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

Total phosphorus budget and studies on dissolved inorganic nitrogen concent rations 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 expecte d from experiences elsewhere with shallow lakes. Pre-diversion clear water was associated with a high dominance of large-bodied Daphnia magna due to a n absence of fish in the relatively low-oxygen conditions. Unexpectedly, th e phosphorus and nitrogen concentrations declined rapidly after effluent di version (92% and 91%, respectively) and the lake has maintained the pre-div ersion state of clear water. Little Mere provides evidence for importance o f biological structure in determining the extent of chemical resilience. Th e laboratory sediment release rates of N and P were considerably higher tha n the net release rates, calculated from mass balance of the lake chemistry , as found elsewhere. Probably, lack of phytoplankton sedimentation, phytop lankton and plants uptake were the reasons for several fold high release ra tes that were observed in laboratory experiment. Therefore, it appeared to approach the gross release rates.