d13C and d15N variations in organic matter pools, Mytilus spp. and Macoma balthica along the European Atlantic coast

Stable carbon (d13C) and nitrogen (d15N) isotope (SI) values of sedimentary organic matter (SOM), seston and two dominant bivalves, Mytilus spp. and Macoma balthica, were studied at 18 stations along the European coast in spring and autumn 2004. Threemain regions, the Baltic Sea (BS), the North Sea and English Channel (NS), and the Bay of Biscay (BB), were tested for possible geographic (latitudinal) differences in the SI values. In spring, only BS showed lower d13C values of seston and Mytilus spp., and higher d15N values of SOM, than NS and BB. No significant differences between the 3 regions were found in autumn. Irrespective of season and regions, Mytilus spp. was more 13C-depleted than M. balthica. d13C values of M. balthica, but not those of Mytilus spp., were significantly correlatedwith SOM. These results are consistent with differences in feeding behavior of Mytilus spp. and M. balthica, as the two species are known as obligatory-suspension and facultative-deposit feeders, respectively. In contrast, no differences in the d15N values of Mytilus spp. and M. balthica were found at individual stations, indicating the same trophic level of the two bivalves within the foodwebs. At some stations, irrespective of geographic location, both bivalves showed d15N values up to 18-20 %. These were two trophic levels higher than those found at the other stations, indicating local and/or episodic eutrophic conditions, probably due to waste water discharge, and the effectiveness of both Mytilus spp. and M. balthica as bio-indicators of anthropogenic eutrophication. Overall, our results suggest that pathways of energy flow from OM pools to dominant bivalves is more related to local environmental conditions than to geographic regions across the European coastline. This has implications for food web studies along the Atlantic coast because most of the values are consistent over a large area and show no significant differences. Therefore, the present study can be used twofold for the determination of trophic baselines and for the correction of the trophic position of consumers higher up in the in the case of differences in waste water discharge.