10. Reverse switches (biomanipulation) and restoration

Observations that fish could exert a major effect on the ecosystem [1,23,33,57,78, 101,124,157] led to the concept of biomanipulation, which is now the lynchpin of shallow lake restoration. Essentially biomanipulation [12-16, 51, 63, 72-74, 96, 103, 104, 107, 114, 119, 131,132,134,135,140,148-150] means the alteration of the fish community to favour Daphnia development and increased grazing pressure on the algae. This may mean removal of all fish, or selective removal of zooplanktivores, with predators being left, or enhanced stocking of predators [17,139]. These measures, if completed efficiently, lead to increased Daphnia biomass and clarification of the water. In the improved light climate, plant propagules, if still present, are able to develop and re-establish the buffer mechanisms which are necessary to maintain plant dominance. An appropriate fish community can then be reinstated and the system should then be self-sustaining.

A strategy has been developed [112] which represents the ideal approach, with current understanding, for such restorations. Few actual restorations have been able to follow the strategy completely, and the shallow lake system is complex enough for events to go awry because of unexpected complications. However, every restoration attempt adds to understanding, if it is properly monitored, and improves the strategy.