Laurence Carvalho

Vertically-challenged limnology; contrasts between deep and shallow lakes

Previous work on a set of small lakes, of varying depth, the meres of North West England, has shown that nitrogen availability controls the summer phytoplankton populations in the deeper ones (max depth > 3 m) and zooplankton grazing in shallow ones. The meres have generally high total phosphorus concentrations and this may be a natural phenomenon dependent on the local geochemistry. Some anthropogenic eutrophication has occurred, however, and from a chain of three meres, sewage effluent was diverted in 1991. The upper lake, Mere Mere, lying above the point of discharge, has not changed in any systematic way since effluent diversion. The middle lake, the very shallow Little Mere, has changed markedly in water chemistry but not fundamentally in ecosystem structure. It was and remains a clear-water, macrophyte dominated lake. The third lake, the deep Rostherne Mere, has shown no response in chlorophyll aconcentrations in four years since effluent diversion though in the past two years there appears to be a downward trend in total phosphorus. The reasons for this are explored in terms of our understanding of lake eutrophication. Comparisons are made with White Mere, a deep groundwater fed lake with a long retention time and a very high total phosphorus concentration. The deep meres may add a new dimension to our understanding of natural and anthropogenic eutrophication.

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.