Explanation:

Freshwater algal blooms occur when there is a combination of suitable environmental conditions including:
-Nutrients:
Nutrients encourage the growth of blue-green algae.The process of nutrient enrichment in a waterway is called eutrophication. The main nutrients contributing to eutrophication are phosphorus and nitrogen. Runoff and erosion from fertilised agricultural areas, erosion from river banks, river beds, land clearing (deforestation), and sewage effluent are the major sources of phosphorus and nitrogen entering water ways. Phosphate attaches to sediments. When water is low in dissolved oxygen (anoxic), sediments release phosphate into the water column. This encourages the growth of algae.
-Temperature:
Blue-green algal blooms usually develop during the warmer months of the year or when the water temperature is higher and there is increased light.Temperatures of 25oC are optimal for the growth of blue-green algae. At this temperature, blue-green algae have a competitive advantage over other types of algae. In temperate regions, blue-green algal blooms generally do not persist through the winter months as the low water temperatures are less favourable for growth, although exceptions do occur.
-Light:
Blue-green algae populations are diminished when they are exposed to long periods of high light intensity but have optimal growth when intermittently exposed to high light intensities. This ability to adapt to variable light conditions gives blue-green algae a competitive advantage over other algal species.
-Turbidity:
Turbidity, or muddiness of water, is caused by the presence of suspended sediments and organic matter in the water column. Low turbidity occurs when there is only a small amount of suspended matter present in the water column. Low turbidity can be due to the influence of the surrounding geological environment and/or slow moving water that allows particulate matter to settle out of the water column. When turbidity is low, more light can penetrate through the water column. This creates optimal growth conditions for blue-green algae.
-Stable Conditions:
Blue-green algae prefer stable water conditions with low flows, long retention times, light winds and minimal turbulence. Drought, water extraction for irrigation, human and stock consumption and the regulation of rivers by weirs and dams all contribute to decreased flows of water in our river systems.Water moves more slowly or becomes ponded, which encourages the growth of algae. In some river valleys the environmental flow rules enable water to be released from the storage to control algal growth.Another consequence of stable conditions is thermal stratification of the water body. Thermal stratification occurs when the top layer of the water column becomes warmer and the lower layer remains cooler. When the two layers stop mixing,the upper layer becomes more stable and the growth of blue–green algal blooms is encouraged.Often water with low levels of oxygen (anoxic) resultin bottom waters when a water body is stratified,which may lead to increased nutrient release from the sediments.