Diet of intertidal bivalves in the R�a de Arosa (NW Spain): evidence from stable C and N isotope analysis

Abstract

We used natural abundance measurements of stable carbon and nitrogen isotopes and a short-term 15N-enrichment experiment to explore the importance of river-derived and autochthonous sources to the diet of intertidal suspension-feeding bivalves in the Ria de Arosa, an oceanic-dominated system in northwest Spain. Highly seasonal and intense phytoplankton "bloom" events occur in this estuary. However, relatively low phytoplankton standing crop and productivity are present during much of the year, suggesting that food sources other than phytoplankton must support bivalves during these periods. Bivalves (Cerastoderma edule, Tapes decussatus, Mytilus galloprovincialis) were sampled quarterly at four intertidal stations along the length of the estuary with increasing distance from the Ulla River. Muscle tissue was consistently enriched in 13C and 15N relative to stomach contents. The stable C isotope values of bivalves indicated that river-derived inputs were, at most, a minor component of diet. These values suggested the potential importance of benthic microalgae to bivalve diet much of the year and the incorporation of 15N label applied to the sediment surface into stomach contents and muscle tissue in a field experiment supported this conclusion. Other labeled components of the microbenthos such as bacteria could also have been ingested. 13C-depleted values of bivalves in May coincided with elevated offshore chlorophyll a concentrations, suggesting the increased use of a phytoplankton source during the spring. We propose that intertidal suspension-feeding bivalves in this estuary use primarily resuspended microbenthos during periods of low phytoplankton concentration, but that phytoplankton increases in relative dietary importance during bloom events.