Analysis of δ13C and δ15N isotopic signatures to shed light on the hydrological cycle's influence on the trophic behavior of fish in a Mediterranean reservoir

Stable isotope analysis (SIA) of carbon and nitrogen sheds light on the origin of the food resources exploited by the fish and provides basic information on the trophic relationships among taxa. In this study, SIA of C and N was used to investigate the trophic behavior of fish species in a small Mediterranean reservoir, Lake Sos Canales (SC) in Sardinia, Italy, during an annual hydrological cycle. Fish were caught approximately every two months, and baseline isotopic C and N levels in the pelagic and littoral area were analyzed to establish the origin of fish food sources, considering suspended particulate matter, planktonic crustaceans and littoral macroinvertebrates. To assess the relative contribution of the two different sources using SIA, a Dynamic Baseline Mixing Model (DBMM) was applied and the results were compared with the fish gut contents. Our aim was to chart the seasonal trophic behavior of the fish species inhabiting an anthropogenic aquatic environment under considerable stress due to water level fluctuations. Isotopic results showed a seasonal trend with 13C levels depleted more in autumn-winter and less in spring-summer, while an inverse trend was recorded for 15N, both in the isotopic baseline values and in the fish. Isotopic results and gut content analysis highlighted a year-round strict dependence on littoral food sources only for the brown trout, whereas the mosquitofish changed their trophic behavior seasonally, shifting from littoral (high water level period) to pelagic (low water level period) food sources, mirroring the hydrological conditions of Lake SC.