Biomanipulation of Furnas Lake, Azores: effects of repeated fish removal

Furnas Lake is a small meso- to eutrophic water body located in an ancient volcanic crater of Sao Miguel Island, Azores. In addition to agricultural nutrient inputs, high densities of herbivorous fish (mainly common carp), as well as (irregular) hypolimnetic aeration contribute to maintain high levels of turbidity in the lake through resuspension of the nutrients accumulated in the sediment. SCHEFFER et al. (1993) suggested that turbidity in shallow lakes is not a smooth function of their nutrient status. Lakes are thought to have 2 distinct equilibria: a clear state dominated by macrophytes and a turbid state dominated by algae. A shift from turbid to clear water conditions is favoured by (1) reduction of nutrient concentrations. reducing algal growth; (2) increased light availability, favouring vegetation, which retains sediments, protects zooplankton and consumes nutrients; and (3) reduction of planktivorous and benthos-feeding fish stock, reducing consumption of zooplankton, which grazes on phytoplankton, and reducing sediment resuspension with release of nutrients from the sediment pool (JEPPESEN et al. 2007). To improve water quality in the Furnas lake, a substantial part of the herbivorous and bottom- feeding (thus sediment resuspending) fish population, mainly common carp, was removed. Water quality was analysed in terms of turbidity, chlorophyll a, trophic state index values, macrophytes and zooplankton.