The effects of water quality and invertebrate predators on food web management of lakes

In food web management of lakes, the aim is to improve water quality by enhancing grazing pressure on phytoplankton through fish stock manipulations. Our results from Lake Hiidenvesi suggested that the theories behind food web management may not be valid in clay-turbid lakes. This is because clay turbidity seems to facilitate high biomasses of invertebrate predators, which may negate the effects of food web management. The earlier part of the project suggested that increasing clay turbidity may a factor behind high biomasses of pelagic invertebrate predators. This latter part of the project concentrated on the causes and physical-chemical consequences of elevated sediment resuspension (the return of bottom material to the water column) on water quality and on the effects of humic water on the coexistence of pelagic invertebrate predators and fish. In the first subtopic, studies in L. Hiidenvesi showed that sediment resuspension substantially increases the availability of soluble phosphorus for phytoplankton, especially during algal blooms when the pH of the water is high. Additionally, field studies revealed a previously unknown link between cyanobacterial blooms, wave action and sediment quality. The studies demonstrated that in late summer enhanced sediment resuspension simultaneously de¬creased the N:P ratio in the water column and increased light extinction although wave action at the time was low. The results suggested that sediment resuspension was the main cause of cyanobacterial blooms. In the second subtopic, field studies and experimental studies suggested that humic water protects invertebrate predators from fish predation, but probably not as efficiently as clay-turbidity.