Feeding strategies of co-occurring suspension feeders in an oligotrophic environment

Suspension-feeders predominate in the vast majority of the coastal marine benthic ecosystems, with several species co-occurring at low spatial scale. Understanding how these species cope with competition for trophic resources has been the core of numerous studies, mostly in coastal shallow systems where food supplies are diverse and abundant. Oligotrophic systems have received less attention. The aim of the present study was thus to investigate the trophic relationships established between 9 suspension feeders collected in an oligotrophic zone (Bay of Marseille, French Mediterranean). Species displayed similar isotopic ratios, consistently with the use of one main source, identified as nanophytoplankton and diazotrophic bacteria as the δ15N values were low, in contrast to interspecific differences generally observed. The seasonal variations of the isotopic ratios and isotopic niche indices were explained by differential sorting abilities, higher for bivalves than for ascidians and the polychaete Chaetopterus variopedatus. The present results demonstrate that resource availability in oligotrophic systems is a major driver of trophic competition. It precludes a general conclusion about suspension feeders trophic patterns drawn exclusively from highly productive systems, and stresses the need for an extensive assessment of those patterns in a vast range of ecosystems.