A few phytoplankton species are deadly. In suitable conditions they can grow and reproduce in great abundance, creating what is called a toxic bloom. They produce poisons that accumulate in the bodies of filter-feeding shellfish such as oysters, mussels, pipi and cockles. Usually the shellfish remain unaffected, but the fish, shore birds and marine mammals which eat them can be poisoned and die. The poisons cannot be destroyed by cooking, and in humans they can cause four nasty illnesses, which may result in paralysis, respiratory difficulty, memory loss or diarrhoea.
New Zealand has the dubious distinction of being one of the few countries in the world to have recorded four types of toxin, with at least 34 different species responsible. Following a widespread outbreak of neurotoxic shellfish poisoning in 1993, shellfish toxins have been regularly monitored. It is becoming increasingly common for areas of the North Island coast and the Marlborough Sounds to close shellfish gathering during spring and summer. Some toxic blooms are so severe that one medical officer warned that eating shellfish from the east coast of Northland was like playing Russian roulette.
In 1998 there was a massive loss of marine life in Wellington Harbour. Fish, shellfish, starfish, sponges and seaweeds all succumbed. The culprit, a new and deadly dinoflagellate (Karenia brevisulcata), reached concentrations of 30 million cells per litre of sea water. It took two years for life in the harbour to recover.
Toxic blooms can also occur in lakes and rivers. Lake Taupō and the Rotorua lakes have been increasingly plagued by blooms of poisonous cyanobacteria species such as Microcystis and Anabaena. These organisms are common in nutrient-rich lakes, where they form a dense green scum. Cattle that drink the affected water can become sick and die. People who bathe in it may suffer skin rashes and liver damage.
Cause of toxic blooms
Marine toxic blooms are natural events; quite a number have been reported in the seas around New Zealand since the 1860s. Some occur irregularly, like the so-called Nelson slime, a blanket growth of mucilage produced by microflagellates. For the 50-year period up to 2002, most toxic outbreaks were associated with the El Niño weather pattern, where strong summer winds stir up cold nutrient-rich waters, promoting phytoplankton growth.
Toxic blooms in fresh water are associated with raised nitrogen and phosphorus levels from sewage or fertilisers.