MONITORING LAKE RECOVERY FROM POINT-SOURCE EUTROPHICATION - THE USE OF DIATOM-INFERRED EPILIMNETIC TOTAL PHOSPHORUS AND SEDIMENT CHEMISTRY

1. Diatom and geochemical responses to reduced nutrient loading were f ollowed in a small, monomitic eutrophic lake in Northern Ireland by us e of short sediment cores taken c. 15 years after redirection of cream ery waste away from the lake. 2. Epilimnetic total phosphorus (TP) con centrations (mug TP 1(-1) were estimated for the period 1850-1990 usin g weighted averaging regression and calibration. Background TP levels, inferred using the diatom model, were c. 35 mug TP 1(-1) and increase d to > 140 mug TP 1(-1) the late 1960s to early 1970s. Total P concent rations dropped to 80 mug TP 1(-1) within 5 years of waste diversion ( c. 1978-79), but varied between 1980 and 1990 (range 70-140mug TP1(-1) , perhaps due to internal loading, occasional continued disposal from the creamery and natural variations in stream P load. 3. Diatom-inferr ed TP concentrations were compared with monitored data where available , and the diatom model tended to overestimate TP concentrations by abo ut 25 mug TP 1(-1). Possible reasons for this are discussed (errors in the diatom model, stratigraphic variability, variability in the monit oring data). 4. Post-1980 geochemistry profiles (concentrations and ac cumulation rates) indicated some changes when compared with sediments deposited before 1980, perhaps reflecting the redirection of the cream ery waste and reduced productivity of the lake (e.g. reduced calcium d eposition). Phosphorus concentrations in the sediments changed very li ttle over the last 150 years and, while sedimentary TP fluxes (g cm-2 yr-1) increase steadily up-core, they do not record the effluent redir ection in the mid-1970s. There is, however, some indication of a sligh t lowering of P retention in the most recent sediments (1985-90). 5. T he general implications of such an approach to monitoring (i.e. the us e of short cores) are discussed and the value of diatom-inferred TP as sessed. Diatom models offer the possibility of determining background TP concentrations and indicate that, despite the redirection of the cr eamery waste over 15 years ago, the pre-creamery epilimnetic TP concen trations have not yet been reached.