Zooplankton Ecology is far too complex a topic to address on a general information web site, and represents the main focus of FAS 6932, which will be offered in Spring 2006.
Here are just a few bullets that reflect important aspects of zooplankton ecology:
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The biomass and taxonomic structure of zooplankton is affected by physical and chemical conditions, resource quantity and quality, and predation by fish and invertebrates. Some key water chemistry attributes that directly affect zooplankton are pH, aluminum, heavy metals, and calcium. Attributes that indirectly affect zooplankton by altering the biomass and composition of their food (algae, bacteria, protozoa) include nutrients such as N and P. The extent to which resources vs. consumers control zooplankton is highly variable among lakes.
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The food web in which zooplankton occurs is quite complex, with some carbon and energy flows directly from algae to zooplankton, some from bacteria to zooplankton, and some indirectly from algae and bacteria to zooplankton by way of intermediate consumers including ciliates and flagellates (protozoa). That indirect route is often referred to in the literature as the microbial web, or microbial loop. The term loop is used when the pathway 'recaptures' organic carbon excreted by algae back into the food web via bacterial uptake and subsequent grazing.
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Zooplankton grazing is influenced by the type of algae occurring in the water. Large filaments of blue-green algae, particularly those that produce or contain toxins, can interfere with grazing, especially in generalist feeders like Daphnia.
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During certain times of the year, grazing by large zooplankton like Daphnia can 'clear' the water of phytoplankton. This most often occurs during spring in temperate lakes when the phytoplankton is dominated by small edible species. In nutrient-rich (eutrophic) lakes, such clear water periods often are followed by a crash in biomass of the large zooplankton and a shift in the phytoplankton to less edible blue-green algae. During that time of the year, the zooplankton tends to be dominated by rotifers and other small zooplankton, which may largely feed on bacteria and protozoa.
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Zooplankton food webs containing many links (e.g., bacteria --> flagellates --> ciliates --> crustaceans) are less efficient in transferring energy to higher trophic levels such as small fish than simple food webs (e.g., phytoplanton --> crustaceans) because they lose considerably more energy in respiration.
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Manipulating food webs to favor dominance by large Daphnia is a potential way to improve water quality in eutrophic temperate lakes. Plankton-eating fish are removed from a lake, Daphnia (the taxa most susceptible to fish predation) increase, they rapidly graze algae, and algal biomass is reduced. This method of lake management has been called 'biomanipulation' and the underlying ecological process has been called a 'trophic cascade.' There continues to be discussion in the literature as to whether or not trophic cascades can persist, vs. the plankton simply switching to dominance by grazer-resistant algae. It has been suggested that biomanipulation cannot work in tropical and sub-tropical lakes because there are no large Daphnia (perhaps due to intense fish predation or high water temperature). However, the data are sparse and this is an open area for future research.
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There is ongoing discussion in the literature regarding the extent to which ratios of essential nutrients (nutrient stoichiometry) in algae influence the biomass and growth of zooplankton grazers, vs. other properties of algae such as size, lipid content, presence / absence of toxins, etc.
These topics and many others are covered in the Zooplankton Ecology course.