MARÍLIA CUNHA LIGNONCananeia mangrove swamp: vegetation is thicker far away from Valo GrandeMARÍLIA CUNHA LIGNON
The town of Iguape, on the extreme south of the São Paulo state coast, bears witness to the fact that one cannot always control the impact of small interventions upon the environment. Rooted in the longest continuous and well-preserved stretch of the Mata Atlantica rainforest in the state, after 1827, a canal only four kilometers long appeared at Iguape. It was built to shorten the path of the rice grown along the banks of the Ribeira de Iguage river to the town’s port, from where this rice was carried to other parts of the country. Over the last almost 200 years, the town has also monitored the environmental changes, now revealed by researchers from the Institute of Oceanography at the University of São Paulo (IO-USP), that this shortcut, between the river and the channel separating Iguape from the Ilha Comprida Island, created in the region.
When it was opened in 1855, the Valo do Rocio canal was only four meters wide and two meters deep. Very soon, however, the deflected waters of the Ribeira de Iguape and the traffic of canoes and boats deepened the canal and eroded its banks. Now known as Valo Grande, the canal is as much as seven meters deep and almost 300 meters wide at some points. It spills into the so-called Mar Pequeno, the sea channel between Iguape and Ilha Comprida, almost 70% of the waters of the Ribeira de Iguape, which previously only reached the Atlantic some 40 kilometers further north, where the river runs into the ocean. All this fresh water has changed the physical, chemical and biological characteristics of Mar Pequeno, which is part of a group of natural lagoons, estuaries, bays, islands and channels that form the lagoon and estuary complex of Iguape-Cananeia-Paranaguá, one of the South Atlantic’s most important nurseries of fish and crustaceans.
“When the canal was opened, environmental legislation didn’t exist and the level of awareness was very different,” comments the geologist Michel Michaelovitch de Mahiques, the IO-USP director and coordinator of the studies conducted at Iguape. “It is undeniable that Valo Grande gave rise to many of the environmental changes identified in the region.”
Some three years ago, Mahiques’ team collected samples of sediments from 14 spots in the Mar Pequeno. With drilling equipment, the researchers extracted columns of mud that went as far as two meters deep. This material deposited at the bottom of rivers and seas contains organic and inorganic indicators that allow one to estimate past environmental conditions. The deeper the layer, the older the information stored.
Analyzing the sediment, they found that the salt content of the Mar Pequeno water tapered off considerably after Valo Grande was opened, so much so that in many spots the current salt content is zero: in the town of Cananeia, which lies 60 kilometers south of the canal, the water is practically fresh during periods of heavy rainfall. Since then, the type of sediment that reaches Mar Pequeno has also changed. The waters of the Ribeira de Iguape carries finer grains and more organic material, explain the researchers in an article published in late 2009 in the Brazilian Journal of Oceanography.
These changes affected the benthon, the organisms that live at the bottom of rivers and seas. For example, the diversity of calcareous foraminifers, unicellular beings that are sensitive to changes in the salt content, dropped off substantially and even disappeared at certain times. The variety of species has increased recently, but it has a different make-up: what prevails today are fresh water species.
What occurs at the bottom of Mar Pequeno also seems to influence life along its banks. In the last decade, the biologist Marília Cunha Lignon, who used to be on another team at the Institute of Oceanography and who now works at Inpe, the National Space Research Institute, has been tracking transformations in the region’s vegetation and landscape. She found that 80 kilometers south of Valo Grande the typical mangrove swamp trees, such as the Laguncularia racemosa, the Rhizophora mangle, and the Avicennia schaueriana, form well-conserved groves. However, near Valo Grande, where water salinity is lower, fresh water vegetation proliferates and may hinder mangrove plants from taking root. “The growth of mangroves seems to be different in the two regions,” says Marilia.
Besides changes in the water’s salt content and in the organic composition of the sediments of Mar Pequeno, Mahiques and his team also found a worrisome chemical change: high levels of heavy metals, in particular of lead. Toxic, polluting and hard to degrade, lead can enter the marine food chain, build up in the organisms of species that have high commercial value, such as the robalo, pescada and manjuba fish, prawns, oysters and mussels, and reach people, damaging the central nervous system.
The sediment extracted from the vicinity of Valo Grande had a lead content as much as 20 times higher than it did prior to the opening of the canal; 20 kilometers away from the canal the level of lead in the sediment is lower, but it is still five times higher than before. The concentration of lead has also changed over time. The levels of this metal became higher in the second half of the twentieth century, when the Plumbum mining company was active. The firm operated from 1945 to 1995 in Adrianópolis, in the state of Paraná. “Residues of this metal reached the Ribeira de Iguape river and were carried into the lagoon, which they penetrated via Valo Grande,” states Mahiques. With the shutting down of Plumbum, the sediment’s lead content dropped off somewhat, but is yet to return to its pre-industrial level. At present, the lead content is five times higher than what one would expect in the region.
It is an established fact that many people who lived near Plumbum have a high lead concentration in their bodies. In 2003, a team coordinated by Bernardino Figueiredo and Eduardo de Capitani, from the State University of Campinas, tested blood samples taken from 335 children aged 7 to 14, who lived in two districts on the outskirts of Adrianópolis: in about 60% of the cases, the levels of lead were higher than that considered safe for health. Even so, Mahiques plans to check whether the plants and animals in the Iguape and Cananeia region did not absorb part of the lead that is still in the Mar Pequeno sediment, which would increase the risk of human contamination. “It could be,” says Mahiques, “that we not only have an environmental and geological problem, but a public health one as well.”
The project
The geological record of anthropic activity in the estuary and lagoon system of Cananeia-Iguape (nº 2006/04344-2); Type Regular Regular Research Awards; Coordinator Michel Michaelovitch de Mahiques – IO-USP; Investment R$ 129,948.01.
Scientific article
MAHIQUES, M. M. et al. Anthropogenic influences in a lagoonal environment: a multiproxy approach at the Valo Grande mouth, Cananeia-Iguape system (SE Brazil). Brazilian Journal of Oceanography. v. 57, p. 325-37. Oct/Dec 2009.
