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Whole-lake food-web manipulation as a means to study community interactions in a small ecosystem
Authors:van Donk  E  Grimm  M P  Gulati  R D  Klein Breteler  J P G
Institution:(1) Provincial Waterboard of Utrecht, P.O. Box 80300, 3508 TH Utrecht, The Netherlands;(2) Present address: Department of Nature Conservation, Aquatic Ecology Section, Agricultural University, P.O. Box 8080, 6700 DD Wageningen, The Netherlands;(3) Witteveen & Bos, Consulting Engineers, P.O. Box 233, 7400 AE Deventer, The Netherlands;(4) Limnological Institute, ‘Vijverhof’ laboratory, Rijksstraatweg 6, 3631 AC Nieuwersluis, The Netherlands;(5) Organization for Improvement of the Inland Fisheries, P.O. Box 433, 3430 AK Nieuwegein, The Netherlands
Abstract:Whole-lake food-web manipulation was carried out in the hypertrophic Lake Zwemlust (The Netherlands), with the aim of studying the effects on the lake's trophic status and to gain an insight into complex interactions among lake communities. Before manipulation this small (1.5 ha) and shallow (1.5 m) lake was characterized byMicrocystis blooms in summer and high chlorophyll-a concentrations were common (ca. 250 μg 1?1). In March 1987 the planktivorous and benthivorous fish species in the lake were completely removed (ca. 1000 kg ha?1), a new simple fish community (pike and rudd) was introduced and artificial refuges were created. The effects of this manipulation on the light climate, nutrient concentrations, phytoplankton, zooplankton, fish, macrophytes, and macrofauna were monitored during 1987, 1988 and 1989. Community interactions were investigated in phytoplankton bioassays and zooplankton grazing experiments. After the manipulation, despite the still high P and N loads to the lake (ca. 2.2 g P m?2 y?1 andca. 5.3 g N m?2 y?1), the phytoplankton density was low (Chl-a<5μg l?1), due to control by large-sized zooplankton in spring and N-limitation in summer and autumn. A marked increase in the abundance of macrophytes and filamentous green algae in 1988 and 1989, as well as N loss due to denitrification, contributed to the N limitation of the phytoplankton. Before manipulation no submerged macro-vegetation was present but in 1988, the second year after manipulation, about 50% of the lake bottom was covered by macrophytes increasing to 80% in 1989. This led to substantial accumulation of both N and P, namely 76% and 73% respectively of the total nutrients in the lake in particulate matter. Undesirable features of the increase in macrophytes were: 1) direct nuisance to swimmers; and, 2) the large scale development of snails, especiallyL. peregra, which may harbour the parasite causing ‘swimmers' itch’. But harvesting of only about 3% of the total macrophyte biomass from the swimmers' area, twice a year, reduced the nuisance for swimmers without adversely affecting the water clarity.
Keywords:biomanipulation  whole-lake experiments  lake restoration  food-web  phytoplankton  zooplankton  Lake Zwemlust
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