Long term data about δ15N in otoliths show increasing trophic position of perch (Perca fluviatilis) with increasing aridity related to ongoing climate change

Fish otoliths are unique archives of fish life history. To explore changes in aquatic food webs, δ15N and δ13C in perch otoliths (Perca fluviatilis) sampled in the Římov reservoir (Czechia) every summer from 2004 to 2021 were studied. δ13C in otoliths correlated positively with δ15N in otoliths and chlorophyll-a concentration in the epilimnion, and negatively with annual rainfall. δ15N in otoliths correlated positively with chlorophyll-a, total zooplankton, temperature, and negatively with total rainfall and also with Lang's rain factor (annual rainfall divided by annual temperature). The strongest correlation was found between chlorophyll-a and Lang's rain factor. With increasing chlorophyll-a, otoliths become heavier in δ15N, and also become heavier with decreasing Lang's rain factor, which suggests higher aridity. These two factors are also interconnected: when chlorophyll-a is higher, Lang's rain factor is lower (higher aridity). Both these factors are likely to correspond with ongoing climate change, since with higher aridity, more nutrients can be flushed into a reservoir, which leads to a higher phytoplankton growth as indicated by higher values of chlorophyll-a. δ15N in otoliths also correlates with zooplankton N but zooplankton N did not correlate with other measured parameters. The difference between the highest and lowest δ15N value is about 4 ‰, which represents nearly one trophic level, and suggests a substantial shift in food web structure. We propose that with increasing aridity, phytoplankton gets more abundant due to higher nutrient concentrations. Due to high temperature and food availability, young perch shift sooner from plankton to fish. This is supported by a marginally significant negative correlation between temperature and difference between age-class 3+ fish and older fish. Our study shows that increasing aridity, associated with ongoing climate change, causes a shift in food web structure in a temperate reservoir.