Longitudinal and temporal variation in fish assemblage structure along lowland prairie Neotropical streams and their relationship with environmental variables

The structure of fish assemblages in lotic systems usually registers variations from the headwaters to the mouth related to geomorphological and physicochemical changes. We evaluated the main processes that structure the fish assemblages on the spatial, seasonal, and decadal (2009–2010; 2019–2020) scales and their relationship with environmental variables in two low-impacted Neotropical Pampean streams tributaries to the Río de la Plata river (RdlP). Environmental variables related to the longitudinal variation in the fish assemblage were depth, macrophyte coverage, and altitude, showing higher values in the upper stream sectors compared to the lower ones. Specific richness increased towards lower stream sectors due to the migration of riverine species from the RdlP. The beta diversity partitioning analysis registered that the greatest proportion of variability was explained by the turnover component in both the longitudinal and seasonal analyses. The upper sectors showed a more stable composition, consisting of species of the order Characiformes associated with aquatic macrophytes, while the lower sectors recorded greater instability due to the entry of migrant Siluriformes from the RdlP, highlighting the essential ecological function of Pampean streams for RldP fish assemblages. The decadal comparison showed a sampling site arrangement consistent with the longitudinal position and poorly related to the study period. Therefore, the consistent pattern of longitudinal variation in fish assemblages and its explicative variables have applicability in the conservation and biomonitoring of lowland streams, as well as a reference for recovery efforts.