The dynamics of zebra mussel (dreissena polymorpha) populations in Lough Key, Co. Roscommon, 1998-2003
Dreissena polymorpha (Pallas), the zebra mussel, arrived in Ireland in the early 1990s. The species established itself in the lower Shannon River and was discovered in Lough Key (upper Shannon River catchment) in April 1998, two years following the estimated initial colonisation there. This research on zebra mussel populations began in 1998 and continued until 2003, utilising a variety of different techniques on larval (pelagic), juvenile (recently settled) and adult (benthic) life stages. Variations were determined in seasonal larval densities, larval size distributions and juvenile settlement patterns among sampling weeks, years and monitoring sites from 1998 to 2003. High levels of 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 zebra mussel numbers, densities and biomass on the three main settlement substrates, stone, Anodonta anatine (live and shells) and aquatic plants. Snorkel survey results indicated that the population remained stable between 2001 and 2003 despite low settlement in 2002. High recruitment in 2003 was associated with warm water temperatures during that summer. Total zebra mussel population in Lough Key was assessed in 2002 using a transect survey, combined with bathymetric data from hydroacoustic mapping (RoxAnnTM). The total population was assessed as 3.3 x 109 Dreissena, with an estimated biomass of 4.4 x 106 kg. The colonisation of Anodonta by Dreissena as a preferential substrate resulted in the extirpation of the native mussel from Lough Key by summer 2000. Anodonta shells continued to provide a substrate for zebra mussels but were less available by 2003 due to sinkage in soft substrates. Aquatic plants, particularly the perennial Phragmites australis were noted as settlement substrates for zebra mussels in the early years of invasion. By 2003 densities on reeds had reduced significantly, possibly linked to the reduction in overall recruitment of zebra mussels in 2002. Transparency and chlorophyll a levels changed significantly between 1998 and 1999 (first year of significant population expansion), with an increase in the former and a decrease in the latter. This was due to a reduction in phytoplankton levels by zebra mussel filtration. Once water temperatures exceed 100C, the Lough Key zebra mussel population is estimated to be capable of filtering the entire lake volume in 10 days. Total phosphorus levels in the lake have reduced significantly due to two drivers – the zebra mussel population and the new phosphorus removal system at Boyle sewage treatment plant. The lake remains within the mesotrophic status according to OECD classification. It is suggested that the impact of zebra mussel filtration should be built into a trophic model for the lake. Summer algal blooms persist in the lake despite an overall annual reduction in phytoplankton. There is a possibility that zebra mussels may increase the density of Cyanobacteria, due to selective rejection during feeding. Careful monitoring should be carried out due to public health concerns associated with the presence of Microcystis aeruginosa toxic strains. Productivity in Lough Key has switched from the pelagic to a benthic zone, as typical for a Dreissena invested lake. This has resulted in increased Cladophora, blanket weed and emergent macrophytes in the lake. The research carried out for this thesis, details the invasive success of Dreissena polymorpha in Lough Key from the early stage of colonisation, through an exponential growth phase, to a relatively stationery phase of growth. This intensive study highlights the fact that, even within a small lake, zebra mussel populations vary between sites and years. The extirpation of Anodonta anatine and the reduction in phytoplankton biomass by zebra mussels establish the position of Dreissena as a keystone species in Lough Key. Further survey work is required to determine long-term changes in population size and any associated ecological impacts.
- Theses - Science ITS 
The following license files are associated with this item: