Isotope and fatty acid trends along continental shelf depth gradients: Inshore versus offshore hydrological influences on benthic trophic functioning

Anthropogenic activities and land-based inputs into the sea may influence the trophic structure and functioning of coastal and continental shelf ecosystems, despite the numerous opportunities and services the latter offer to humans and wildlife. In addition, hydrological structures and physical dynamics potentially influence the sources of organic matter (e.g., terrestrial versus marine, or fresh material versus detrital material) entering marine food webs. Understanding the significance of the processes that influence marine food webs and ecosystems (e.g., terrestrial inputs, physical dynamics) is crucially important because trophic dynamics are a vital part of ecosystem integrity. This can be achieved by identifying organic matter sources that enter food webs along inshore–offshore transects. We hypothesised that regional hydrological structures over wide continental shelves directly control the benthic trophic functioning across the shelf. We investigated this issue along two transects in the northern ecosystem of the Bay of Biscay (north-eastern Atlantic). Carbon and nitrogen stable isotope analysis (SIA) and fatty acid analysis (FAA) were conducted on different complementary ecosystem compartments that include suspended particulate organic matter (POM), sedimentary organic matter (SOM), and benthic consumers such as bivalves, large crustaceans and demersal fish. Samples were collected from inshore shallow waters (at ∼1 m in depth) to more than 200 m in depth on the offshore shelf break. Results indicated strong discrepancies in stable isotope (SI) and fatty acid (FA) compositions in the sampled compartments between inshore and offshore areas, although nitrogen SI (δ15N) and FA trends were similar along both transects. Offshore the influence of a permanently stratified area (described previously as a “cold pool”) was evident in both transects. The influence of this hydrological structure on benthic trophic functioning (i.e., on the food sources available for consumers) was especially apparent across the northern transect, due to unusual carbon isotope compositions (δ13C) in the compartments. At stations under the cold pool, SI and FA organism compositions indicated benthic trophic functioning based on a microbial food web, including a significant contribution of heterotrophic planktonic organisms and/or of SOM, notably in stations under the cold pool. On the contrary, inshore and shelf break areas were characterised by a microalgae-based food web (at least in part for the shelf break area, due to slope current and upwelling that can favour fresh primary production sinking on site). SIA and FAA were relevant and complementary tools, and consumers better medium- to long-term system integrators than POM samples, for depicting the trophic functioning and dynamics along inshore–offshore transects over continental shelves.