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Newsletter and Technical Publications Effects of Eutrophication Algal Blooms A pervasive result of enrichment of lakes with nutrients is increased growth of algae. Cyanobacteria are an especially troublesome group that are known to form unsightly surface scums, to cause severe oxygen depletion and fish mortalities, and to lead to death of cattle and other animals from ingestion of algal toxins. Filamentous species of cyanobacteria or green algae (chlorophytes) can clog filters in water treatment or industrial facilities. Dinoflagellates are another group of phytoplankton that can cause toxic conditions. One by-product of algal blooms can be high concentrations of dissolved organic carbon (DOC). When water with high DOC is disinfected by chlorination, potentially carcenogenic and mutagenic trihalomethanes are formed. Growth of Aquatic Plants Dense mats of floating aquatic plants, such as water hyacinth (Eichhornia crassipes), an aquatic fern (Salvinia molesta) and Nile cabbage (Pistia stratiotes ), can cover large areas near-shore and can float into open water. These mats block light from reaching submerged vascular plants and phytoplankton, and often produce large quantities of organic detritus that can lead to anoxia and emission of gases, such as methane and hydrogen sulfide. The material derived from these plants is usually of low nutritional quality and is not often an important component of the food for zooplankton or fish. Accumulations of aquatic macrophytes can restrict access for fishing or recreational uses of lakes and reservoirs and can block irrigation and navigation channels and intakes of hydroelectric power plants. Anoxia A by-product of increases in the abundance of algae and aquatic macrophytes is generation of more organic matter. As this organic matter decomposes in the water column or in the sediments, the concentration of dissolved oxygen decreases. In shallow lakes and where plant production is large, complete deoxygenation of the sediments and water can occur. Such conditions are not compatible with the survival of fishes and invertebrates. Moreover, under anoxic conditions, ammonia, iron, manganese and hydrogen sulfide concentrations can rise to levels deleterious to the biota and to hydroelectric power facilities. In addition, phosphate and ammonium may be released into the water from anoxic sediments, further enriching the lake. Species Changes Shifts in the abundance and species composition of aquatic organisms often occur in association with alterations of ecosystems caused by eutrophication. Reduction in underwater light levels because of dense algal blooms or floating macrophytes can reduce or eliminate submerged macrophytes. Changes in food quality associated with shifts in algal or aquatic macrophyte composition, and decreases in oxygen concentration often alter the species composition of fishes. Elevated Nitrate Concentrations High concentrations of nitrate resulting from nitrate-rich runoff or nitrification of ammonium within a lake can cause public health problems. The inhibition of the ability of infants to incorporate oxygen into their blood can result in a condition called blue baby syndrome (methylhaemoglobinaemia) if nitrate levels are above 10 mg per liter in drinking water. The condition can be life-threatening. Increased Incidence of Water-related Diseases In some situations eutrophication stems from untreated human sewage reaching lakes and reservoirs. If a portion of the population producing the sewage suffers from infections transmitted directly or indirectly via water, the spread of human diseases occurs. While such situations are especially prevalent in tropical countries where poverty is common and the number of diseases is large, avoiding the spread of disease via water is a concern for all countries. Indeed, municipal water supplies that pipe water throughout a city from central storage reservoirs are highly susceptible to spread of diseases, such as typhoid or cholera, that can be seeded by seemingly negligible fecal pollution from infected persons. Increased Fish Yields Yields of fish tend to increase as primary productivity increases in lakes, reservoirs and in aquacultural systems. Assuming that the fish whose yields are improved are edible and marketable, the increase in primary productivity often associated with nutrient enrichment can have a positive result up to a point (see Figure 1.7 in IETC's Technical Information Series number 11).
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