CHLOROPHYLL-A AND NUTRIENTS IN LOUGH-ERNE

Lower Lough Erne (109km2) was sampled for nutrients and chlorophyll a from 1985 to 1991 at a sampling station over the deepest (maximum dept h 65m) portion of the lake known as the Broad Lough. The results were as follows. 1. Phytoplankton in the Broad Lough were unlikely to be nu trient-limited; winter soluble reactive phosphorus concentrations of c . 35mug P l-1 were rarely depleted to less than 10mug P l-1 by spring and summer phytoplankton, the annual nitrate minima were greater-than- or-equal-to 130mug N l-1, and the soluble reactive silica minima were greater-than-or-equal-to 0.4mg SiO2 l-1. 2. Light attenuation was high owing to water colour and non-algal particulates, with ratios of phot ic zone volume and mixed depth volume close to 0.2. Variations in the mixed depth between years owing to temperature stratification had litt le influence on the underwater light regime and could not be related t o inter-annual differences in mean summer chlorophyll a which ranged b etween 3mug and 15mug l-1. 3. Total phosphorus concentration averaged 48.5mug P l-1 between 1985 and 1991 compared to 47.5mug P l-1 between 1974 and 1975. 4. Deoxygenation of the hypolimnion was insufficient to cause anoxia and year-to-year variations in dissolved oxygen of the h ypolimnion were related to the duration of thermal stratification. The re was no evidence to indicate that nutrient release from the sediment into the hypolimnion or into the lough as a whole was a significant f actor in the nutrient balance of the lough.