Monitoring lake recovery from point‐source eutrophication: the use of diatom‐inferred epilimnetic total phosphorus and sediment chemistry

Abstract

SUMMARY1. Diatom and geochemical responses to reduced nutrient loading were followed in a small, monomictic eutrophic lake in Northern Ireland by use of short sediment cores taken c. 15 years after redirection of creamery waste away from the lake.2. Epilimnetic total phosphorus (TP) concentrations (fig TP 1‐−1) were estimated for the period 1850–1990 using weighted averaging regression and calibration. Background TP levels, inferred using the diatom model, were c. 35μg TP 1‐−1 and increased to >140μg TP 1‐−1 in the late 1960s to early 1970s. Total P concentrations dropped to 80 μg TP 1‐−1 within 5 years of waste diversion (c. 1978–79), but varied between 1980 and 1990 (range 70–140 μg TP 1‐−1), perhaps due to internal loading, occasional continued disposal from the creamery and natural variations in stream P load.3. Diatom‐inferred TP concentrations were compared with monitored data where available, and the diatom model tended to overestimate TP concentrations by about 25 fig TP 1‐−1. Possible reasons for this are discussed (errors in the diatom model, stratigraphic variability, variability in the monitoring data).4. Post‐1980 geochemistry profiles (concentrations and accumulation rates) indicated some changes when compared with sediments deposited before 1980, perhaps reflecting the redirection of the creamery waste and reduced productivity of the lake (e.g. reduced calcium deposition). Phosphorus concentrations in the sediments changed very little 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 redirection in the mid‐1970s. There is, however, some indication of a slight lowering of P retention in the most recent sediments (1985–90).5. The general implications of such an approach to monitoring (i.e. the use of short cores) are discussed and the value of diatom‐inferred TP assessed. Diatom models offer the possibility of determining background TP concentrations and indicate that, despite the redirection of the creamery waste over 15 years ago, the pre‐creamery epilimnetic TP concentrations have not yet been reached.