水生生物外语教材.pdfVIP

206 THE BIOLOGY OF LAKES AND PONDS large-scale field exclosures. The potential for strong cascading effects could also be shown, however, in small-scale laboratory experiments. Performance Set-up a series of containers (e.g. aquaria or even large buckets) and fill them with lake/pond water that has been filtered to remove zooplankton. Add a few drops of commercial plant nutrient to each container to enable algae growth. Divide the containers into three groups (treatments) and let the first group remain as it is (i.e. have only algae; one trophic level). Collect large daphnids (easiest in a pond without fish) and add them to the second group of containers (density: 6 Biodiversity and environmental about 30 l1) which then has both algae and zooplankton (two trophic levels). To the third group of containers, which should have three trophic threats levels, add zooplankton as well as one or two small fish (sticklebacks, guppy). Quantify the zooplankton density and phytoplankton biomass when the experiment is terminated after two weeks. Differences in dominant zooplankton and phytoplankton species could also be studied. The experiment could be done either with a low or a high input of Introduction nutrients to determine if top-down effects are equally strong at different nutrient loads