Occurrence of blooms of the cyanobacterium Cylindrospermopsis raciborskii (Woloszynska) Seenayya and Subba Raju in a north Queensland domestic water supply

This paper describes seasonally recurring blooms of the potentially toxic cyanobacterium Cylindrospermopsis raciborskii in relation to some limnological characteristics of Lake Julius, a large man-made water impoundment in Australia's semi-arid tropics. These blooms have occurred each year since 1991, with subsurface concentrations of >50 000 cells mL–1. Periods of greater cyanobacterial abundance are characterized by reduced rates of vertical mixing of the water column, reduced mixed:euphotic depth ratios and high epilimnetic temperatures (>25°C). Surface scums were not observed and, in general, this species displays a fairly uniform distribution throughout the euphotic zone and below. An isolate of C. raciborskii taken from Lake Julius during a bloom in November 1995 and grown in pure culture produced no symptoms of poisoning when tested by mouse bioassay, and absence of detectable concentrations of the hepatotoxin cylindrospermopsin was confirmed by HPLC/MS-MS. Low concentrations of cylindrospermopsin (~1–2 g L–1) were detected in the lake during blooms of C. raciborskii