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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.3. Impacts of Eutrophication

1.3.2. 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 can attain high levels and are known to form unsightly surface scums, to cause severe oxygen depletion and fish mortalities upon their die-off, and to lead to death of cattle and other animals from ingestion of algal toxins. Gastrointestinal disorders in humans can be associated with the drinking of water that contained blooms of cyanobacteria. Contact with water or even aerosols emitted from cyanobacterial blooms can cause allergic reactions in some people. Cyanobacteria and filamentous species of chlorophytes, or green algae, can cause off-flavors and odors in water and fish, as well as clogging of filters in water treatment or industrial facilities. Dinoflagellates are another group of concern that is known to develop so-called red tides, which can include toxic strains. One by-product of dense algal blooms is high concentrations of dissolved organic carbon (DOC). When water with high DOC is disinfected by chlorination, potentially carcenogenic and mutagenic trihalomethanes are formed.

Algal Toxins

Freshwater toxins are produced almost exclusively by cyanobacteria. Various cyanobacterial genera and species produce different toxic compounds generally classified as neurotoxins, hepatotoxins, cytotoxins and endotoxins. Although neurotoxins are highly toxic, in general, their degradation in water column is rapid. However, saxitoxins are an exception with breakdown reactions requiring weeks. Moreover, removal of hepatotoxins from reservoirs containing toxic cyanobacteria is difficult because some forms are stable and resistant to chemical hydrolysis or oxidation and may persist for months or years and remain potent even after boiling.

The alkaloid cylindrospermopsin is considered a cytotoxin because it attacks cells throughout the body. Gastroenteritis, renal malfunction and hepatitis have been observed in animals or human population intoxicated by water containing cyanobacteria with cylindrospermopsin production. Lipopolysaccharide endotoxins have been implicated in irritations of skin and allergic responses in human and animal tissues that come in contact with these compounds.

Several environmental factors, i.e., light, temperature, nutrient concentration, or pH, can influence the degree of toxin production, but the genetic structure of a bloom seems to be the major factor determining its toxicity. Typically, about half of all blooms tested are toxic, and the occurrence of toxic blooms is becoming more frequent. Toxin content is highest within actively growing cyanobacterial cells, and release to the water appears to occur during cell senescence, death and lysis.

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 usually 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. The effect of eutrophication of Lake Dianchi, China, indicated by the growth of dense mats of the water hyacinth is shown in Figure 1.5.

Figure 1.5. Water hyacinth in Lake Dianchi, China.

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 deeper water can occur. Such conditions are not compatible for 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 are 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 the multifaceted 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. For example, less desirable species, such as carp, may become dominant. However, in some situations, such changes may be deemed beneficial.

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