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<Planning and Management of Lakes and Reservoirs:
An Integrated Approach to Eutrophication>


1.3. Impacts of Eutrophication

1.3.3. Case Studies of Effects of Eutrophication

Tropical Reservoirs

The series of reservoirs in the middle Tiete River, Brazil, receive inputs from sugar cane processing plants and discharges from the city of Sao Paulo at their upstream end, hence the most polluted is the first reservoir in the cascade, Barra Bonita. Barra Bonita Reservoir, completed in 1963, has approximately 325 km² of surface area and a maximum depth of approximately 25 m. Studies beginning in the late 1970s have demonstrated a rapid eutrophication of the reservoir and the importance of flushing rate and winds on water quality. Thermal stratification is transient throughout the year, and the water column remains oxygenated. These conditions, in combination with high concentrations of iron entering via tributaries, tend to maintain low phosphate levels in the water, while the sediments have elevated concentrations. In contrast, nitrate concentrations are high, and enrichment experiments do not detect nitrogen limitation.

Reservoirs within the confines of cities often exhibit advanced states of eutrophication and other signs of pollution. For example, Parano Lake was constructed in 1959 to beautify Brasilian, the capital of Brazil, provide recreation and generate hydroelectric power. Instead, discharge into the lake from sewage treatment plants caused eutrophication with blooms of the cyanobacteria, Microcystis aeruginosa . After numerous studies over the past two decades, a restoration program is proceeding. Tertiary sewage treatment to remove phosphorus has reduced loading to the lake by approximately two thirds, but has not lead to significantly lower concentrations of total phosphorus. Biomanipulation of planktivorous fish populations to reduce phytoplankton biomass is being implemented. Microcystis aeruginosa appears less conspicuous and water quality is improving.

Lakes and Reservoirs in Argentine

Problems with water quality in lakes and reservoirs in Argentine have recently increased due to agricultural activities, deforestation, logging, animal production, mining activities, urban run-off, and, particularly, the discharge of untreated sewage. As a result of these activities, many lakes and reservoirs are receiving high quantities of nutrients and are suffering eutrophication. The increasing occurrence of algal blooms is showing this reality.

Algal blooms occur especially in reservoirs and ponds, spread over at least twelve provinces of Argentine. Fifteen aquatic environments were identified at risk of poisoning by toxins of Cyanobacteriae. Eighty percent of these environments are reservoirs, used for drinking water supply and recreational activities. Major impacts on the water quality of the Argentine's reservoirs, in which a water quality monitoring programme has been carried out, are related to at least one of the following issues: eutrophication, dissolved oxygen depletion, microbiological pollution, toxic pollution, aquatic biota impairment, and salinisation.

Mechanical problems at water purification plants, bad taste and odd flavour of drinking water caused by cyanobacterial blooms, were detected and reported in San Roque Reservoir since 1971, and in Paso de Piedras and Cruz de Piedra Reservoirs. Scum cloggs filters in the water purification plants and produces odour and disagreable taste of the drinking water. However, its toxic effect is not sufficiently considered. In Argentine, bioassays and toxin analysis are largely absent, although more than three million people are potentially exposed to Cyanobacteriae toxins.

Algal bloom toxicity was eveluated in Cruz de Piedra and San Roque Reservoirs. In the first one, blooms were tested by mouse bioassay, and no positive signals of toxicity were found. On the other hand, San Roque Reservoir is one of the environments at most risk, due to the density and long duration of the blooms, the number of inhabitants potentially affected, and the positive results of the toxicity tests. The eutrophication of this reservoir is the direct cause of alterations in the qualitative and quantitative composition of the phytoplankton, which is very sensitive to the change. More than 50 algal species were reported in the reservoir in 1948. A later study carried out in 1971 indicated a decrease in algae diversity. The presence of summer algal blooms was also reported. During this period, Microcystis aeruginosa and Anabaena were the main components of the phytoplankton.

The results of a first study, related to the algal toxicity in the reservoir, pointed out that two out of the ten most striking blooms, occurring during 1989 to 1993, showed certain toxicity determined by the mouse bioassay. However, no toxicity was found in drinking water samples although the presence of the toxin could be determined by analysis of the water.

Due to current and permanent presence of Microcystis blooms over the year (Figure 1.9.) and the use of the reservoir by the local population, seasonal variation of the heptapeptide Microcystin was evaluated. The results of the evaluation indicated that water blooms are toxic during the four seasons. High values were observed even in winter when the weather conditions are not favourable for algal growth. Although high concentrations of Microcystin were detected in the lake water, low concentrations (<1 :g/l) were observed in drinking water. The maximum amount of the toxins was detected at the entrance of the main tributary, the San Antonio River, which represents the sampling station with most polluted water, as indicated by other water quality parameters.

The above indicates that the San Rogue Reservoir requires a particular attention because of the permanent density and toxicity of the algal blooms. To obtain a better quality of the drinking water, the water purification plant is presently testing a new pilot plant with activated carbon and ozonization process. However, no preventive measures have been taken in San Rogue watershed. The main pollution source is the untreated sewage discharges; however, no sewage treatment plant has yet been constructed.

Figure 1.9. Algal bloom in San Roque Reservoir, Argentine.


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