Occurrence and possible causes of a severe cyanobacterial bloom in Lake Cargelligo, New South Wales

Although smaller cyanobacterial blooms had occurred in Lake Cargelligo in previous summers, a severe bloom of Anabaena circinalis occurred in the lake in November 1990. Cell numbers exceeded 100 000 cells m L -1 , and toxicity tests revealed the bloom to be highly hepatotoxic. This resulted in the first known closure of a town water supply due to cyanobacteria in New South Wales. Blooms of Microcystis aeruginosa, Aphanizomenon issatschenkoi, Oscillatoria rnougeotii and Cylindrospermopsis raciborskii also occurred in the lake at similar very high cell numbers during the summer and autumn of 1990-91. All five species persisted until May 1991, although there was no detectable toxicity from January onwards. Severe flooding in the Lachlan River valley upstream of Lake Cargelligo during the winter of 1990 led to nutrient enrlched inflows to the lake. These elevated nutrient concentrations would have been a major factor contributing to the bloom. However, other physico-chemical factors were also suitable for cyanobacterial growth.