Biogeographic variation in [delta] 13C and [delta] 15N values of Mytilus spp. and Macoma balthica along the European coast

In order to assess biogeographic variation in isotopic composition of dominant marine bivalves, [delta]13C and [delta]15N values of Mytilus spp. and Macoma balthica, and their potential food sources, were studied along the European coast in spring and autumn 2004. Baltic Sea, North Sea and Atlantic Sea regions, characterized by strong differences in (benthic) biodiversity, were compared. Mytilus spp. showed more depleted [delta]13C values than M. balthica, most evident in the Baltic Sea in spring, where also particulate organic matter and macroalgae [delta]13C values were found to be depleted. This suggested differences in food sources among the two species, consistent with the wider spectrum of food items known for M. balthica. No differences in [delta]15N values between Mytilus spp. and M. balthica, however, indicated the same trophic level within a food web. Moreover, enriched [delta]15N values, up to 2 trophic levels higher, were found at some stations for both bivalves, as an indication of local eutrophic conditions. The results suggest similar pathways in energy flow from primary producers to bivalves, irrespective of biogeographic differences related to environmental conditions and biodiversity levels.