Restoration of two lowland lakes by isolation from nutrient-rich water sources with and without removal of sediment.
Experiments were carried out to restore aquatic plant communities to 2 of the phytoplankton dominated Norfolk Broads. Alderfen Broad was isolated by diversion of an inflow stream, without removal of its recently-deposited P-rich sediments. For Cockshoot Broad sediment was removed and dams were placed against the effluent-rich River Bure, but the Broad continued to be fed by a small stream draining an agricultural and fen catchment. In 1979-82 after isolation of Alderfen Broad, the phytoplankton crop was greatly reduced and the water became clear. Net release of PO4-P from the sediment ceased and the Broad became dominated by Ceratophyllum demersum. The reduction in turbulence of the water column by the plants and the OM supplied to the sediment surface in their decay led to the release of PO4-P from the sediment which in 1984, supported a large phytoplankton crop in the spring. The aquatic plant population declined. In 1985 there was both spring and summer phytoplankton growth, the latter probably supported by N-fixation, and the aquatic plant growth was negligible. In Cockshoot Broad a reduction in phytoplankton growth followed soon after isolation and sediment-removal, and the water became very clear and has remained so. Part of the Broad has already recolonized with a diverse collection of aquatic plants. e.g. Elodea canadensis, Lemna minor, Potamogeton pectinatus. The consequences of these results for the management of the rest of the Broadland waterway are discussed.