Dr Frank Gleason
The recent and massive death of fish in the Murray-Darling Basin caused by harmful algal blooms is an environmental catastrophe.
During the last few years there has been severe drought in the Murray-Darling Basin. The water has disappeared from the large parts of the river system because of lack of rain fall and the use of water for irrigation. In the lower parts of the basin large numbers of fish have perished.
The state government reports that the primary cause of fish death is drought. However during the lifetime of some of the larger and older fish, there have been other major droughts and the fish survived. When rivers dry up, the concentration of nutrients increases and often cyanobacteria grow in these ecosystems. Cyanobacteria can be digested by parasitic bacteria and fungi and the organic carbon produced can be added to the river water. This addition allows more fungi and bacteria to grow in that river. Some of the species of bacteria and fungi produce toxins, which are capable of killing fish.
Bacteria and fungi use oxygen when they grow in water, which results in increased biological oxygen demand. The oxygen which fish need for survival therefore disappears. The loss of oxygen will certainly cause fish to die, but another reason for cause of death can be the presence of toxins in the water.
Inorganic nutrients which are required for the growth of cyanobacteria can come from disturbed soils in agricultural and forested ecosystems, but it can also come from sewage released by municipalities and industrial effluents. Organic nutrients can also originate from these same sources. The primary cause of fish death is eutrophication. The secondary cause of fish death is anaerobic conditions. The growth of large number of microorganisms results from the decomposition of tissues of dead fish. These microorganisms remove the oxygen from the water. Also these microorganisms produce toxins, which are accumulated in the food chain. Toxins can be absorbed by both plant and animal tissues and contaminate human food supplies.
The drought simply accelerates the process. The fish will survive if inorganic and organic substrates are removed from the water and prevented from entering the river systems.