Populations and Production of Benthic Animals in two Contrasting Shallow Lakes in Norfolk

marked and rapid loss of aquatic vegetation from most of the broads, with an increase in turbidity, caused primarily by excessive eutrophication (Mason & Bryant 1975c; Mason 1976a). A corresponding loss in the diversity of the fauna of the open broads has occurred. The littoral zones, in contrast, remain very diverse in animals and emergent aquatic plants and are highly productive (Mason & Bryant 1974, 1975b, c; Mason 1976a, b). With this change from a primary production system dominated by angiosperms (which are highly seasonal) to one dominated by algae (with an extended growing season in the broads), a change in the pattern of production in the zoobenthos is to be expected. Algae, whether living or dead, are probably more readily utilizable as food by benthic animals, while the decrease in faunal diversity may lead to reduced competition for those species able to tolerate the new conditions. The present study compares the populations (from November 1971 to June 1975 inclusive) and production (from June 1973 to May 1975 inclusive) of the macroinvertebrates of a broad which has been subject to recent cultural eutrophication (Alderfen Broad) with those inhabiting a broad which still retains clear water and a good growth of macrophytes (Upton Broad).