Impact of two invasive species, roach and the zebra mussel, on pelagic zooplankton species abundance in Lower Lough Erne

Abstract Long-term results allowed the impact of two invasive species, roach and the zebra mussel, on pelagic zooplankton species abundance in Lower Lough Erne, Northern Ireland, to be established. The main observed change was an average sixfold reduction in the annual mean densities of total larger Rotifera, Cyclops strenuus , Eudiaptomus gracilis , total larger nauplii and Diaphanosoma brachyurum over the period 1987–1994. The densities of these taxa remained low thereafter, and the densities of four other taxa ( Daphnia , Bosmina longirostris , Bythotrephes longimanus and Mysis relicta ) were low throughout the period 1987–2013. The main changes in the lake properties were related to fish, and predation by roach was shown to be the main cause of the decrease in zooplankton abundance, with the possibility that perch also contributed to this. Predation by larval, juvenile and larger roach and perch on zooplankton is supported by the collation of fish diet results from the literature. The lack of a measurable impact of the zebra mussel on zooplankton abundance due to the latter’s already low abundance is an example of an antagonistic effect of two invasive species.