Restoration of two lowland lakes by isolation from nutrient-rich water sources with and without removal of sediment

Published source details Moss B., Balls H., Irvine K. & Stansfield J. (1986) Restoration of two lowland lakes by isolation from nutrient-rich water sources with and without removal of sediment. Journal of Applied Ecology, 23, 391-414.
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Summary

The Norfolk Broads in eastern England comprises about 50 small (1-20 ha), shallow (1-2 m deep) lakes formed by 9th-14th century peat-digging and subsequent flooding. The lakes (Broads) are mostly inter-connected by man-made channels (dykes) to main river channels which drain some of the most intensive agricultural land in Britain, from which nutrients derived from heavy fertilizers inputs, have entered the water system, as well as carrying treated sewage effluent. Until the 1950s, most of this wetland had clear water, sparse phytoplankton populations and abundant submerged plants. Increasing eutrophication led to increased water-weed growth in the 1950s but the aquatic plants were rapidly replaced by dense phytoplankton communities which now dominate. This has accordingly led to decreased aesthetic and conservation interest, reduced fish stocks and diversity, greater sedimentation rates and reduced bank protection against erosion.

One way to reverse this trend in the Broads would be to pump out the accumulated nutrient-rich sediments, but removal and disposal are very expensive. In this study, Alderfen Broad was isolated, by diversion of an inflow stream rich in nutrients, without removal of its recently-deposited phosphorus-rich sediments, in an attempt to restore water quality.

Study site: Alderfen Broad is 4.7 ha in area with an average depth of about 80 cm. Until the 1960s it was dominated by stands of rigid hornwort Ceratophyllum demersum and water lilies. Water plants had disappeared almost completely by the 1970s. At this time the Broad was receiving large quantities of nutrients, via the soakway of a small sewage-treatment works and release from the sediments of phosphorus during summer.

Broad isolation: The wetland surrounding the Broad has a network of dykes which allowed diversion of the main inflow stream (carrying nutrient-rich water) away from the Broad into the nearby River Ant. Diversion of the inflow stream was completed in early 1979 and the water chemistry of the Broad and of the diverted water has since been regularly monitored. The dam that diverts the original inflow stream has been examined regularly and has not been breached, or overtopped, even at periods of high water.

In four years (1979-82) after isolation of Alderfen Broad, the phytoplankton crop was greatly reduced and the water cleared. Net release of PO₄-P from the sediment ceased and the Broad became again dominated by rigid hornwort. However, due to reduced water turbulence due to the plant growth, and the organic matter supplied to the sediment surface in their decay, PO₄-P release from the sediment was reactivated. In 1984, the lake supported a large phytoplankton crop in the spring and the aquatic macrophyte population declined. In 1985 there was both spring and summer phytoplankton growth and aquatic macrophyte growth was negligible.

Conclusions: Isolation initially brought about the desired improvement in Alderfen Broad within four years. This was followed by a regression back to its original state in the following three years. This cycle was due to the interaction of the plant and phytoplankton populations with the phosphorus-rich sediments laid down in the Broad over the past several decades.

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