Whole-lake food-web manipulation as a means to study community interactions in a small ecosystem

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 by Microcystis blooms in summer and high chlorophyll-a concentrations were common (ca. 250/~g 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 -l and ca. 5.3 g N m -2 y ~ ), the phytoplankton density was low (Chl-a < 5 lug 1-~), due to control by large-sized zooplankton in spring and N-limitation in summer and autunm. A marked increase in the abundance ofmacrophytes 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 80~0 in 1989. This led to substantial accumulation of both N and P, namely 76~o and 73~o 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, especially L. peregra, which may harbour the parasite causing 'swimmers' itch'. But harvesting of only about 3 ~o of the total macrophyte biomass from the swimmers' area, twice a year, reduced the nuisance for swimmers without adversely affecting the water clarity. 276

Impact of submerged macrophytes on fish‐zooplanl phytoplankton interactions: large‐scale enclosure experiments in a shallow eutrophic lake

“…Although this has been explained by the more quiescent environment within the plant beds favouring algae with low sinking rates (Losee & Wetzel, 1993), the same community structure has also been observed in macrophyte-free lakes with high zooplankton grazing pressure, for instance in biomanipulated lakes in which the planktivorous fish stock has been artificially reduced (e.g. Leah, Moss & Forrest, 1980;Reinartsen & Olsen, 1984;Van Donk et al, 1990;Sondergaard et al, 1990). The high contribution of motile forms to the phytoplankton biomass in macrophyte beds may therefore alternatively reflect reduced fish predation.…”

“…Thus the phytoplankton community in macrophyte beds is generally dominated by small and motile forms such as cryptophytes, while the abundance of algae with high sinking rates (e.g. diatoms and green algae) is generally low Van Donk et al, 1990). Although this has been explained by the more quiescent environment within the plant beds favouring algae with low sinking rates (Losee & Wetzel, 1993), the same community structure has also been observed in macrophyte-free lakes with high zooplankton grazing pressure, for instance in biomanipulated lakes in which the planktivorous fish stock has been artificially reduced (e.g.…”

Total phosphorus thresholds for regime shifts are nearly equal in subtropical and temperate shallow lakes with moderate depths and areas

“…Assuming higher settling and lower resuspension rates in smaller lakes, the model of Janse et al (2008) suggested that the thresholds of phosphorus loading for regime shifts should decrease with increasing fetch, but such a prediction evidently does not hold for lakes with area of 0.1-355 km 2 . In the northern temperate zone, mini lakes with area <0.05 km 2 (0.7-1.5 m in depth) were found to be in the clear-water state when TP was 650-2500 mg m À3 (Jeppesen et al, 1990;Van Donk et al, 1990;Carvalho, 1994;Perrow et al, 1994). Such a 'mini-lake phenomenon' may be attributed to various reasons (Jeppesen et al, 1990), among which frequent fish-kills and shallow water depth seem to be most important (Van Geest et al, 2003;Scheffer & van Nes, 2007).…”

First attempt to apply whole-lake food-web manipulation on a large scale in The Netherlands

“…The efficiency of the small trawl for fry was 45 ~'oNutrient enrichment experiments (bioassays), zooplankton grazing (t4C-tracer technique) and primary production rates were measured to assess the factors limiting the growth of the phytoptankton and to quantify the role of zooplankton grazing. These methods are outlined in Van Donk et aL (1990).…”