What causes algal blooms
A range of different conditions can lead to freshwater or marine 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.
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.
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 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.
A further 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. Blooms under ice are even possible in some parts of the world.
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 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) result in bottom waters when a water body is stratified, which may lead to increased nutrient release from the sediments.
Marine algal blooms
Marine algal blooms are a common natural phenomena along the NSW east coast and can result from upwelling of colder nutrient rich water. Studies from a long term coastal station off Sydney found that marine phytoplankton blooms appeared to correspond with upwelling/uplifting or slope water intrusions lasting 2 to 22 days and occurring from September to February.
Nutrient fluctuations as a result of anthropogenic changes can also influence the presence of algal blooms and species succession and may even influence when toxins are generated. Marine blooms may threaten fish resources, human health, ecosystem function and recreational amenity of beaches and bays. Marine algal blooms fall into the classes of:
- BACILLARIOPHYCEAE (diatoms)
- DINOPHYCEAE (dinoflagellates)
- PRYMNESIOPHYCEAE (golden–brown flagellates)
- CHRYSOPHYCEAE (golden–brown algae)
- RAPHIDOPHYCEAE (chloromonads)
- DICTOCHOPYCEAE (silicoflagellates)
- CYANOPHYCEAE (marine blue–green algae) (Hallegraef,1991).
Marine algal blooms can appear as red water discolourations commonly referred to as ‘red tides’ or a range of other discoloured water, from green, yellow and brownish to an oily or milky appearance. These algal blooms are commonly mistaken by the public for sewage or some other form of pollution. Other blooms can show no discolouration but be highly toxic at low levels. It is important that samples of marine algae are analysed, as relatively harmless algae and potentially toxic algae cannot be differentiated by the naked eye. However, some nuisance red tide blooms can be differentiated in the field by the trained eye or easily identified by microscopy. This includes Noctiluca scintillans, a dinoflagellate, that is easily identified under a crude microscope due to its size and distinct balloon–like shape.
Similar to freshwater blue–green algae, some estuarine and marine algal species produce irritants that can cause respiratory irritation and severe contact dermatitis. The major route for human exposure is through consumption of seafood and shellfish as some species produce potent toxins that can be accumulated in fish and shellfish. Even low densities of toxic algae may be sufficient to cause illness or death in humans, while some species can selectively kill fish by inhibiting their respiration. Not all potentially toxic algal species are toxic in every situation.
Although the distribution of marine and estuarine algae is uncertain in Australia , the number and intensity of marine algal blooms is believed to be increasing world–wide due to:
- Expansion of aquaculture in coastal areas
- Coastal eutrophication and unusual climatic conditions
- Movement of shellfish stocks and transport of resting cysts in ballast water.
Unlike freshwater algal species that may be present for extended periods and normally occur where water movement is minimal, marine algal occurrence responds to nutrient enrichment, water circulation such as tides and currents, and wind patterns. As such, their occurrence is often short–lived in a particular area and difficult to predict.