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


CHAPTER 6. TECHNOLOGICAL AND MANAGERIAL ASPECTS OF EUTROPHICATION

6.11. Restoration Methods

6.11.1. Examples of Remediation of Reservoirs

Restoration of a tropical urban reservoir: Lago Parano< (BrasRlia, Brazil)

Lago Paranoá (4,000 ha, 14 m mean depth) is an urban man-made lake created in 1959 to improve leisure and climate conditions, as well as to receive wastewater and run-off from the new capital of Brazil, Brasilia. In the early 1970s, a significant deterioration of water quality was already evident, and the reservoir became highly eutrophic due to the inflow of inadequately treated domestic sewage from two sewage plants. In 1978, an ecological disaster occurred in the Southern Branch of the reservoir as a severe bloom of Microcystis aeruginosa promoted a massive fish mortality, the decomposition of which widespread an unpleasant odour throughout the city. Eutrophic condition of the Lago Paranoá was also characterised by a permanent bloom of filamentous cyanobacteria (Cylindrospermopsis raciborskii), poorly utilized by dominant, small-bodied zooplankton (i.e., rotifers and small cladocerans), a low Secchi depth (< 50 cm), and high nutrient levels (30 to 80 mg/l total phosphorus, and 1.8 to 2.5 mg/l total nitrogen during the 1980 to 1990s period).

Lago Paranoá Restoration Program started in 1975 through an UNDP Co-operation Program (Project UNDP BRA/75/03 WHO BRA 2341), involving The Brasília Water and Sewage Corporation (CAESB) and the University of Lund (Sweden), with a total cost of US$ 800,000. The main outcomes of the project were: (1) training of CAESB staff by visiting Swedish research scientists; (2) building of a Limnological Laboratory at CAESB for a permanent monitoring program of water quality evolution; and (3) implementation of sewage collection, based on the identification of domestic sewage as the main source of eutrophication, and phosphorus as the limiting factor for excessive phytoplankton growth.

A second UNDP Co-operation Program entitled "Evaluation of Sanitary Aspects of Lago Paranoá and Rio São Bartolomeu Watersheds" was carried out from 1987 to 1991 (Project BRA/87/11/B/01/01). The aim of this project involving a total budget of US$ 1 million, was the training of CAESB technical team in the scientific fields of sanitation and environmental impact.

The most decisive step towards the restoration of Lago Paranoa was taken in 1993 to 1994 by the construction of two new sewage plants capable of processing, at tertiary level, 2400 l/s of sewage produced in the city of Brasilia prior to its release into the reservoir. The so-called PHOREDOX biological process with final chemical polishing is based on the principle of bacterial luxury uptake of phosphorus followed by the transformation of ammonia into gaseous nitrogen through specific microorganisms. The efficiency in terms of N and P removal by such biological process is higher than 90%. In addition, a complementary sewage collecting system composed of 46 km of intercepting sewers, 15 pump stations, 13 km of pressure sewers and 417 m of underwater crossing pipelines was constructed. The overall cost of this new sewage treatment reached US$ 200 million, half of it provided by the Inter-American Development Bank and the other half as Brazilian Government counterpart.

As detected by the Limnological Monitoring Program, conducted by CAESB since 1976 and averaging an annual cost of US$ 3 millions, there has been a gradual reverse in eutrophication process of Lago Paranoá. This followed a drastic reduction in external point-sources total phosphorus loading from 418 kg/day in 1992 to 74 kg/day in 1998. The most important water quality improvements occurred for total phosphorus and chlorophyll-a, as overall mean values dropped respectively from 67 and 90 mg/l in 1992 to 31 and 41 mg/l in 1998. In addition, the use of algicide (i.e., copper sulphate) to control blooms of cyanobacteria was also reduced from 10.2 tons/year in 1992 to 1.7 tons/year in 1998. Reservoir areas suitable for bathing and aquatic sports increased from 50% in 1991 to above 90% in 1998, as monitored by the weekly sampling program of bacterial contamination conducted by CAESB. Some important signs of oligotrophication were observed within the planktonic community level: (1) the re-appearance of Daphnia gessneri in 1995 following three decades of its previous record, (2) short-term dominance of colonial floating green algae Botryococcus braunni replacing Microcystis aeruginosa in some hypertrophic areas of the reservoir in 1996, (3) recent appearance of a typically oligotrophic calanoid copepod species (Notodiaptomus cearensis) throughout the reservoir branches in 1997, and (4) succession in phytoplankton community with the virtual disappearance and replacement of Cylindrospermopsis raciborskii by green algae and diatoms by the end of 1998.

Complementary internal management strategies such as biomanipulation and ecohydrological measures (i.e., reduction of residence time) are currently being adopted to increase the success of the Lago Paranoá Restoration Program. Management of omnivorous fish populations was shown to be capable of bringing further improvements in water quality. In addition, a drastic, intense flushing and lowering of water level at the end of the dry season followed by a filling up with rain water washed out nuisance cyanobacteria and improved transparency to a level as high as 6 meters. In addition, it reduced chlorophyll levels down to 10 mg/l during a 2-months period in 1998.

The case study of Lago Paranoá clearly illustrates that a careful assessment and identification of main sources of eutrophication combined with an efficient long-term limnological monitoring program represent key factors to implement a successful restoration program which directly benefits 2 million inhabitants in the capital of Brazil.

Development of a biomanipulation strategy for the remediation of eutrophication in lakes and reservoirs at low latitudes

Biomanipulation was originally defined as any deliberate intervention on a key component of the food chain aiming at improving water quality (Shapiro et al., 1975). During the past three decades, a considerable number of experimental studies plus whole lake manipulations have demonstrated that reduction in visual-feeding planktivorous fish allows an increase in the abundance of large-bodied herbivorous zooplankton (mainly Daphnia) and their grazing pressure, resulting in a suppression of phytoplankton biomass and an enhancement of water clarity (Lazzaro, 1987; de Bernardi and Giussani, 1995). The principles and applications of such food-web manipulation have developed mainly in temperate regions and only few studies have investigated its applicability to contrasting freshwater ecosystems in the tropics and subtropics (Lazzaro, 1997).

Pioneering experimental studies on the potential for biomanipulating a tropical urban reservoir, Lago Paranoá, Brasília, Brazil, have been carried out during the past 10 years. The combination of laboratory measurements of fish feeding rates on plankton and phosphorus excretion rates, plus the assessment of planktivores impacts in enclosures, limnocorrals and partitioned reservoir areas have demonstrated the following:

(1) The control of exotic omnivorous fish overpopulation (mainly Tilapia rendalli and Oreochromis niloticus) in shallow areas significantly reduces internal phosphorus loading by slowing down recycling from bottom sediment. This results in lower abundance of nuisance cyanobacteria.

(2) The concomitant introduction of a moderate biomass of sterile filter-feeding silver carp (Hypophthalmichthys molitrix) dramatically increases grazing pressure on such nuisance phytoplankton and improves water quality by up to 70%.

The first step towards implementing this promising fisheries management to counteract eutrophication as a complementary measure within Lago Paranoá Restoration Program, was an echo-sounding campaign carried out in 1998. It aimed at estimating overall reservoir fish stock and identification of specific areas with unbalanced increased fish populations. This study revealed a total reservoir fish stock of 1,500 tons, and identified the hypereutrophic Riacho Fundo Branch as an area with a fish abundance three times higher than the average from other parts of the reservoir, tilapia making up to 90% of fish biomass. The excessive fish abundance in this hypereutrophic area, enriched with a thick layer of organic bottom sediment (i.e., up to 3 meters), made it especially susceptible to massive fish kill during winter circulation of water mass. This was observed in 1997 and 1998, when 300 tons of dead fish were removed only from this area.

Taking into account this fragile situation, a pilot experiment of massive fish removal was carried out on May 1999 in the isolated mouth of Riacho Fundo tributary. A total of 20,000 adult fish weighting 4,000 kg (85% tilapias) were removed by 20 professional cast-net fishermen in 6 days. From mark-recapture estimates involving more than 2,000 tagged fish, the stock of commercial size fish in this 78 ha area was estimated to reach 100 tons. These results are currently subsidizing a management plan for this hypereutrophic area in which professional cast-net fisheries will be legalized as a tool to prevent future massive fish kill and simultaneously improve. Controlling tilapia overpopulation will prevent a considerable amount of phosphorus accumulated during four decades of accelerated eutrophication process from being available to primary production. In addition to the ecological advantage of reducing internal phosphorus loading from a permanent control of bottom feeding tilapia overpopulation and a smaller need for copper sulfate application to control cyanobacteria blooms, there will also be important socio-economical benefits from legalizing professional cast-net fisheries for the low-income population living in Brasilia suburbs. The achieve capture of up to 100 kg of marketable tilapia by a single fishermen during few hours of work, could provide a US$ 200 daily income which would increase job offers in the Federal District. Additionally, new job opportunities could also be created by establishing an environment friendly silver carp cage culture on a commercial scale in low landscaping valued hypereutrophic areas of Lago Paranoá.

There is a great potential for applying these alternative biomanipulation strategies to many other tropical and subtropical urban lakes suffering from eutrophication and consequent excessive omnivorous fish populations. However, the control of external nutrient loading to the ecosystem is a pre-requirement to ensure the success of such food web manipulation.

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