Determination of zooplankton dietary shift following a zebra mussel invasion, as indicated by stable isotope analysis

Summary 1. Freshwaters with established zebra mussel populations typically exhibit reduced chlorophyll a concentrations, but the subsequent impacts upon zooplankton are varied. We hypothesised that in an invaded system with less phytoplankton but available allochthonous subsidy, zooplankton may utilise greater proportions of allochthonous matter and that this could be traced by analysis of stable carbon and nitrogen isotopes. 2. We used archived zooplankton samples which had been consistently preserved and which spanned the invasion period of an Irish lake, Lough Erne. Increasing reliance upon allochthonous resources would be reflected in an increase in zooplankton δ 13 C away from phytoplankton which is relatively 13 C‐depleted in humic‐stained L. Erne. 3. Analysis of a series of monthly samples (1992–96, 1999–2003) revealed…