The Aratu Bay ecosystem (12°48′S; 38°28′W), located in an industrialized area, was compared with a similar but undisturbed ecosystem, Iguape Bay, in order to determine the impact of industry on water quality during the last forty years. Comparison was based on simple and low cost methodologies using bioassays on oyster embryos, validated by a survey of a benthic activity index.

A bioassay method based on oyster embryo development was used to compare water quality from the study areas. A high percentage (93.2 and 97.3) of abnormal oyster embryo development was observed in water samples from one area (south and southeast) of the bay where, besides receiving heavy discharges of industrial effluents, water circulation is restricted. At the north end of the bay where the area is shallow, and some industrial discharges are present, water quality data indicated that the area was stressed. Little effect of industrial impact has been noted around the Cotegipe channel, a deep area where the water circulation pattern is heavy and the water well mixed.

Validation of this survey has been achieved by the simple method of investigating benthic activity in the area. Quadrat counts of active benthic galleries were used as an index of community activity for comparative analysis between sampling stations and for discriminating between polluted and unpolluted sites. This research has shown that the quadrat counts of active benthic galleries, at least in areas of comparable homogeneous substrate, can be used as a viable index of environmental impact, when interpreted together with other source of data, such as early-life-stage tests. This new approach proved to be effective and led to the recommendation of remedial policies in the area.

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