An algal bloom has reappeared in the River Derwent in the past fortnight sparking a number of calls from the public to the Environment Division and Tas Ports about a "red tide".
The Environment Division has confirmed the presence of the dinoflagellate Noctilluca scintillans in the River Derwent around the Hobart area.
Reports came from Lindisfarne Bay, Montagu Bay, Kangaroo Bay, Battery Point, and the small beach southern side of Casino.
Similar algal blooms have been reported in the past few years from Falmouth near St Helens to Prion Bay on the south coast. There were 11 notifications in 2002 and 24 in 2003. The blooms are occasionally mistaken as oil spills because of the pinkish-reddish colour and appearance in quite large "slicks".
It is found in temperate, subtropical and tropical coastal waters and accumulations result from a combination of physical processes such as currents and biological processes. It occurs especially in the vicinity of river mouths and after heavy rain.
While some dinoflagellate blooms can cause the release of toxins, there are no known toxins in the dinoflagellates that form this algal bloom.
However, if individuals are concerned they may wish to avoid swimming in areas where the blooms are present.
The bloom may discolour oysters and mussels, but the Director of Public Health has advised that there is no additional risk to humans when consumed.
However, the general rule is that the public should not consume any naturally occurring shellfish because of the possible presence of other pollutants.