An evaluation of alternative management strategies for shallow eutrophicated lakes and reservoirs
External phosphorus loadings on a range of freshwater Sites of Special Scientific Interest (SSSIs) thought to be affected by eutrophication were predicted utilising export coefficients. The effect of such loadings on lake trophic status was evaluated using the Vollenweider-OECD eutrophication model. Estimates of the relative contribution of phosphorus from various sources enabled the selection of possible reduction strategies on a site specific basis. The effect of reduction strategies on trophic status was predicted with the aid of the model. It was established that diffuse agricultural losses of phosphorus are the most common source of enrichment. However, consented discharges of sewage efHuent appear to affect a significant number of sites. Phosphorus in urban runoff is a notable source for lakes situated in less rural areas. Agricultural point sources significantly influence a small number of lakes. A critique of the methodology concluded that the use of separate export coefficients for organic and inorganic sources may be useful for identifying appropriate management strategies, but that the scientific basis for such an approach is dubious. In addition, the employment of agricultural returns for data on livestock levels may introduce an unacceptable degree of error into the calculations. The Vollenweider-OECD model appears to predict the trophic status of the lakes under assessment reasonably well, but there is a need for a reliable method of ascertaining loading reduction objectives. A review of current legislation and policy applicable to the alleviation of eutrophication of freshwater SSSIs encompassed laws relating to nature conservation, to water quality, and to agricultural extensiflcation. It confirmed that legislation which directly addresses the problem is nonexistent, but that certain laws may be applied in a piecemeal manner. In general, the form of nature conservation protection adopted in this country is not designed to prevent deterioration of water quality. An aspect of eutrophication control which may prove to be the most problematical in legislative teniis is the regulation of diffuse agricultural sources of phosphorus. Proposals for changes in law and policy on this issue included the establishment of a catchment-wide scheme, specifically designed to reduce diffuse agricultural lossesof phosphorus, and targeted at eutrophicated SSSIs.