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Bill Harding (Dept. of Scientific Services, Vlaeberg, South Africa) has sent in an account of a cyanobacterial attack on the vitality of a community and the bold measures that were used to fight back.
During late 1997, a partial collapse of sago pondweed in the Wildevoelvlei wetland, situated on the west coast of South Africa, resulted in a dense bloom of Microcystis aeruginosa. The wetland is a shallow 25 hectare system that is perennially hypertrophic, owing to an influx of between 4 and 7 megaliters of treated wastewater effluent per day.
Shortly after the onset of the bloom, the hepatotoxins microcystin-YR and -LR were detected in freeze-dried algal extracts. Subsequently, the same toxins were detected in the tissue of mussels collected from the reef adjacent to the wetland outflow to the sea. An immediate ban was placed on the collection of shellfish by a community that normally relies on this resource for both food and income.
Bill proposed a vigorous response to the challenge, based on his experience that M. aeruginosa has a low tolerance for salinity, while desirable components of the wetland biota can withstand an elevation of salinity to between 7 and 10 parts per thousand (ppt) for a short period of time.
After lowering the water level of the wetland, 600 tons of course rock salt were added to approximately 50,000 cubic meters of water in two applications spaced seven days apart. Effluent from wastewater treatment was curtailed for the duration of the operation. The salinity rose to 3.5 ppt after the first application and to 8 ppt after the second. There was an immediate decline of the M. aeruginosa bloom, with the concentration of chlorophyll-a falling from 700 to 150 µg/l during the first week.
Thereafter, the cyanobacterium was replaced by a bloom of Kirchneriella, followed by a clear water phase (chlorophyll-a less than 5 µg/liter). At this point, the wetland became dominated by chlorophytes and diatoms, together with the zooplankton Daphnia longispina. Salinity began to decline 14 days after the initiation of the operation, when it became necessary to resume the release of effluent. The ambient salinity of the system had returned to zero after about 50 days. The restrictions on harvesting were lifted at the same time.
The bold increase of salinity on this scale demonstrated the value of this environmentally-sensitive intervention for use in coastal lakes and estuaries where conditions allow. The short-term elevation of the ambient salinity of this previously estuarine system resulted in the rapid and total eradication of the toxin- producing cyanobacterium and the concomitant alleviation of the risks to human and animal health in both the freshwater and marine environments. The application further highlighted the intrinsic value of retaining estuarine character and tidal interaction in coastal lake systems that have been altered through human development.