Trophic niche characterization of zooplankton and planktivorous fish: a comparative approach using stable isotopes in bulk and amino acid samples

Quantification of the size and overlap of trophic niches of species with similar diets is crucial to understand food web dynamics. However, there are difficulties in determining all food resources used by each species over time and resolving competition suggested by niche overlap. In this study, we compare niche estimations in zooplankton and four species of planktivorous fish using stable carbon and nitrogen isotopes analyzed in bulk tissue samples with those using stable nitrogen isotopes in amino acids. Samples were collected in spring 2021 and 2022 in two areas of the northwestern Iberian shelf (Galicia) and southern Bay of Biscay (mar Cantábrico) with different influence of the seasonal upwelling. Estimations of trophic niches using both methods were complementary, revealing interspecific differences among fish species in the exploitation of food web resources. For instance, marked differences in the sources of carbon and nitrogen and low niche overlap indicated local feeding for zooplankton and some fish species (sardine and chub mackerel), while other species showed large variability in sources and niche overlap across zones (anchovy and mackerel). The apparent competition of fish species for resources suggested by these results can be explained by differences in their mobility, particularly in mackerel and anchovy, when the isotopic signal is not equilibrated with the new diet.