Rapid recovery of a shallow hypertrophic lake following sewage effluent diversion: lack of chemical resilience

Total phosphorus budget and studies on dissolved inorganic nitrogen concentrations have been made for a small, hypertrophic, shallow lake, Little Mere, for a year prior to effluent diversion and three years following effluent diversion. Considerable resilience in phosphate concentrations was expected from experiences elsewhere with shallow lakes. Pre-diversion clear water was associated with a high dominance of large-bodied Daphnia magna due to an absence of fish in the relatively low-oxygen conditions. Unexpectedly, the phosphorus and nitrogen concentrations declined rapidly after effluent diversion (92% and 91%, respectively) and the lake has maintained the pre-diversion state of clear water. Little Mere provides evidence for importance of biological structure in determining the extent of chemical resilience. The laboratory sediment release rates of N and P were considerably higher than the net release rates, calculated from mass balance of the lake chemistry, as found elsewhere. Probably, lack of phytoplankton sedimentation, phytoplankton and plants uptake were the reasons for several fold high release rates that were observed in laboratory experiment. Therefore, it appeared to approach the gross release rates.