Thresholds Database > Lake eutrophication, River Bure, England

Background

Alternate Regimes

1. Clear water, submerged macrophytes, abundant fish, few Daphnia

2. Eutrophic, phytoplankton, few fish, abundant Daphnia

Mechanism

High phosphorus and nitrogen input from sewage increased the density of phytoplankton and zooplankton and turbidity of the water. The amount of dissolved oxygen decreased with an associated loss of benthic fish species and macro-invertebrate prey species. There is a cyclic shift between the alternate states.

Management Decisions in Each Regime

State 2: Cockshoot Broad (3.3ha. x 1m. deep) was isolated from sewage-effluent in 1982 and 70cm of sediment was pumped out. Total phosphorus concentrations fell quickly and dense macrophytic vegetation became re-established in one part of the Broad. In one area, the macrophyte population collapsed after 8 years of stable high biomass and remained low for two years. Fish were removed and the plants recovered, but conditions are still cyclic.