NUMBER TWENTY NINE - OCTOBER 1998

The 'normal' functioning of natural systems is often illuminated when they are pushed to extremes. Until 1980, Little Mere, an artificial lake in North Cheshire, UK, was polluted with effluent from a small sewage treatment works that discharged directly to it, and which was greatly overloaded. This arose from much development in the area, owing to the increased prosperity of the nearby Manchester conurbation. Two consequences of this have been a greater understanding of the link between nutrients and alternative states in shallow lakes, and the socially reprehensible hazards of a large number of impatient people driving cars aggressively on the roads around the lake!

Despite a water chemistry in the lake that resembled a laboratory culture solution for algae, with milligram per litre quantities of ammonium-nitrogen and phosphate-phosphorus, the water of Little Mere was crystal clear in summer and the lake had a substantial coverage of plants. This was remarkable as the concentrations of total nitrogen and total phosphorus were two orders of magnitude greater than those which would be associated with dense algal growths in many lakes. The reason became clear when, one June morning in 1980, my graduate student, Laurence Carvalho and I noticed that the water was full of bright red objects. These turned out to be large individuals of a Daphnia species, D. magna, which were full of haemoglobin. This species is so big (up to 4mm) that it cannot coexist with fish, who see it very readily (especially when its haemoglobin content makes it stand out like a bar sign in Las Vegas!)

Daphniamagna was abundant in the lake because the effluent discharge into it was of poor quality and was deoxygenating the water so severely, especially at night, that few or no fish could survive. In turn the Daphnia> prospered and effectively prevented any algal growth in the summer, except for a few elegant colonies of Volvox, which were too big for it to eat but which grew so slowly that they had little effect on light penetration to the underlying plants. Thus we had an excellent example showing that a clear water, plant-dominated lake could exist despite very high nutrient concentrations. Whether or not this situation can be found where fish are able to persist remains a question to be answered as more lakes are investigated.