Spatial variability in Mussels used to assess base level nitrogen isotope ratio in freshwater ecosystems

Abstract

Freshwater mussels have been used to establish base level nitrogen isotope ratio values (δ15N) used in trophic position and food web studies in freshwater ecosystems. In this study, we assess the variability introduced when using unionid mussels in this manner by investigating the spatial variation in δ15N values in mussels from different locations in shallow freshwater ponds, and also differences in δ15N in different tissue types in the mussels. Results from the analysis of adductor, foot and a section of the mantle tissues from mussels across all ponds showed that adductor tissue was consistently enriched by about 1‰ versus the mantle and foot. The foot showed the least variability which, coupled with ease in obtaining consistent samples, led us to select this tissue type for subsequent analysis. The six ponds included in the study had average mussel δ15N values ranging from 4.9‰ to 11.9‰. Four of the six ponds showed no significant within pond differences between δ15N values from mussels collected at different sampling sites. The range of mussel δ15N values obtained from within ponds showed that using data from a single mussel to assess baseline δ15N values would result in the introduction of a maximum error of 0.56 of a trophic level to subsequent trophic position calculations.