Niche segregation amongst sympatric species at exposed sandy shores with contrasting wrack availabilities illustrated by stable isotopic analysis

Wrack supplies (macroalgae, seagrasses and carrion) are a common feature of sandy beaches worldwide. These allochthonous inputs are a potential high-quality food subsidy for beach fauna, but little is known about the feeding ecology and niche segregation strategies of these species in beaches with limited wrack availabilities. We used stable isotopic ratios of nitrogen and carbon to examine the diets and niche segregation among three sympatric crustaceans, the amphipods Talitrus saltator and Talorchestia brito, and the isopod Tylos europaeus, in two temperate beaches, Cabedelo and Quiaios, on the Portuguese Atlantic coast, with contrasting wrack availabilities, over winter and summer. In the beach with limited wrack (Quiaios), consumers showed significantly different interspecific and temporal isotopic signatures, suggesting a distinct foraging habitat and niche differentiation. Mixing models outputs (SIAR) revealed in this case a seasonal shift in the diet of consumers from terrestrial sources in winter towards marine-based sources (phytoplankton) in the summer. In contrast, at Cabedelo, consumers showed clear overlap in isotopic signatures, with similar contributions of the available wrack to their diets. As a whole, an opportunistic and generalist feeding behaviour was described for these species, namely for T. saltator. Isotopic profiles support the occurrence of spatial overlap during summer but different foraging strategies of the three species in these beaches during winter. Our results show that on temperate beaches with ephemeral wrack supplies, consumers might need to adopt different foraging strategies and niche segregation for their subsistence, and additionally illustrate the viability of using stable isotopes to capture subtle changes in trophic niches.