ECN site code: L17
ECN monitoring by: Agri-Food and Biosciences Institute

Lough Erne is the collective name given to Upper Lough Erne (34.5 km2) and Lower Lough Erne (109.5 km2), which are connected by the River Erne in County Fermanagh, Northern Ireland. The Upper Lough and the shallower regions of Lower Lough Erne present an example of a flooded drumlin landscape which has created an intricate mosaic of land and water. As a consequence of differences in depth and area, the water retention time of the Upper Lough is less than one month while that of the Lower Lough is four months. Passing from the shallow Upper Lough (mean depth 2.3m) and through Lower Lough Erne (mean depth 11.9m), phytoplankton abundance is reduced and algal composition alters, as do nutrient and temperature cycles. Phosphorus concentrations (100 mg P l-1) in the Upper Lough create eutrophic conditions, with high summer chlorophyll a concentrations, but phytoplankton abundance in the deep open water of the Lower Lough is more typical of a mesotrophic water body despite comparatively high phosphorus concentrations (60 mg P l-1). The paucity of phytoplankton in this region is attributed to a high back-ground light attenuation from the peat-stained water and the greater depth of the mixed water zone (>35m). The lake supports a fish population dominated by a recent introduction, the roach, as well as pike, perch, bream, trout and the pollan (Coregonus autumnalis pollan Thompson). The Zebra Mussel (Dreissena polymorpha Pallas) is a recent introduction (1996) which now has only a limited distribution in the Lower Lough. As this species expands its range and abundance throughout the Erne system, it may impact significantly on the lake ecology. Water monitoring is undertaken at the deepest portion of Lower Lough Erne with limited samples taken along a gradient towards the main river inflow of the lake.