NUMBER TWENTY NINE - OCTOBER 1998

It is essential to carry out fully controlled, well replicated experiments in ecology, so that mechanisms can be understood. Often replicated enclosures which isolate parts of a lake are used with each enclosure having a particular treatment. Terms like mesocosm, microcosm, exclosure, and limnocorral (in North America) are also used for the same structures, which, depending on the purpose, may be made of polyethylene, rubber or fine netting. Enclosures have the advantage that many can be constructed to give adequate replication but the disadvantage that they are small relative to the lake. They cannot therefore contain a completely representative community that includes wide ranging piscivores like birds, for example. To do that, replicated large experimental ponds, which would be extremely expensive, would have to be used, or whole lakes. The latter is the usual approach, but the treatments cannot be replicated and are statistically less reliable.

Enclosure experiments involve careful planning, extremely hard work in sampling over a period of a month or two, especially where experimental designs may use thirty or forty enclosures, and months of follow-up analyses of samples collected. However, they provide considerable insight. In the enclosures shown, an experiment [109] was carried out by myself, Dr Ryszard Kornijow and Dr Gavin Measey in which a water lily stand was thinned to a half or a third of its normal density and the effects of this on relationships between perch and Daphnia were studied. We showed that at normal high densities, the perch were ineffective at finding and consuming the Daphnia, but that if the stand was thinned, the refuge that the lilies had provided was greatly weakened. Where the Daphnia were taken by the perch, the phytoplankton populations increased.