\u0026rsquo;s Executive Order in 1924, which designated it as an agricultural drainage collection site. In 1968, the California Legislature adopted a statute to further establish the Salton Sea primarily for the collection of agricultural drainage [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Under this policy, freshwater from the Colorado River is first channeled via aqueducts to irrigate agricultural fields in the Coachella and Imperial Valley regions before draining into the lake. In the arid climate of California\u0026rsquo;s desert, large amounts of water are used to flush excess salt residue from crops, resulting in tributaries and agricultural canals having significantly higher concentrations of dissolved solutes, nutrients, and pesticides compared to the original Colorado River water [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. This untreated input has sustained the lake for the past century but has also led to hypereutrophication due to the influx of high nutrient levels (phosphate: \u0026gt;\u0026gt;0.5 mg/L; nitrate: \u0026gt;\u0026gt;1.5 mg/L [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]). Consequently, the lake has suffered from harmful algal blooms, anoxia, massive fish kills that have led to the local extinction of fish [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], and foul odors from sulfide volatilization [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhile hydrological models often focus on major drivers of lake elevation decline, such as reduced Colorado River inflows due to State water transfer agreements [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], they may overlook crucial patterns and changes in biogeochemical cycling. These ecosystem interactions, including water depth, inflow volume, nutrient dynamics, and temperature changes, are essential for understanding lake stratification and associated ecological responses, which can have significant public health and economic impacts [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Current research largely focuses on how climate-driven temperature rises affect thermal stratification in lakes [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. However, there is a gap in integrated research addressing how lake shallowing\u0026mdash;another major consequence of climate-induced drought\u0026mdash;impacts water column biogeochemical cycling and thus lake ecology. Our study addresses a critical knowledge gap by examining the Salton Sea's biogeochemical regime shift in response to rapid shallowing. We focus on how increased mixing frequency has altered thermal stratification and redox processes over a 20-year period. Our analysis captures the progression of annual and seasonal variations in water column redox, driven by the complex interplay of microbial metabolisms and elemental cycling processes along redox gradients. These processes include primary producer photosynthesis, organic matter decay, and sulfide production by anaerobic sulfate reducers at depth. To document recent, incremental, and fundamental changes, we synthesize multiple data sources: our in-situ measurements, previous studies [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], public sulfide emissions data [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], and satellite observations of redox-sensitive phenomena [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. This comprehensive approach, integrating physical, chemical, and biological factors, provides a holistic view of the Salton Sea's evolving biogeochemistry under climate change and anthropogenic pressures.\u003c/p\u003e"},{"header":"III. Results","content":"\u003cp\u003e \u003cb\u003eShift in Thermal Stratification and Its Impact on Chemo-stratification\u003c/b\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates changes in temperature (\u0026deg;C) and dissolved oxygen (mg/L) during representative summer and winter days in 1998 [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], 2006 [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], and 2020. We focus on July and August for summer analysis, with July representing typical baseline conditions and August showcasing mixing events driven by monsoon winds and seasonal temperature shifts [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. Summer stratification patterns have evolved significantly: July thermoclines transitioned from three distinct layers in 1998 and 2006 to barely two in 2020, while August profiles show a reduction from three layers in 2006 to minimal layering in 2020. In terms of dissolved oxygen profiles in July, there is a shrinking layer of bottom anoxic water over time. In July 1998 and 2006, anoxia extended 4\u0026ndash;5 meters up from the bottom (in regions with depths of 10m and 7m, respectively, or greater). By 2020, this bottom anoxic layer had shrunk to just 2 meters (starting at 9m depth). During August mixing events, patterns also changed: in 2006, waters below surface 2m depth consistently became anoxic, while in 2020, oxygen levels typically stayed at summer baseline levels (i.e., in July), with complete oxygen depletion occurring only rarely. Supplementary Fig.\u0026nbsp;1 compares water column oxygen data from two periods (2005\u0026ndash;2007 and 2020\u0026ndash;2022), confirming this shift in lake conditions. The earlier period (2005\u0026ndash;2007) shows frequent occurrences of completely oxygen-depleted water columns just below surface waters, while recent data (2020\u0026ndash;2022) reveal a trend toward low dissolved oxygen levels (i.e., \u0026lt;\u0026thinsp;3 mg/L) in the water column but without full anoxia. This longer-term dataset strengthens our understanding of how seasonal oxygen patterns have changed over time. Winter conditions, represented by December profiles, show similar temperature patterns between 2006 and 2020 with no distinct layering but lower dissolved oxygen levels in 2020 (~\u0026thinsp;5 mg/L) compared to 2006 (~\u0026thinsp;7 mg/L).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eShift in Sulfur Cycling with the Expansion of Low-Oxygenated Zones\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe sulfur cycle plays a crucial part of water column biogeochemistry because of its relationship to nutrient and metal cycling, oxygen levels, and microbial community structure [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Hydrogen sulfide, a reduced sulfur species, is produced by anaerobic microbial sulfate reduction (MSR) and accumulates in completely anoxic waters. Excessive sulfide in the water column can lead to toxicity to aquatic life and the production of foul odors through sulfide volatilization [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. The high sulfate concentrations (~\u0026thinsp;190mM in the Salton Sea, more than 6 times the levels of seawater), large organic loading, and periods of anoxia have historically led to ample accumulation of sulfide in the deep water during summer months. We examined water column sulfide concentrations and volatilization patterns over the past two decades. Data for sulfide concentrations in the water column from 2005\u0026ndash;2007 [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] and 2020\u0026ndash;2022 (this study) were compiled (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; bottom right). During the period 2005\u0026ndash;2007, bottom water sulfide concentrations reached approximately 0.6 mM during summer anoxia. In contrast, the maximum recorded sulfide concentration in 2020\u0026ndash;2022 dropped to about 0.06 mM, a tenth of the value recorded a decade earlier.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eData on sulfide emission from the lake surface for the period 2014 to 2018 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; top right) were obtained from the South Coast Air Quality District [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e] at the Nearshore Monitor (33.5183, -116.075356), located approx. 1.6 km from the northern shoreline of the lake. Although these data, which became available starting in 2014, are from different years than the sulfide concentration data presented earlier, the sulfide emission patterns show a trend like that observed for the bottom water sulfide concentrations. In the summers of 2014\u0026ndash;2016, sulfide emissions showed a single large peak, suggesting a strong late summer influence on sulfide release at the water-air interface. However, in 2017\u0026ndash;2018, multiple smaller peaks were detected as early as spring and spanning across the summer, indicating a shift in the timing, frequency, and magnitude of sulfide volatilization. In other words, there was a shift from a single large release to many smaller releases at different times of the year, not just the late summer. However, these emissions data are constrained by availability and the location of the monitoring station, which limits a comprehensive understanding of the sulfide volatilization mechanisms related to water column mixing.\u003c/p\u003e \u003cp\u003e \u003cb\u003eShifting Biogeochemical Cycles\u003c/b\u003e \u003c/p\u003e \u003cp\u003eDuring hot summer months, accelerated bacterial respiration of decaying algae leads to oxygen depletion and anoxia in bottom waters, promoting MSR and hydrogen sulfide accumulation. When these sulfidic bottom waters rise with subsurface currents and mix with oxic surface waters, sulfide irruptions coincide with oxidation and gypsum precipitation events often referred to as \"greentides\". This term originates from the high-reflectance gypsum crystals that emit green light in the visible spectrum, specifically at the 469 nm and 645 nm bands, oversaturating the surface waters [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. These gypsum crystals typically settle after 2\u0026ndash;3 days [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The NASA MODIS satellite can observe these distinctive features, with archives available from 2000 to the present day (NASA). Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows that since 2007, the spatiotemporal relationships of greentides have changed significantly, specifically as expressed in shifts in their seasonal distributions and intensities. Originally, gypsum blooms had their largest peaks for surface area coverage during the summer months. Now, peaks are smaller but more frequent throughout the spring to fall, with significant transitions starting around 2007. This shift, specifically to lower intensities but more frequent occurrences, aligns with the expansion of low-oxygenated conditions throughout the water column (rather than anoxia confined to bottom waters; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), as well as the lower sulfide accumulation in bottom waters and more frequent sulfide release throughout the year as lake mixes more often (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). While the exact mechanisms behind greentides remain unclear, they are closely linked to key factors in biogeochemical systems, such as oxygen availability, MSR rates and locations (sediment v. water column), water upwelling and mixing, wind conditions, and the physicochemical controls on gypsum precipitation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"IV. Discussion","content":"\u003cp\u003e \u003cb\u003eImpacts on Ecology\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThroughout Earth's history, shifts in the redox state of the atmosphere, oceans, and biosphere have driven biogeochemical cycling [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. Since the industrial revolution, anthropogenic modifications have significantly amplified the sensitivities and dynamics of those processes [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. The Salton Sea captures many of the causes and consequences of these changes, serving as a valuable microcosm for observing biogeochemical shifts on human timescales. The lake's physical and chemical dynamics are primarily controlled by its mixing patterns. Historically, it defied its common classification as a polymictic (well-mixed) lake [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. While mixing completely in winter, summer brought strong thermal stratification and substantial hydrogen sulfide accumulation in bottom waters\u0026mdash;a characteristic typically found in meromictic lakes where persistent density layers create isolated zones for sulfide formation and accumulation. This summer stratification created a clear division between oxygen-rich surface waters and anoxic bottom layers, driven by the temperature differential between sun-heated surface water and cooler bottom water. Multiple factors influence the properties of this layering: nutrient availability, lake level decline, temperature differential, wind stress, and water depth. Their interplay determines how seasonal biogeochemical cycles are maintained or disrupted in eutrophic lakes [47, 48]. Recent changes in these dynamics in the Salton Sea are evident in phenomena like gypsum precipitation events, which, governed in part by thermodynamic principles [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e], signal broader shifts in catchment-scale processes since 2007 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e; [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e50\u003c/span\u003e, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e51\u003c/span\u003e]).\u003c/p\u003e \u003cp\u003eToday, the Salton Sea has transformed significantly. Its historical pattern of seasonal stratification has given way to true persistently polymictic conditions with frequent mixing throughout the year. This shift has fundamentally altered the lake's chemical character, resulting in more persistent low-level sulfide and expanded oxygen minimum zones due to increased summer mixing (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; Supplementary Fig.\u0026nbsp;1). The ecosystem has transitioned from one with cyclical periods of thriving wildlife in surface waters interrupted by occasional catastrophic events to one with consistently unfavorable conditions, especially during summer months. These ecological challenges are exacerbated by the dramatic coeval increases in salinity [52]. This transformation poses significant challenges for restoration efforts. The persistent poor conditions could severely impede efforts to restore fish populations and other wildlife habitats, potentially rendering local extinction for fish year-round. This new reality contradicts the State Waterboard's designated use for the Salton Sea in habitat preservation and recreation [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e], alongside agricultural drainage collection, necessitating a reevaluation of restoration strategies and management goals.\u003c/p\u003e \u003cp\u003e \u003cb\u003eImpacts on Local Communities\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe Salton Sea, situated in California's agricultural heartland of Coachella and Imperial Valleys and near the Los Angeles Metropolitan area, significantly impacts regional air quality and public health. Local agricultural activities and water diversion policies heavily influence the lake's water supply and quality. As the lake continues to shallow (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), its internal nutrient cycling is projected to intensify. Phosphorus, typically retained in sediments when overlaying bottom waters are oxic, are released when the overlaying waters become anoxic [54], such as during the summer in the Salton Sea. Changing seasonal patterns of water column mixing and redox are certain to impact nutrient cycling and thus the spatiotemporal patterns of algal and bacterial production in the surface waters, including the frequency and magnitude of bloom events. We are just beginning to see and characterize these effects, including shifts in feedbacks related to primary production controls on water column oxygen distributions. Further, while single-pulse sulfide release events may become less common [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e], changing patterns of biological production, and blooms specifically, could have dire consequences. For example, the link to public health is evident by rising hospitalization rates in the region on days of algal/bacterial blooms [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e56\u003c/span\u003e], and this phenomenon is particularly alarming in an area where childhood asthma prevalence is already double that of state and national averages [\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. Further complicating the situation is recent interest in the geothermal field of the lake\u0026rsquo;s southern shore as a potential lithium source [58, 59]. While this development could bring economic opportunities, the influx of workers settling into communities already susceptible to harm from the polluted lake risks exacerbating existing environmental and health challenges.\u003c/p\u003e \u003cp\u003e \u003cb\u003eGlobal Implications for Arid Lakes in Rapidly Changing Anthropogenic Environments\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThis 20-year observation of biogeochemical cycling in the Salton Sea offers valuable insights for managing global inland arid lakes facing rapid shallowing and low water supply, particularly because many of those are similarly struggling with eutrophication [\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e]. Our findings highlight the profound influence of nutrient delivery on lake biogeochemistry and suggest a potential global trend in water quality deterioration prior to complete lake desiccation. Specifically, our study reveals a sequence of redox shifts that should be common in shallowing eutrophic lakes around the world:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eInitially, lakes are strongly stratified with distinct oxic and anoxic layers,\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAs shallowing progresses, lakes experience expanded low-oxygenated zones throughout the water column (the current stage of the Salton Sea),\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eOnly when lakes become shallow will highly eutrophic settings develop full and persistent oxygenation at all depths, with a new suite of potential challenges via remobilization of sequestered metals and pesticides, for example.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eThis sequence contradicts the common expectation that reducing lake level and corresponding increasing efficiency in mixing would consistently increase oxygenation [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e]. Our observations also challenge the predictions made by Rueda and Schladow (2009) [\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e] that as the Salton Sea becomes shallower and its surface area decreases, it should experience stronger stratification, based on energy partitioning in horizontal scaling models. Our findings indicate that vertical depth is more crucial in determining stratification stability. Contrary to the common understanding that smaller lakes have weaker mixing due to reduced surface shear from diurnal winds, we found that shallower depths lead to more frequent mixing because of weakened vertical temperature differentials.\u003c/p\u003e \u003cp\u003eThe Salton Sea's most pressing, yet often overlooked, management priority is the reduction of nutrient inputs to improve water quality. This approach is important for managing similar eutrophic, shallowing systems worldwide [63, 64]. However, effective lake cleanup is complicated by broader societal trends: climate change, drought, population growth, and agriculture and urbanization that often lead to water transfer policies that prioritize economic benefits or human consumption over sustainable ecosystem preservation. Further, the Federal Clean Water Act does not regulate non-point source pollution from agriculture, leaving much of the assessment and mitigation to individual states. Martin et al. (2020)[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] highlights critical knowledge gaps in our understanding of ecological tipping points, particularly the concept of social lag, which cautions the delay between ecological deterioration and society's recognition and response to those changes. This lag impedes our ability to identify and prevent critical transitions in ecosystems [\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e]. To address this, we need more focus on thresholds and transitions between ecological states in polluted environments. This approach emphasizes the practical application of scientific findings into timely policy and management to address real human tragedies associated with ecosystem degradation.\u003c/p\u003e"},{"header":"V. Methods","content":"\u003cp\u003eWater quality monitoring data shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Supplementary Fig.\u0026nbsp;1 were collected at the deepest portion of the southern basin (33.26265, -115.739), along with transects at shallower depths. Temperature, pH, and dissolved oxygen were measured in-situ using a calibrated YSI EXO2 multi-parameter sonde probe (YSI Incorporated, Yellow Springs, OH, USA) to determine the temporal variation of lake stratification. Schroeder et al. (2002) [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e] and Reese et al. (2008) [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] used comparable methods at the same location, albeit during different time periods in 1998 and 2005\u0026ndash;2007, respectively. Water column samples for sulfide concentration determination (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) were collected from 2020 to 2022 in the same location using a battery-powered peristaltic pump with in-situ filtering capabilities at 0.4 micron. Samples were preserved with powdered zinc acetate and analyzed spectrophotometrically using the Cline method [\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e66\u003c/span\u003e]. This approach is consistent with the methods reported by Reese et al. (2008) [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] for the period 2005\u0026ndash;2007. SCAQMD sulfide emissions data was collected at the Nearshore Monitor (33.5183, -116.075356) on an hourly basis [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Due to the frequent occurrence of 0 ppb readings (indicating no sulfide detection), the monthly maximum was chosen as a representative measure. The calculation of gypsum surface areas in the Salton Sea was conducted by Ma et al. (2020) [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e] using MODIS/Terra [\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e67\u003c/span\u003e] Level-1A satellite data from 2000 to 2018. The process involved converting the data to Level-1B format, applying partial atmospheric correction, and calculating the Rayleigh-corrected reflectance. A Gypsum Bloom Index (GI) was developed based on the spectral characteristics of gypsum-infested waters, particularly the difference between the baseline spectral curve and the peak value at 555 nm. To determine gypsum-covered areas, a threshold GI value of 0.01986 was established using the maximum gradient method. Pixels exceeding this threshold were classified as gypsum bloom-infested waters, and their total area was calculated to quantify the gypsum surface area. The accuracy of this method was validated through cross-comparison with Landsat TM/ETM+/OLI images, using 58 same-day matched image pairs.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eConceptualization: C.H., C.D., T.L. Methodology: C.H., C.D., T.L. Investigation: C.H., C.D., T.L. Writing: C.H., T.L. The authors declare no competing interests.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThe authors are grateful to the Sonny Bono Salton Sea National Wildlife Refuge, who made fieldwork possible. The Coachella Valley Mountains Conservancy grant 38500000P010017 and the Lewis and Clark Fund for Exploration and Field Research in Astrobiology supported this project. Field supplies were partially funded by the EDGE Institute at the University of California, Riverside.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLenton, T. M. et al. Tipping elements in the Earth's climate system. Proc. Natl. Acad. Sci. U. S. A. 105, 1786\u0026ndash;1793 (2008)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eScheffer, M. et al. Early-warning signals for critical transitions. Nature 461, 53\u0026ndash;59 (2009)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDakos, V. et al. 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Evol. 8, 275\u0026ndash;279 (1993)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eScheffer, M. \u0026amp; van Nes, E. H. Shallow lakes theory revisited: various alternative regimes driven by climate, nutrients, depth and lake size. Hydrobiologia 584, 455\u0026ndash;466 (2007)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBentley, R. A. et al. Social tipping points and Earth systems dynamics. Front. Environ. Sci. Eng. China 2, (2014)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCline, J. D. SPECTROPHOTOMETRIC DETERMINATION OF HYDROGEN SULFIDE IN NATURAL WATERS1. Limnology and Oceanography 14.3, 454\u0026ndash;458 (1969)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNASA. NASA Worldview. NASA Earth Observing System Data and Information System \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://worldview.earthdata.nasa.gov/\u003c/span\u003e\u003cspan address=\"https://worldview.earthdata.nasa.gov/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2023)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5688203/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5688203/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe Salton Sea, California's largest lake, faces severe environmental degradation from agricultural runoff and diminishing water supply. Here we show the lake's shifted biogeochemical regime from these anthropogenic pressures. Field, historical and satellite data over the past 20 years reveal a breakdown of the lake's thermal stratification. As the lake level drops, the thermal barrier between oxygen-rich surface waters and anoxic bottom waters weakens, leading to a more mixed water column. Unlike its historical pattern of extreme anoxic events during late summer overturn, the lake now maintains poorly oxygenated, low-sulfidic waters, which alters seasonal algal production patterns. These conditions create a consistently unfavorable environment for wildlife and have regional implications for air quality. The lake's water quality evolution, categorized into three stages (stratified oxygenated-deoxygenated layers, expanded low-oxygen zones, and potential full oxygenation in shallow, nutrient-rich waters), highlights the utmost importance of targeted nutrient input reduction for effective management of shallowing arid lakes worldwide.\u003c/p\u003e","manuscriptTitle":"Rapid shallowing can trigger dramatic ecological changes and challenges within and beyond arid, nutrient-rich lakes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-09 11:22:18","doi":"10.21203/rs.3.rs-5688203/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-earth-and-environment","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commsenv","sideBox":"Learn more about [Communications Earth and Environment](https://www.nature.com/commsenv/)","snPcode":"","submissionUrl":"","title":"Communications Earth \u0026 Environment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"8ee7bbf7-544a-453a-9f9b-ee053f7660f3","owner":[],"postedDate":"July 9th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":42174805,"name":"Earth and environmental sciences/Environmental social sciences/Climate-change impacts/Environmental health"},{"id":42174806,"name":"Earth and environmental sciences/Biogeochemistry/Element cycles"},{"id":42174807,"name":"Earth and environmental sciences/Environmental sciences/Environmental chemistry/Pollution remediation"}],"tags":[],"updatedAt":"2025-07-09T11:22:19+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-09 11:22:18","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5688203","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5688203","identity":"rs-5688203","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}