Over a wide range of nutrient loadings or concentrations shallow lakes appear to have two main alternative states - clear water, macrophyte-dominated and turbid, phytoplankton-dominated, each with numerous detailed variants. This model is reviewed in terms of defining the problem, broad understanding, detailed understanding and wider meaning. For the former two, our information is reasonably good but there are many details yet to be investigated. Some aspects of this are the mechanisms by which plant beds may act as refuges for grazer Cladocera such as daphnids, the role of non-daphnid (plant-associated) Cladocera in grazing phytoplankton, the role of periphyton grazers and the detailed influence of mixed fish communities in predation. Finally it is suggested that the model has analogies in alternative ways of organising human communities and offers guidance in managing future human affairs.

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