Experiments with Large Enclosures in a Fertile, Shallow, Brackish Lake, Hickling Broad, Norfolk, United Kingdom

Abstract

Events in the plankton community of a 120 ha, shallow (1.2 m), brackish lake have been studied with the help of two watertight enclosures, each of which isolated about 300 m3 of water and its underlying sediment. Analyses of soluble reactive phosphorus, total phosphorus, ammonia-N, nitrate-N, silicate-Si, chloride, photosynthetic pigments, phytoplankton and zooplankton counts are reported and are related to inputs of nitrogen and phosphorus into the lake from adjoining basins and the catchment, and to the hydrologic regime. The inputs of phosphorus are budgeted and a very large roost of black-headed gulls appears to provide most of the phosphorus supply. The results are discussed with reference to the eutrophication of the lake and decline of aquatic macrophytes which have occurred in the past ten years.

The composition and ecology of periphyton communities in freshwaters. II. Inter-relationships between water chemistry, phytoplankton populations and periphyton populations in a shallow lake and associated experimental reservoirs (‘Lund tubes’)

“…2) was comparatively low in the lake in winter (50-80/~g P 1 -x) but steadily increased to a maximum of 183/~g P 1 -~ in autumn 1978. This pattern, which reflects that of previous years (Holdway et al 1978;Leah et al 1978), is believed to reflect a steady input of phosphorus from gull excreta decomposing at the sediment surface in spring, resulting in a summer and autumn accumulation in the water, then a dilution in winter by incoming land drainage water with low total phosphorus levels. The water in the tubes was not subject to such great winter dilution and had considerable loading of phosphorus from the excreta of birds perching on the tube rims as successive scaring devices failed.…”

“…Much background information and maps of Hickling Broad, showing the location of the experimental tubes and geographical and limnological features are given in Leah et al (1978), Holdway et al (1978) and Eminson & Moss (1980). Surface water (the lake is only about 1 m deep and does not stratify) was sampled at 2 weekly intervals from each of the tubes and from the Broad in a bay, Heigham Corner, where the tubes are situated.…”

“…The major source of phosphorus loading on both lake and tubes was gull excreta (Leah et al 1978) and phosphate released from decomposition of their droppings and entering the lake from the catchment area is readily absorbed by the phytoplankton, which persists as a sizeable population even in winter (see below). Total phosphorus (Fig.…”

“…The biology of the periphyton populations on these plants was therefore of much interest, particularly since a few plants of Potamogeton pectinatus L. had also persisted in two experimental 20 m diameter "Lund tubes" placed in the Broad (Leah et al 1978). The isolated water in these tubes had received a phosphorus loading probably greater than that of the Broad itself owing to the failure of "bird scarers" on the tube rims to discourage gulls perching on them in 1976-1977, (Leah et al 1978). The lack of flushing of the tubes also meant that the added phosphorus was not diluted as much as it was in the Broad.…”

Algal and other fossil evidence for major changes in Strumpshaw Broad, Norfolk, England in the last two centuries

“…To varying degrees the ecosystems of the Broads have been altered in the last few decades or, in some cases, longer periods. The major change has been a rise in nutrient loading (eutrophication) by agricultural changes, expanding population and volumes of sewage effluent, and, in one area, increasing seagull populations (Moss, 1977a,b;Leah et al, 1978). The eutrophication has been traced in broads of the R. Thurne valley (Moss et al, 1979), R. Ant valley (Moss et al, 1979, Osborne & Moss, 1977 and R. Bure valley (Moss et al, 1979) with dated sediment cores.…”