Phytoplankton blooms
| For more information about chlorophyll 'a' see section 4.4 - Parameter 9 |
Eutrophication (nutrient enrichment) can cause the excessive growth of aquatic plants. Their growth is stimulated by increased levels of nitrogen, phosphorous and other nutrients. Problems from increased growth are many. First, the increased growth of macroalgae may lead to clogging or choking of a waterway. This makes it unusable for recreation such as swimming and boating, and also harms the natural workings of the waterway. Second, the increased growth of phytoplankton can lead to an algal bloom.
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The following phytoplankton are the major types in Western Australian waterways.
The following information is supplied by the Australian Water Resources Council, the National Health and Medical Research Council and the Agricultural Council of Australia and New Zealand.
Blue-green algae are the natural inhabitants of many inland waterways and marine waters. Sometimes these algae may release toxins into the water which can kill animals and cause illness in human beings. They may also release compounds which give musty or earthy tastes and odours to the water. In still water such as ponds, channels and reservoirs, these algae may multiply sufficiently in summer to discolour the water so that it appears green, blue-green or greenish brown.
During calm weather the blue-green algae can rise to the water's surface to form a scum which may look like paint or jelly, and may form flocs (soft, wool-like tufts). The scum can be blown around the surface and appear on different parts of the waterbody at different times depending on wind direction and strength.
Blue-green algal blooms have been a regular occurrence in many Western Australian water-ways for some years. Affected waterways include urban wetlands, poorly flushed south-western estuaries, river pools, farm dams and the coast. Over 35 species of blue-green algae have been found in the Peel-Harvey estuarine system.
Dinoflagellates are single-celled organisms that photosynthesise. Most of them have two flagella (thin, thread-like tentacles used for feeding and moving). Most dinoflagellates are marine, where they are often abundant in the plankton, but some occur in fresh water. They are often found in wetlands, especially estuaries, where they sometimes form toxic red `tides' (they create a thin `wash' through the water) associated with other algal blooms. Red tides are extremely harmful to animals and plants. Toxic dinoflagellates are stored in the bodies of shellfish (molluscs), which strain them out of the water. The molluscs themselves are not killed, but they become poisonous to humans and animals that eat them.
| For more about indicator species see section 2.2 - Environmental indocator species |
Diatom blooms are common in the Swan River Estuary.
| For more about deoxygenation see section 4.4 - Parameter 4 |
During a bloom, the amount of algae can become so big that they take up all the dissolved oxygen. In these circumstances, fish can die of asphyxiation and plants may die through lack of nutrients. The death of fish on a large scale is known as a fish kill. Dead fish and vegetation then tend to wash up on the shores, where they decompose and produce unpleasant odours.
When a waterway is eutrophied (there are too many nutrients in the water), the numbers and types of plants and animals that live there changes. Similarly, when an algal bloom occurs some plants and animals leave or die, and others thrive. Some invertebrates seem to thrive in the presence of a decaying algal bloom. For example, midge larvae eat living and dead algal cells, and an algal blooms provides them with abundant food. However, algal poisoning is usually caused by algal bloom, and that harms a lot more animals and plants than it helps. Algal poisoning is usually caused by cyanobacteria (blue-green algae) and dinoflagellates that produce toxins that affect animals and plants. Cyanobacteria are mainly responsible for poisoning in fresh water. Dinoflagellate poisoning is common in marine and estuarine environments. Some strains of algal blooms produce toxins that kill birds and mammals. Algal toxins can result in stock deaths when they occur in farm dams. Other species of algae, fish and invertebrates can also be affected, usually harmfully, and this changes the structure of wetland communities, reducing the numbers of certain species and allowing others to thrive.
Waterways are good to look at, but most people don't find them pretty when an algal bloom is happening. The water is discoloured and turbid. Poor water clarity is less attractive and less safe for swimming and diving.
Phytoplankton are important because they make oxygen and are eaten by filter-feeding shellfish and the larvae of commercially important crustaceans and finfish. The spread of microscopic algae can help aquaculture and wild-fisheries because commercially important fish eat them and grow. When phytoplankton grows a lot and spreads too far, someimes this can cause severe losses to aquaculture, fisheries, native birdlife and tourism. Humans can become ill and even die if they are affected by particular phytoplankton blooms.
There has been a worldwide increase in problems associated with harmful phytoplankton, toxic algae and red tides. People have fallen ill who have eaten contaminated wild and farmed fish and shellfish, or had direct contact with waters contaminated by toxic phytoplankton blooms. In Western Australia the number of intense red tides (dinoflagellate) and blue-green (cyanobacterial) blooms has increased. Recently, species of these organisms that are very toxic to humans have also appeared in Western Australia. Health authorities have been posting more and more health warnings at recreation areas affected by blooms and red tides.
Direct contact with algae-infested water can be harmful. Symptoms such as skin rashes, eye irritation, vomiting, diarrhoea, fever, muscle weakness or cramps and pains in muscles and joints have occurred in people who have swallowed or swam through algal scum. There have been no reports of long-term effects or deaths in humans, but in some cases people had to go to hospital for treatment. Algal scum or visibly discoloured water may be toxic to livestock and domestic animals. But algae-affected water can usually still be used for irrigation.