What causes algal blooms?
What causes algal blooms?
Freshwater algal blooms
Freshwater algal blooms occur when there is a combination of suitable environmental conditions including:
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.
The major sources of phosphorus and nitrogen entering water ways are:
- runoff and erosion from fertilised agricultural areas
- erosion from river banks, river beds, land clearing (deforestation)
- sewage effluent.
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.
Blooms of blue–green algae can also occur when the concentration of nutrients is fairly low, but blooms are more frequent when the concentration of nutrients is high.
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 around 25°C 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. Higher water temperatures in tropical regions may cause blue–green algal blooms to persist throughout the year.
Another factor related to temperature is the stability of the water column. In spring, summer and autumn in deeper lakes, reservoirs and weirpools, the surface water heats while the deeper waters remain cool. This temperature difference creates a density stratification, known as thermal stratification, that imparts considerable stability to the water column and reduces turbulent mixing. A combination of warm water and a stable water column with a lack of mixing is very favourable for blue-green algal growth, provided other growth promoting conditions are also present. Winter blooms have also been shown to occur when stratification, even if weak, can also be present and water column mixing reduced, despite the water being cold.
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.
Even under low light conditions, or in turbid water, blue–green algae have higher growth rates than other algae. This ability to adapt to variable light conditions gives blue–green algae a competitive advantage over other algal species.
The cells of some blue–green algal species contain gas vesicles that can provide positive buoyancy. Changes in buoyancy occur in response to light and nutrient conditions. In deep water with low light availability these cells become positively buoyant as they utilise intracellular metabolites. After floating towards the surface, new metabolites produced by photosynthesis under higher light conditions can make the cells heavier again so that they sink. This mechanism, known as buoyancy regulation, can assist blue-green algae vary their position within a stable water column, and obtain optimal light conditions.
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.
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 changes in the flows of water in our river systems. Water may move more slowly or becomes ponded, which encourages the growth of algae. In some river valleys environmental flows may enable water to be released from 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) result in bottom waters when a water body is stratified, which may lead to increased nutrient release from the sediments.