Is δ15N of freshwater amphipods related to their body size?

Background/Question/Methods

Stable isotopes have been frequently used to decode and understand the complexities within populations and communities of freshwater ecosystems.   As animals consume their food they become increasingly enriched in heavier isotopes such as ¹⁵N.  Furthermore, as animals develop and increase in size the dietary changes associated with ontogenetic changes in morphology, foraging tactics and habitat use often lead to increasing enrichment.  Conversely, for species that are opportunistic and omnivorous strong trends between size and isotopic composition can be obscured.  Here we explored if and how δ¹⁵N is related to body size of amphipods, a widespread and important group of benthic invertebrates in freshwater lakes and streams.  We sampled a total of eight lakes in Central Ontario for all available sizes of amphipods in July and August, 2008.   Amphipod samples were analysed for δ¹⁵N and POM δ¹⁵N was used for baseline correction.  Our isotopic data were statistically analysed to examine trends between amphipod δ¹⁵N and size within and among lakes. 

Results/Conclusions

At the lake level, relationships between isotopic composition and size were highly variable.  Amphipod δ¹⁵N were positively related to their body size at varying degrees in seven of the eight study lakes and negatively related to size in one lake.  Among these relationships only one positive relationship was significant (r²=0.87, p>0.05). There were significant differences in the slopes of these relationships (p>0.05).  When data for all lakes were combined there was no relationship between amphipod size and their δ¹⁵N isotopic composition (r²<0.001, p=0.86) among the study lakes. Together, the general absence of strong relationships and variability of the relationships found between lakes indicate that amphipods have omnivorous feeding habits and that their ontogenetic changes are not closely related to their δ¹⁵N.