MAJOR CYANOBACTERIAL BLOOM IN THE BARWON-DARLING RIVER, AUSTRALIA, IN1991, AND UNDERLYING LIMNOLOGICAL CONDITIONS

The occurrence of a severe cyanobacterial bloom is described. This blo om affected almost 1000 km of the Barwon-Darling River, New South Wale s, Australia, in November and December 1991 and was dominated by Anaba ena circinalis Rabenhorst. This cyanobacterium was present in concentr ations of around half a million cells per millilitre at some localitie s during its peak in mid November. Moderate to very high toxicity was demonstrated by mouse bioassay at many localities during this time. Th e bloom was attributed to very low flow conditions and high nutrient c oncentrations, especially of total phosphorus. However, warm water tem peratures, elevated pH, reduced turbidity, and improved water transpar ency would also have been contributing factors. Very high ammonia conc entrations were also observed during the bloom. The bloom declined dur ing December and was eventually flushed from the river by increased fl ows following heavy catchment rainfall between mid December and early January.