Research points to a strategy for managing aquatic plants in reservoirs

Uncontrolled proliferation mainly results from the disposal of sewage in river basins. Findings have been published in book form available for free download.

By José Tadeu Arantes  |  Agência FAPESP – Excessive proliferation of aquatic plants is a serious problem in reservoirs. The plants grow uncontrollably and form an extensive barrier of biomass, which hinders reservoir use, blocks sunlight, and sucks up oxygen from the water during decomposition, eventually leading to the death of aquatic animals. Frequent episodes of intense growth have been reported in Brazil.

Marcelo Luiz Martins Pompêo, a professor at the University of São Paulo’s Bioscience Institute (IB-USP), has investigated the phenomenon and has proposed strategies either to prevent excessive plant proliferation, or to remedy it if prevention proves impracticable.

The results are published in the book Monitoramento e manejo de macrófitas aquáticas em reservatórios tropicais brasileiros (“Monitoring and Management of Aquatic Macrophytes in Brazilian Tropical Reservoirs”), which is available as a PDF file that can be downloaded free of charge from ecologia.ib.usp.br/portal/macrofitas. The research was supported by FAPESP and SABESP through FAPESP’s Public Policy Research Program (PPP) and by the National Council for Scientific & Technological Development (CNPq).

“A lot of people still take an unfavorable view of aquatic plants. The news media don’t help educate the general public. The problem isn’t macrophytes but bad reservoir management,” Pompêo told Agência FAPESP.

“If they’re present to an appropriate extent, free-floating or rooted aquatic plants provide a broad diversity of habitats, creating refuges for fish and other animals. In addition, they assimilate nutrients that would otherwise be trapped in sediment, and by excreting compounds or decomposing, they release these nutrients into the aquatic medium, ensuring the necessary supply to ecosystems. The main problem, caused by sewage disposal, arises when large amounts of nitrogen and phosphorus are dumped into the reservoir, leading to the explosive proliferation of macrophytes.”

Excessive amounts of nutrients generally find their way into reservoirs due to inadequate river basin management. 

“Where there’s no adverse human intervention, macrophyte populations tend toward equilibrium because they’re regulated by natural cycles, such as rain and drought, among others. However, where there’s large-scale deforestation and uncontrolled urban growth, with industrial and domestic waste disposal in the vicinity of reservoirs or in river basins, the negative consequences become far more intense and harmful due to the large amounts of nitrogen and phosphorus that enter the reservoirs,” Pompêo said.

“When physical or chemical processes release these nutrients into the water column, it becomes difficult to contain the growth of aquatic plants and phytoplankton, and explosive growth is common. Laboratory studies performed in Brazil have shown that nutrient superabundance can lead to 15% daily growth in the macrophytic biomass.”

With this rapid growth, if control is neglected for three weeks, the reservoir is transformed into a sea of aquatic plants, and it becomes very hard to eliminate them. The superabundance of nutrients combined with a tropical climate in which both sunlight and temperature levels are often high almost always affords excellent conditions for the plants to grow.

When the plants die, the nutrients that they had absorbed return to the liquid environment, fueling a vicious cycle. Moreover, in the most critical situations, oxygen levels can reach zero in a significant proportion of the reservoir during decomposition, leading to the death of many other organisms.

As a preventive strategy, Pompêo’s book proposes the creation of centers responsible for monitoring specific groups of reservoirs in metropolitan São Paulo, such as the Billings-Guarapiranga complex – these two reservoirs are interconnected, so what happens in one affects the other – and the Cantareira system, with five reservoirs linked by canals and tunnels. 

According to Pompêo, these centers would require a certain amount of autonomy, but they would not need sophisticated laboratories. Simple labs, one installed in each center, would enable them to solve minor, immediate monitoring problems. More sophisticated labs with expensive equipment and highly trained operating personnel would also be needed. Their use could be shared by several centers.

“Of course, the centers wouldn’t just monitor macrophytes, as the investment would be too large to justify no more than that,” Pompêo said. “My suggestion is that in addition to macrophytes, they should also monitor zooplankton, phytoplankton, fish populations, and so on, and both water and sediment. Above all, they should study and understand land use in the areas surrounding the river basins concerned, because many of the solutions required to maintain water quality can come from the reservoirs’ environs. In particular, sewerage and wastewater treatment systems are the leading causes of the injection of large amounts of nutrients into reservoirs.”

Guarapiranga reservoir, Pompêo recalled, has been in use for approximately 100 years. “If appropriate action had been taken during this long period, especially in terms of monitoring and management, the reservoir would be quite different now, and it would offer the public a far higher level of water quality. The operating cost would also be much lower,” he said.

Where the explosive growth of macrophytes has not been prevented, what can be done to remedy the situation? “In my view, the best approach is to remove the plants from the reservoir completely,” Pompêo said. “Using chemicals or shredding the plants and leaving them in the water kills them, they decompose, and this process removes even more oxygen from the water. Removing them manually preserves what oxygen remains and eliminates buds that would otherwise resprout, as well as the nutrients that would feed the living plants.”

Complete removal is no easy task, however, he added. In some parts of Brazil, there have been cases where more than 200 trucks’ worth of plants, each bearing five cubic meters, have been removed from a single reservoir.

“That’s why a monitoring and management program is advisable,” he stressed. “If you have systematic monitoring, at the first sign of abnormal growth, you can bring in preventive measures. Prevention is far cheaper than correction.”