THE NORFOLK BROADLAND: EXPERIMENTS IN THE RESTORATION OF A COMPLEX WETLAND

Abstract

Summary1. The Norfolk Broadland comprises wide river valleys, floored with deep deposits of peat and clay. Over forty mediaeval peat pits (the Broads) became flooded after the fourteenth century and were mostly connected with the rivers by navigation channels. Between about 1400 A.D. and 1800 A.D. the valleys supported a diverse wetland ecosystem, partly maintained by deliberate cropping of wetland plants. Some of the wetland was gravity‐drained, but extensive aquatic habitats held diverse fens and submerged vegetation dominated by short‐growing aquatic plants in very clear water.2. The Enclosure Acts passed around 1800 encouraged more intensive agriculture in the catchments and this, coupled with pumped drainage, particularly of the wetlands in the lower valleys, caused them to sink and so they were embanked against the rivers. The drained land, intersected by channels (dykes), was grazed.3. In the late nineteenth century, migration of human populations to the towns led to pressures for improved sewage disposal; consequently, increasing amounts of raw sewage, and later, treated effluent, both major sources of phosphorus, were discharged to the rivers and Broads, which were thus progressively eutrophicated. Agricultural changes led to an increased nitrogen supply. A tourist boating industry also began then.4. Increasing eutrophication caused replacement of low‐growing submerged aquatic plants together with ranker species, and then, particularly after the Second World War, submerged plants were lost from much of the waterway and replaced by phytoplankton‐dominated communities. The mechanism by which this change took place concerns growth of epiphytic and filamentous algal communities and probably interactions with phytoplankton‐grazing animals associated with the plants. The loss of submerged plants is linked with decreased invertebrate diversity, and changes in fish and water bird populations. Eutrophication is also associated with fish‐kills caused by a toxic flagellate, Prymnesium parvum, oubreaks of avian botulism and decreased fen diversity through flooding with enriched water. Increased boat activity has led to severe bank erosion. Erosion is exacerbated by loss of bank protection, because of loss of submerged plants and of fringing reedswamp through damage by coypu, an exotic rodent.5. Lack of labour‐intensive management of the undrained fens has led to a decrease in their diversity, through natural succession to alder swamp.6. Restoration of an aquatic‐plant‐dominated waterway is desirable and has been attempted through various means of reducing phosphorus input. Complete isolation of Broads or dykes has proved generally useful, but complications with release of phosphate from sediments, though such release eventually declines, are delaying success of an attempt to reduce phosphorus levels by precipitation of phosphorus from sewage effluent. Such a technique is the only one widely practicable in Broadland.7. Simple nutrient reduction may not always be sufficient to cause a return from phytoplankton‐dominated, turbid water to clear water with aquatic plants. It may be necessary to reinstate a higher intensity of zooplankton grazing than at present possible because of predation of grazers by fish in a habitat lacking refuges for the zooplankton from such predation.8. The drained grazing marshes contain, in their dykes, a rich aquatic plant and invertebrate community, and provide nesting sites and grazing for birds. Changed political and economic factors may lead to future diminution of these communities through intensified drainage for arable cultivation.