The Microcystis cyanobacteria bloom in the Swan River--February 2000

In February 2000 the Swan-Canning estuary in Western Australia experienced a record bloom of the toxic cyanobacteria Microcystis aeruginosa. At its height, concentrations of M. aeruginosa reached integrated water column cell counts of 15,000/ml and formed bright green scums in sheltered bays, where counts of 130 million cells/ml were recorded. Due to public health concerns parts of the river were closed from 10 to 22 February 2000. Two unseasonably large summer rain events in early and late January 2000 created conditions for the bloom. Freshwater runoff, estimated at 270 GL, was enough to fill the Swan-Canning estuary five times over and brought with it high levels of nutrients, mainly nitrogen (> 2.0 mg/L TN) and phosphorus (> 0.15 mg/L TP). A number of methods to reduce bloom accumulations were tried, including an attempt to increase the salinity of the surface water above the critical 10 ppt level for Microcystis; using a bentonite clay and poly-aluminium chloride mixture to flocculate and sink the algae; and sucking up scums using oil spill equipment. Over 900 tonnes of M. aeruginosa were removed and safely disposed using sewage treatment facilities. The bloom collapsed when the freshwater flush subsided and seawater intrusion from the Indian Ocean re-established itself, raising the salinities above the tolerance of Microcystis.