Niche partitioning among marine mammals inhabiting a large estuary as revealed by stable isotopes of C, N, S and O

Abstract

Detailed knowledge on habitat use by marine mammals is critical to understand their role in the ecosystem. The stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) have been widely used to study the trophic ecology of marine mammals, but the stable isotope ratios of other elements such as sulfur (δ34S) and oxygen (δ18O) can better inform about habitat use in areas with strong salinity and redox gradients. The Río de la Plata estuary represents the largest freshwater runoff in the south-western Atlantic Ocean and supports a rich community of marine mammals. Here, we analyzed δ34S values in bone from seven marine mammal species inhabiting the estuary and the adjacent Atlantic Ocean, in order to complement previous isotopic data (δ13C, δ15N and δ18O) and compare their resolution as habitat tracers. As expected, δ34S and δ18O offered relevant insights into the characterization of the habitat used by marine mammals and allowed a better delineation of habitat partitioning between them. Bottlenose dolphins, South American sea lions and South American fur seals seem to be frequent users of the less saline areas of the estuary, whereas Burmeister´s porpoises, franciscana dolphins and false killer whales seemed to prefer the saltier marine waters close to the bottom. Fraser´s dolphins were the only inhabitants of true offshore waters. Our findings demonstrate how the integration of different stable isotope ratios can help disentangle fine habitat partitioning between marine mammals living in a complex ecosystem such as Río de la Plata.