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B. Modification of algal succession to limit toxic algal blooms

Problems with toxic cyanobacterial bloom formation

The storage of good quality water is the primary objective of the reservoir management. Drinking water supplies especially have a requirement for high water quality. But even when reservoir waters are impounded for industrial use, fisheries, or recreation, the impounded water have greater utility if they are relatively clean. The design features of reservoirs, such as surface area and volume of reservoirs relative to river flows and the locations of inlet and outflow structures, can influence these water quality conditions.

Small (volume = 10⁶- 0⁸ m³), shallow lowland reservoirs, especially those used as sources of drinking water, and for fish cultivation, irrigation, and recreation, are sometimes characterised by long water retention times. These reservoirs are ecosystems that are especially susceptible to eutrophication (Taub 1984, Staskraba et al. 1999a, Zalewski 1999). This is caused by:

• runoff of agrochemicals from agricultural land uses in catchment;
• high catchment area to reservoir area ratios, and high nutrient inputs;
• intensive sedimentation of both particulate and dissolved matter in the reservoir;
• disruption of the biotic structure as a consequence of frequent and/or significant changes in water levels during a year; and
• the absence of a properly working littoral zone as a consequence of the water level fluctuations.

Eutrophication in reservoirs leads to the formation of intensive phytoplankton blooms. During recent years, both the incidence and intensity of such blooms appear to be increasing, when examined at the global scale. This increasing severity of algal blooms may be a consequence of increasing levels of nutrient enrichment as a result of sewage disposal, increased agricultural runoff, and changes in hydrological regimes potentially related to climate change.

The term, "bloom," describes a phytoplankton biomass significantly higher than the average biomass during a year. According to Nebaeus (1984), phytoplankton biomass greater than 3 mg/l, or chlorophyll-a concentrations above 20 mg/m³, constitute a bloom. Cyanobacterial, or blue-green algal, blooms can occur in different portions of the water column (Figure 8.2). In tropical water bodies, blooms also can take place throughout the year. In temperate regions, however, there is often a characteristic seasonal development of bloom-forming algal populations. These blooms usually comprise one or two species identified by the dominant phytoplankton type or genera; e.g., diatom blooms, cyanobacterial blooms, Microcystis blooms, Scenedesmus blooms, etc.

Phytoplankton blooms can degrade water quality in several ways by:

• decreasing water column transparency,
• affecting the taste and odour of the water,
• depleting deep water oxygen concentrations¹,
• changing fish community composition,
• decreasing the abundance of submerged macrophytes and shifting aquatic plant communities toward those dominated by a few species of bloom-forming algae,
• producing toxic compounds that pose an health hazard to people and animals.

One of the most dangerous effects of cyanobacterial blooms is the release of secondary metabolites, many of which are very toxic (Carmichael 1992, Codd and Bell 1998, Chorus and Bertram 1999), into the environment. This consequence is often exacerbated by water treatment techniques that cause the algal cells to lyse, releasing the cell contents including the toxins into the treated water. In addition, when these compounds are chlorinated, some can produce trihalomethanes, which are human carcinogens.

Cyanobacterial biotoxins fall into four classes: hepatotoxins, neurotoxins, dermatotoxins, and lipopolysaccharides (Table 8.1). These substances are often algal genera, species-, or strain-specific. Hepatotoxic microcystins are the most common of the cyanobacterial toxins. They have the form of cyclic peptides, and can cause death through haemorrhages of the liver. Mortality can occur over a time period ranging from a few minutes to a few days after intoxication. Microcystins also are carcinogens, which have tumour-causing properties, primarily leading to liver cancers (Ueno et al. 1996). Hepatotoxic cyanobacterial extracts can induce chromosomal aberrations in human cells (Figure 8.3), and can occur as a consequence of long-term, chronic exposure of humans to cyanobacterial toxins. Such exposure can occur via drinking water, as conventional water treatment processes are ineffective in removing toxins during water purification process (Chorus and Bertram 1999).

¹ Note: Many tropical systems naturally stratify as a consequence of the temperature-density relationships pertaining within those systems, and often develop anoxic or anaerobic hypolimnia as a consequnce of this stratification, whereas temperature systems generally develop anoxic or anaerobic hypolimnia as a consequence of the decomposition or oxidation of organic matter settling into the hypolimnion as a consequence of high levels of biological production resulting from the eutrophication of those systems. For this reason, Ryding and Rast (1989) suggest that hypolimnetic deoxygenation alone not be considered a eutrophication indicator in tropical lakes and reservoirs. Rather, a combination of indicators is recommended to support the diagnosis of eutrophic conditions in these lakes.

Toxic cyanobacterial blooms have caused massive mortalities among wild and domestic animals (Codd et al. 1997), and have constituted a hazard to human health principally through gastric upsets and diarrhoea arising from the ingestion of polluted water and contaminated foods, and skin irritations. Many data concerning the effects of poisoning from cyanobacterial toxins in humans comes from tropical and subtropical areas in Australia, Africa, and South America (Falconer 1994). The first well-documented human fatalities, in which cyanobacterial toxins were implicated, were described in Brazil in 1996. More then 50 patients died after displaying hepatotoxic and neurotoxic symptoms following treatment in a haemodialysis centre (Puoria et al. 1998). Because of the epidemiological character of cyanobacterial toxins, the World Health Organization recommended safety guidelines for the maximum concentrations of microcystin in drinking water is 1 µg/l (WHO 1998).

Tab. 8.1. Cyanobacteria reported in the literature as having toxic properties

Toxin classes Cyanobacterial genera

Hepatotoxins microcystins Microcystis, Anabaena, Plantotrix (Oscillatoria), Nostoc, Anabaenopsis nodularins Nodularia cylindrospermopsins Cylindrospermopsis, Aphanizomenon, Umezakia

Neurotoxins anatoxin-a Anabaena, Plantotrix (Oscillatoria), Cylindrospermum, Aphanizomenon Anabaena anatoxin-a(s) Aphanizomenon, Anabaena, Cylindrospermopsis, Lyngbya saxitoxins

Dermatotoxins lyngbyatoxin-a Lyngbya aplysiatoxins Lyngbya, Plantotrix (Oscillatoria), Schizothrix

Lipopolysaccharides many cyanobacteria

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