Early life stages of Dreissena polymorpha (zebra mussel): the importance of long-term datasets in invasion ecology

Abstract

The early life stages of Dreissena polymorpha were studied in Lough Key, Co. Roscommon, Ireland during the reproductive season, 1998-2003. This involved weekly sampling of larval/veliger density, size distributions and settlement density. Low larval and adult densities of Dreissena in 1998 indicated that this was the first year of significant reproduction in Lough Key. Variation existed in seasonal larval densities, larval size distributions and juvenile settlement patterns among sampling weeks, years and monitoring sites from 1998 to 2003. In the early years of invasion (1998-2000) annual variations were observed in larval density and juvenile settlement. Increasing levels of larval density and settlement from 1998 to 2000 were typical for the early exponential growth phase of Dreissena invasions. The high level of successful recruitment in those years was evident from the exponential increase of the adult zebra mussel population. In subsequent years, these differences were likely related to environmental variations in summer water temperatures and food availability for veligers. In 2001, settlement estimates were extremely low relative to the larval densities present. Prolonged warm water temperatures in summer 2003, resulted in a long reproductive season and also high settlement rates. The variation between annual datasets observed in this study, implies that long term studies of the early life stages of Dreissena polymorpha is required to monitor the dynamics of this species within lake ecosystems.