Summer flow event induces a cyanobacterial bloom in a seasonal Western Australian estuary

Abstract

In January 2000, record rainfall led to the first recorded bloom of Microcystis aeruginosa in the Swan River estuary. A simple model is used to examine the bloom dynamics and the unusual conditions that produced it. Laboratory trials were conducted to determine the response to salinity of M. aeruginosa, while other parameters for the model were obtained from the literature. Growth was found to be optimal at salinities up to 4, and declined to zero at 25. The unseasonable summer rainfall flushed brackish and marine water from the estuary and produced a surface mixed layer with low salinity. The model simulations show that the hydrological conditions, in combination with high concentrations of inorganic nutrients (dissolved inorganic nitrogen >1.2 mg L^–1, filterable reactive phosphorus >0.02 mg L^–1) in river inflows, high water temperature and high daily insolation, promoted rapid phytoplankton growth, favouring dominance by M. aeruginosa. Doubling rates during the bloom were around 0.35 day^–1 and cell counts exceeded 10⁵ cells mL^–1 within three weeks of the inflow event. Although this doubling rate ultimately determined the total bloom biomass, local concentrations were strongly influenced by physical processes that concentrated M. aeruginosa cells both vertically and horizontally, and advected a seed population from the upper estuary into the lower basin.