Recovery of a polluted estuarine system: a mesocosm experiment

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Abstract
An ecosystem-level experiment was carried out to determine the rate at which water columns in estuarine areas grossly polluted by sewage, heavy metals and hydrocarbons can recover and the rates at which pollutant concentrations within sediments decrease when the input of pollutants is terminated. Grossly polluted sediment from the Providence River (Rhode Island [USA]), measurably polluted sediment from mid Narragansett Bay, and relatively unpolluted sediment from Rhode Island Sound were placed in mesocosms containing clean flowing sea water. After 5 mo. the behavior of the grossly polluted system was not dramatically different from the other 2 situations and seasonal patterns of phytoplankton, zooplankton and benthic fauna were similar. The grossly polluted system was much more similar to the other 2 treatment situations than to in situ conditions at the Providence River sediment source with regard to nutrients, chlorophyll concentrations and O2 concentrations, suggesting that new pollutants dictate environmental conditions much more strongly than old pollutants trapped in sediments. The experiment predicts rapid recovery for temperate estuarine systems, once pollutant loading is discontinued, although pollutant material would remain detectable in the sediments for a very long period of time.