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.

In this study experimental restoration of Cockshoot Broad was attempted through a combination of isolation from nutrient-rich water and sediment sources, and removal of accumulated nutrient-rich sediment.

Study site: Cockshoot Broad (3.3 ha) lies to the south of the River Bure. It was linked to the river via Cockshoot Dyke, at its northern end and through Old Hall
Dyke, Ranworth Broad and Ranworth Dyke to the east. Prior to isolation it acted as a
sedimentation basin for water moving in from the river. By 1981 it had become so infilled that in summer only a few centimetres of water remained. There were no submerged aquatic macrophytes present. Prior to the 1950s the Broad supported a rich flora.

Sediment removal and water quality: In 1981, sediment was pumped out to give a water-depth of about 1 m over three-quarters of the Broad. A dam was built across Cockshoot Dyke and a one-way sluice installed on Old Hall Dyke. This allowed outflow of water entering from seepage and from a small stream (Cockshoot Stream), Water quality was monitored in the Broad, in the stream, and in the River Bure, the original main source of water to the Broad, for two years prior to isolation in 1982 and then subsequently.

The water cleared, nutrient levels dropped and a variety of aquatic macrophytes colonized part of Cokshoot Broad, especially the Dyke area, soon after isolation and sediment removal. Some plant introductions were also made in order to assist this process. By 1985 the following were established: water starwort Callitriche palustris, rigid hornwort Ceratophyllum demersum, Canadian pondweed Elodea canadensis, frogbit Hydrocharis morsus-ranae, common duckweed Lemna minor, ivy-leaved duckweed L.trisulca, yellow water-lily Nuphar lutea, white water-lily Nymphaea alba, amphibious bistort Polygonum amphibium, fennel-leaved pondweed Potamogeton pectinatus, unbranched bur-reed Sparganium emersum and bladderwort Utricularia vulgaris.

By the end of this study period, colonization of the main basin by most of these had not occurred, but plantings of both lily species had established and water-lily beds were increasing in area. The reasons why the Dyke has become colonized so rapidly, compared with the Broad, are not clear. It seems likely that the shortage of plants in the main Broad reflects an initial shortage of inocula.

Conclusions: In Cockshoot Broad a reduction in phytoplankton growth followed soon after isolation and sediment removal, part of the Broad has been recolonized by a variety of aquatic plants and the water became very clear and has remained so. This is in contrast with an isolation-only approach in another similarly affected Broad (for summary see: http://www.www.conservationevidence.com/ViewEntry.asp?ID=851).

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