Dead zones are found in many freshwater lakes including the Laurentian Great Lakes e. Furthermore, such hypoxic events are particularly common in marine coastal environments surrounding large, nutrient-rich rivers e. Hypoxia and anoxia as a result of eutrophication continue to threaten lucrative commercial and recreational fisheries worldwide. Figure 3 Helisoma trivolvis left and Physa acuta right are two of the most common freshwater snails in North America.
Both species use chemical cues to detect predators such as molluscivorous fish and typically respond by hiding under rocks and logs or in shallow water. Some algal blooms pose an additional threat because they produce noxious toxins e. Over the past century, harmful algal blooms HABs have been linked with 1 degradation of water quality Francis , 2 destruction of economically important fisheries Burkholder et al.
Within freshwater ecosystems, cyanobacteria are the most important phytoplankton associated with HABs Paerl Toxigenic cyanobacteria, including Anabaena , Cylindrospermopsis , Microcystis , and Oscillatoria Planktothrix , tend to dominate nutrient-rich, freshwater systems due to their superior competitive abilities under high nutrient concentrations, low nitrogen-to-phosphorus ratios, low light levels, reduced mixing, and high temperatures Downing et al.
Poisonings of domestic animals, wildlife Figure 4 , and even humans by blooms of toxic cyanobacteria have been documented throughout the world and date back to Francis' first observation of dead livestock associated with a bloom of cyanobacteria. Furthermore, cyanobacteria are responsible for several off-flavor compounds e. In addition to posing significant public health risks, cyanobacteria have been shown to be poor quality food for most zooplankton grazers in laboratory studies Wilson et al.
Eutrophication is also associated with major changes in aquatic community structure. During cyanobacterial blooms, small-bodied zooplankton tend to dominate plankton communities, and past observational studies have attributed this pattern to anti-herbivore traits of cyanobacteria e. However, the biomass of planktivorous fish is often positively related to nutrient levels and ecosystem productivity.
Piscivorous fishes e. Thus, an alternative explanation for the lack of zooplankton control of cyanobacterial blooms could include consumption of zooplankton by planktivores. Water resource managers routinely employ a variety of strategies to minimize the effects of cultural eutrophication, including 1 diversion of excess nutrients Edmondson , 2 altering nutrient ratios Downing et al. However, nutrient reduction can be difficult and expensive to control, especially in agricultural areas where the algal nutrients come from nonpoint sources.
However, algaecides are expensive to apply, do not control the primary cause of the problem i. Another alternative for improving water quality in nutrient-rich lakes has been biomanipulation - the alteration of a food web to restore ecosystem health Shapiro et al. The basic premise is that secondary consumers planktivorous fishes are removed either through the addition of tertiary consumers piscivorous fishes or harvesting, which allows for the dominance of large-bodied, generalist grazers e. When planktivorous fishes are abundant and there is no predation refuge e.
Many past studies have shown strong correlations between the size structure of zooplankton communities and phytoplankton abundance. These data support the notion that predatory top-down forces can have important implications for aquatic communities and ecosystems. With that said, fish-centric biomanipulation effects on water quality are typically short-lived i.
Water – a vital nutrient
Figure 5 Daphnia are generalist herbivores that play a powerful role in the structure and function of lake ecosystems and can have large effects on phytoplankton abundance, water clarity, and water quality when abundant. Arend, K. Seasonal and interannual effects of hypoxia on fish habitat quality in central Lake Erie. Freshwater Biology 56 , Benndorf, J. Conditions for effective biomanipulation - conclusions derived from whole-lake experiments in Europe. Hydrobiologia , Boyd, C. Pond aquaculture water quality management. Norwell MA: Kluwer Burkholder J. New 'phantom' dinoflagellate is the causative agent of major estuarine fish kills.
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Crews, J. Agriculture and natural resources U. Auburn AL: Auburn University Diaz, R. Spreading dead zones and consequences for marine ecosystems. Science , Dodds, W. Eutrophication of U. Environmental Science and Technology 43 , Downing, J. Predicting cyanobacteria dominance in lakes. Canadian Journal of Fisheries and Aquatic Sciences 58 , Edmondson, W. Phosphorus, nitrogen, and algae in Lake Washington after diversion of sewage. Huisman J. Changes in turbulent mixing shift competition for light between phytoplankton species.
Ecology 85 , Jeppesen, E. Top-down control in freshwater lakes: the role of nutrient state, submerged macrophytes and water depth. Lehtiniemi, M. Turbidity decreases anti-predator behaviour in pike larvae, Esox Lucius. Environmental Biology of Fishes 73 , Morris, J. Harmful algal blooms: an emerging public health problem with possible links to human stress on the environment.
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Nutritional Freshwater Life
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Gainesville, FL: University of Florida Smith, V. Eutrophication science: where do we go from here? Trends in Ecology and Evolution 24 , As his love grows into an obsession, he attempts to create a goldfish that will capture and reflect her beauty. Some Malays believe that the climbing perch Anabas testudineus has the supernatural power of warding off or warning against the presence of evil spirits in the house. They often inhabited the sacred wells, feeding on the fruits often, hazelnuts of the tree of life. According to the World Association of Zoos and Aquariums WAZA , more than million people visit zoos worldwide, most of which have aquariums, and million people visit aquariums globally each year.