UK ECN site. The shallower regions of Lower Lough Erne (109.5 km2), 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 Lower Lough is four months. The phytoplankton abundance in the deep open water of the Lower Lough is more typical of a mesotrophic water body despite comparatively high phosphorus concentrations (58 mg P l-1, 2017). 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 (>35 m). The lake supports a fish population dominated by the introduced roach, as well as pike, perch, bream, trout and pollan. The Zebra Mussel is a relatively recent introduction (1996) which has impacted significantly on the lake ecology. Water monitoring is undertaken at the deepest portion of Lower Lough Erne.