Executive Summary

The zebra mussel (Dreissena polymorpha) is an aquatic invasive species which spread to Lough Key, Co. Roscommon, from the lower Shannon in the late 1990s. The mussels have been monitored in this 9-km2 lake each year since 1998.

By 1999, the population estimate was 6 billion (6 × 109) with high cover of zebra mussels on stones in near-shore areas of the lake. This project was a 3-year study (2001–2003) on the impact of nutrients on the zebra mussel population in Lough Key (2000-MS-5- M1: ERTDI programme), which included some previous data from 2000.

Zebra mussels are effective filter feeders, with high individual clearance rates of phytoplankton, cyanobacteria and other particles. Successful colonisation of a lake results in increased transparency and decreased chlorophyll levels and often a consequent decrease in total phosphorus.

Since 1999, these changes have been noted in Lough Key. Transparency and chlorophyll levels indicate an improvement in trophic status (to oligotrophic) but total phosphorus levels are still within the mesotrophic range and have not reached limiting levels.

These direct impacts create problems in classifying lakes according to existing (OECD) and developing (Water Framework Directive 2000/60/EC) classification schemes. The construction of the new Boyle Sewage Treatment Plant with a phosphorus removal system has reduced the loading of total phosphorus to Lough Key.

It was believed that the reduction in phosphorus loadings could have a limiting effect on zebra mussels in Lough Key, due to a consequent reduced production of phytoplankton. It is not possible to separate the nutrient impact this species has had on lake water quality from that due to the new sewage treatment plant.

The project aimed to quantify the total number and biomass of zebra mussels and also to assess any changes to the population during the course of the study. Relevant water parameters and phytoplankton samples were analysed; direct ecological changes were also noted during the project.

Zebra mussels were studied at all stages of growth from larva/veliger stage, through juvenile settlement and most intensively in the juvenile and adult stage (0–3 years). The total number of adult zebra mussels in the lake was estimated at 34 billion.

This number is believed to relate to a more effective method of population estimate, rather than any real increase since 1999. This population is capable of filtering the entire volume of Lough Key in a 10-day period.

Biomass estimates taken in 2001 and 2003 were not significantly different – this indicates that changes in total phosphorus did not limit the zebra mussel population during the course of the study. Food availability appears to be the main density-dependent limiting factor in the lake, as many stony areas are not fully colonised.

Ecological changes noted during the study included the extirpation of the native duck mussel, Anodonta anatina, reduced usage of the common reed (Phragmites australis) as a substrate and increased growth of aquatic plants and benthic algae.