CHANGES IN A DEEP LAKE FOLLOWING SEWAGE DIVERSION - A CHALLENGE TO THE ORTHODOXY OF EXTERNAL PHOSPHORUS CONTROL AS A RESTORATION STRATEGY

1. The restoration of deep lakes has traditionally focused on reducing the external phosphorus loading. 2. Following the diversion of sewage effluent, that led to marked reductions in nutrient concentrations in its main inflow, Rostherne Mere has shown no reduction in phosphorus or chlorophyll a concentrations. A shallow lake upstream (Little Mere) , however, has shown a marked response to effluent diversion. 3. Nutri ent budgets for Rostherne Mere reveal that sewage effluent was by far the most significant external source of total phosphorus and that diff use drainage from the catchment was the most significant external sour ce of dissolved inorganic nitrogen. Phosphorus loads from groundwater and a bird roost were insignificant. Internal sources of phosphorus we re, however, considerable and were largely responsible for the observe d delay in recovery. 4. Phosphorus Limitation of phytoplankton biomass may never be attainable because of substantial internal and diffuse s ources of phosphorus, combined with a long retention time. Nitrogen is Likely to be more important in limiting phytoplankton biomass. Contro l of diffuse nitrogen sources may therefore be more effective in the r estoration of the deeper lakes of this region.