Change to a diatom assemblage in a eutrophic lake following point source nutrient re-direction: a palaeolimnological approach

SUMMARY. 1. Lough Augher, Co. Tyrone, Northern Ireland, underwent eutrophication as a result of untreated effluent disposal by a local creamery, from 1900 until 1972–73, when primary sewage treatment began. When this remedial action met with limited success the effluent was redirected to the River Blackwater, downstream of the lake 2. A sediment core taken in July 1981 shows an unambiguous record of the diatom response to this eutrophication; the species succession represents a clear eutrophication gradient, with a shift from mesotrophic plankton forms (Aulacoseira ambigua, Asterionella formosa, Fragilaria crotonensis, Diatoma tenue var. elongatum) to a variety of small Stephanodiscus spp. typical of very eutrophic conditions (S. parvus, S. hantzschii). The succession is in reasonable agreement with that predicted by changing Si:P ratios. 3. A second, short core, taken in September 1985, shows dramatic changes in the diatom plankton after 1981, with resurgences and rapid increases of species present early in the lake's eutrophication, and representative of mesotrophic conditions. There is clear agreement between the biostratigraphic record of the two cores, for the time period during which they overlap, c. 1970–81. 4. The available chemical data post-dates the re-direction of the creamery effluent. However, it indicates that the phosphorus concentration is in equilibrium with the loading, and has stabilized following effluent re-direction. There were no significant differences for chlorophyll a and total phosphorus between the years for which data are available. The mean concentrations were 10, 14 and 10 g 1^?1 chlorophyll a, and 61, 63 and 58 g TP 1^?1 in 1978, 1979 and 1982 respectively. 5. A Correspondence Analysis Joint-plot is used to summarize the biostratigraphy of the two cores. It serves as an ecological summary of the responses of the plankton diatoms to changing nutrient concentrations and ratios, following redirection of the creamery effluent, and demonstrates clearly the new direction taken by the planktonic diatoms. Although the phosphorus and chlorophyll a data suggest that the lake returned quickly to an equilibrium state, the diatom community continued to change. suggesting a time-lag effect and non-equilibrium responses by the planktonic diatoms to their nutrient environment. 6. The potential value of palaeolimnological approaches to long- and medium-term monitoring of diatom plankton changes should not be underestimated; they may provide an important time component for limnological restoration projects.