Long-term phytoplankton monitoring of the Darling River at Burtundy, New South Wales: Incidence and significance of cyanobacterial blooms

Abstract

Changes in the phytoplankton population at Burtundy are described, based on data (1980-92) from the algal, water quality and flow monitoring programmes of the Murray-Darling Basin Commission; cell densities and community composition are discussed in relation to flow, turbidity, temperature and nutrients. Of 102 taxa from seven classes recorded in the 628 phytoplankton samples, only seven taxa occurred in >30% of samples and 55 taxa occurred in <I% of samples. Total cell densities remained below 10 000 cells mL-1 for around 70% of the study period. The population was characterized by the presence of Bacillariophyceae and Chlorophyceae and the irregular development of cyanobacterial blooms. Eight genera of cyanobacteria contributed to 17 population peaks exceeding 10 000 cells mL-1. Ten of these peaks occurred during a low-flow period from April 1985 to June 1988 when turbidity was usually less than 50 NTU, and two were recorded at river temperatures of less than 14°C. Total cell density and community composition had no apparent seasonal pattern but had five distinct phases related to flow and turbidity. Seasonal temperature cycles and availability of nutrients appear to be less important than flow and turbidity in determining the longer-term variations observed in the algal population.