Stomach content and stable isotope analyses provide complementary insights into the trophic ecology of coastal temperate bentho-demersal assemblages under environmental and anthropogenic pressures

Assessing organic matter fluxes and species interactions in food webs is of main interest to understand the ecological functioning in bays and estuaries characterised by a wide diversity of primary producers and con-sumers. Demersal fish and cephalopod assemblages were studied across a network of 24 shallow subtidal stations in the bay of Saint-Brieuc for their diversity, stable isotope compositions and stomach contents. The community was composed of 21 taxa, eight species accounting for 94.4% of the total abundance. Three different assemblages were identified along bathymetric gradient and spatial patterns in fish dredging. Marine POM and SOM were the most likely bases of food webs regarding delta 13C range displayed by fish and cephalopod without differences among assemblages. Amphipoda was the main prey item in stomachs leading to significant diet overlaps among fish species, with some variations in additional items. Sepia officinalis was characterised by a singular diet and very low dietary overlap with other species. Contrasted stable isotope values and niche overlaps among species were evidenced in the delta 13C/delta 15N space. Callionymus lyra and Buglossidium luteum, characterised by the widest isotopic niches, encompassed those of other species, except the singular 13C-depleted Spondyliosoma cantharus. Coupling taxonomic assemblages, stomach contents and stable isotope analyses help disentangling the resources uses and evidencing trophic pathways. Contrasts in fish and cephalopod demersal assemblages occurring at different depths not necessarily imply differences in the trophic resources uses in such complex shallow coastal ecosystems under anthropogenic influences.

Automated summary

Assessing species interactions and food webs in bays and estuaries is crucial for understanding ecological functioning. This study examined demersal fish and cephalopod assemblages in the bay of Saint-Brieuc, finding that they were mainly dependent on marine POM and SOM as the basis of their food webs. Stable isotope and stomach contents analyses revealed variations in diet overlap and resource use among different species. The results suggest that differences in assemblages occurring at different depths may not imply differences in trophic resource use in shallow coastal ecosystems.