Vertically-challenged limnology; contrasts between deep and shallow lakes

Abstract

Previous work on a set of small lakes, of varying depth, themeresof North West England, has shown that nitrogen availabilitycontrols the summer phytoplankton populations in the deeperones(max depth>3 m) and zooplankton grazing in shallow ones. Themeres have generally high total phosphorus concentrations andthismay be a natural phenomenon dependent on the localgeochemistry.Some anthropogenic eutrophication has occurred, however, andfroma chain of three meres, sewage effluent was diverted in 1991.Theupper lake, Mere Mere, lying above the point of discharge, hasnotchanged in any systematic way since effluent diversion. Themiddlelake, the very shallow Little Mere, has changed markedly inwaterchemistry but not fundamentally in ecosystem structure. It wasandremains a clear-water, macrophyte dominated lake. The thirdlake,the deep Rostherne Mere, has shown no response inchlorophyll a concentrations in four years since effluent diversionthough inthe past two years there appears to be a downward trend intotalphosphorus. The reasons for this are explored in terms of ourunderstanding of lake eutrophication. Comparisons are madewithWhite Mere, a deep groundwater fed lake with a long retentiontimeand a very high total phosphorus concentration. The deep meresmayadd a new dimension to our understanding of natural andanthropogenic eutrophication.