Freshwater Inflow Variability Affects the Relative Importance of Allochthonous Sources for Estuarine Fishes

Estuaries are influenced by seasonal and inter-annual variability in marine and freshwater intrusion. This variability affects the abundance and distribution of resources and consumers and may drive estuarine food web dynamics. This study tests for relationships among environmental variables and estuarine consumer δ13C, and assesses whether freshwater-derived sources increase in relative contribution to estuarine consumers during high freshwater inflow events. Data were collected seasonally from 2010 through 2016 for Patos Lagoon, Brazil, and included rainfall and river discharge in the drainage basin, and salinity, chlorophyll-α, and phytoplankton abundance in the estuarine zone of the lagoon. Similarly, stable isotope data (δ13C, δ15N) were collected seasonally for autochthonous sources and consumers (detritivorous and zooplanktivorous fishes) in the estuarine zone, and allochthonous sources of freshwater and marine origin. Mixed-effect models assessed relationships among consumer δ13C and environmental variables, and Bayesian mixing models (MixSIAR) estimated the relative importance of autochthonous and allochthonous sources for estuarine consumers. Hydrology affected estuarine consumer δ13C, corresponding to greater contributions of marine-derived organic material during periods with high freshwater inflow, especially for detritivorous juvenile mullet. This unexpected finding is likely due to high freshwater inflows delaying recruitment of juvenile mullet from coastal areas into the estuarine zone such that data from seasonal sampling reflected marine rather than local feeding and active transport of marine-derived sources into the estuarine zone. In addition to transporting freshwater-derived organic material, high freshwater inflow events may have other important indirect effects on the dynamics of trophic subsidies to estuarine food webs which require further study.