Unionid mussels as stable isotope baseline indicators for long-lived secondary consumers in pelagic food web comparisons

Abstract

Stable isotope signatures of filter-feeding freshwater mussels have been established as a convenient baseline measure at the primary consumer level for comparisons of pelagic food webs. We studied this idea with four unionid mussel species Anodonta sp. (A. anatina), Pseudanodonta complanata, Uniopictorum and U. tumidus in four Finnish lakes. δ 15 N and δ 13 C signatures of mussels were compared with surface sediment and summer averages of their potential food sources and planktonic herbivores. Isotope compositions of Anodonta, Pseudanodonta and Unio differed significantly between lakes but were roughly similar within lakes, except in the smallest study lake. There was considerable deviation in the δ 13 C of mussels relative to that of particulate organic matter (POM) in water or sediment, suggesting selective use of food sources. The δ 13 C of POM also showed considerable temporal variation. Enrichment in δ 15 N mussels over that of POM was less than expected, but the mussel signatures matched closely with those of herbivorous Daphnia. We suggest that unionid mussels can provide a reasonable baseline estimate for pelagic food web comparisons with much less sampling effort than other suggested primary consumers.