An article in the current issue of the Journal of Coastal Research describes water quality experiments by University of Rhode Island oceanographers Diana Stram and Chris Kincaid and US Environmental Protection Agency (EPA) oceanographer Dan Campbell to assess the extent of water quality degradation in this ecologically sensitive area. The project was funded by a URI/EPA cooperative fellowship agreement and by the URI Coastal Resources Center.
The Rio Chone estuary has a history of water quality problems due to the combination of seasonal climate influences and management of natural resources. The estuary is affected primarily by an influx of sediment because of erosion of hillsides and construction of ponds; seasonal climate changes combined with decreased freshwater flow; and shrimp mariculture development resulting in mangrove habitat loss and the flushing of shrimp pond production wastes into the estuary.
The team of scientists characterized the water quality problems in the Rio Chone estuary by modeling the seasonal distribution of biochemical oxygen demand (BOD) and dissolved inorganic nitrogen (DIN) within the water column. The model allows the scientists to simulate which areas of the estuary are at greatest risk for low water quality, as well as which regions are primarily contributing to the pollution of the estuary.
The model was run under varying water conditions, during both wet and dry seasons, and for a variety of pond effluent locations. The development of the shrimp mariculture industry without regard to its environmental impacts has led to increased BOD and DIN, which, in turn, has caused a number of problems in the estuary.
Shrimp mariculture effluents contain high concentrations of BOD, DIN, and salt (from high evaporation). In the dry season, when the estuarine salinity is inverted (with higher salinity at the head than at the mouth) and circulation in the estuary is sluggish, elevated BOD concentrations from the pond effluents are not flushed from the upper estuary leading to areas of extreme oxygen depletion.
Ammonia represents a large proportion of the DIN concentration. Ammonia can impact aquatic life by acute and chronic toxicity as well as ammonia oxidation, which reduces dissolved oxygenlevels. Ammonia in shrimp ponds causes high mortality rates and can also cause low growth rates in shrimp. Low dissolved oxygen concentrations also increase the toxicity of ammonia to shrimp. Thus, a combination of chronic dissolved oxygen problems and an increase in nutrients can lead to serious effects of decreased water quality on shrimp growth. Increased nitrogen can also contribute to the development of toxic phytoplankton blooms such as red tides. Red tides have not occurred in the Rio Chone, but the increased nitrogen loading may allow them to develop.
The average production in shrimp ponds in the Rio Chone has decreased over the last decade. Increased DIN (and therefore increased ammonia) coupled with increased BOD has likely produced this decline.
The conversion of 90% of the area's mangrove forests to shrimp mariculture ponds is of particular importance to water quality issues. Studies have shown that mangroves can be used for removing excess nutrients from coastal waters, and mangrove sediments may also be able to absorb some of the BOD from pond effluent.
Other studies have attempted to link water quality with profitability of shrimp mariculture. In the Rio Chone, the ponds closer to the mouth of the estuary, where there is more well-flushed water than the ponds located in the upper regions of the estuary, enjoy higher growth rates of shrimp.
"Our results suggest that site selection of shrimp ponds as well as restoration plans for mangroves should consider the hydrodynamic regime of the estuary in order to maximize pond production and maintain estuarine water quality," said Stram. "Site selection and the subsequent conversion of mangroves to ponds near the head of the estuary should be avoided in order to sustain water quality and prevent seasonal self-pollution problems in pond production."
The URI Graduate School of Oceanography is one of the country's largest marine science education programs, and one of the world's foremost marine research institutions. Founded in 1961 in Narragansett, RI, GSO serves a community of scientists who are researching the causes of and solutions to such problems as harmful algal blooms, global warming, air and water pollution, oil spills, overfishing, and coastal erosion. GSO is home to the Coastal Institute, the Coastal Resources Center, Rhode Island Sea Grant, the Institute for Archaeological Oceanography, the Pell Marine Science Library, and the National Sea Grant Library.
Journal
Journal of Coastal Research