LAUSR

Estuaries are complex systems where environmental fluctuations occur over distinct timescales due to local meteorological and large-scale climatic factors. Consequently, studies with low temporal resolution and taxonomic coverage may fail to detect isotopic variations in basal sources, providing biased interpretations of isotope mixing models. We investigated the seasonal and El Nino Southern Oscillation (ENSO)-driven interannual variations in δ13C, δ15N and C:N values among distinct basal sources and their implications for mixing models interpretation in a subtropical estuary. δ13C variations among sources differed in their magnitude and timescales, being large enough to confound source-specific values. Macroalgae and POM δ13C varied seasonally, whereas ENSO effects prevailed for C3 and C4 salt marsh plants, highlighting the contrasting influence of local versus remote environmental drivers on short- and long-lived primary producers, respectively. Peaks of δ15N were detected for all sources during short-term anthropogenic nutrient inputs. Isotope mixing model comparisons showed that overlooking isotopic variations in basal sources under distinct ENSO conditions can cause misinterpretation of local trophic interactions and nutrient cycling. The present study contributes to design appropriate sampling delineations in highly variable aquatic environments, emphasizing the importance of comprehensive, long-term monitoring of estuarine primary producers to encompass environmental drivers of stable isotopic variations.