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Algal Bloom

The term algal bloom refers to the rapid increase and extremely high densities of phytoplankton, algae or cyanobacteria in a marine or aquatic environment. These often cause negative impacts to other organisms via production of natural toxins, reduced oxygen levels, or blocking gill tissues in fish, resulting in asphyxiation . These events have been associated with large-scale marine mortality and with various types of shellfish poisonings.

When populations of one phytoplankton species out-compete other co-occurring species in the water, they can dominate the phytoplankton community and are said to "bloom".

Some blooms can easily be seen due to the discoloration of the water which results from the high density of pigmented cells. There doesn’t appear to be an official threshold level, but algae are considered to be blooming at concentrations of hundreds to thousands of cells per milliliter, depending on the severity. Concentrations may reach millions of cells per milliliter and marine events can appear as red water discolourations commonly referred to as "red tides" or other discoloured water, ranging from green, yellow and brownish to an oily or milky appearance, depending on the species of algae.

Algae Sichuan China
In freshwater these events generally occur when there is a combination of suitable growing conditions including:

- Enriched nutrient levels of phosphorus and nitrogen;
- Warmer temperatures of around 25° C creating optimal growth for one type of algae;
- Light conditions favour one strain, e.g. blue green algae will still flourish under low light conditions;
- Low turbidity (or muddiness) of water body means better light penetration;
- Stable water conditions with low flow rate; and
- Thermal stratification of a water body.

Marine algal blooms can result from upwelling of colder nutrient rich water. The NSW Department of Water and Energy state that, “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.”

The Dept also suggests that, “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.”

According to the US NOAA, “algal growth is enhanced when environmental conditions (such as light, temperature, salinity, and nutrient availability) are optimal for growth. Other biological (e.g., grazing by zooplankton) and physical (e.g., transport by ocean currents) factors determine if biomass will accumulate. Some high biomass HABs [Harmful Algal Blooms] have been linked to nutrient over-enrichment of coastal waters. Nutrient over-enrichment from human activities can occur when runoff (e.g., from lawns and farmland), wastewater discharges (e.g., from industry and municipalities), and atmospheric deposition supply nutrients at a rate that "overfeeds" the algae that exist normally in the environment.”

However it is interesting to note that the “red tide” that occurs pretty much every summer along Florida’s Gulf Coast was observed by the Spanish explorers in the 16th century. This would seem to confirm that not all events are caused by human activity.

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