The word plankton comes from the Greek ‘planktos’, which means wandering or drifting. It aptly describes the millions of free-floating organisms living in the ocean and other aquatic ecosystems.
Many kinds of organisms make up plankton; some spend their entire life drifting in the upper ocean, others are members of the plankton community for a time before they develop into stationary or free-swimming adults.
There are three main types of plankton:
Most plankton can only be seen with the aid of a powerful microscope, but some larger animals such as shrimps and jellyfish are also classified as plankton by virtue of their drifting lifestyle.
Although some plankton are capable of slight movement, usually up and down the water column, most are too small and weak to do anything but be carried passively along by the current.
Plant plankton or phytoplankton perform three main functions, crucial to life on earth:
The most well known phytoplankton are microscopic algae. The distribution and quantity of phytoplankton depends on light penetration, the stability of water layers and the availability of nutrients. Around New Zealand there is usually a spring-time bloom of phytoplankton algae in surface waters. At this time, surface temperatures rise, sunlight hours increase and nutrients become abundant following winter cooling and the stirring action of storms. Phytoplankton grow and reproduce rapidly, doubling their population each day and sometimes reaching nuisance proportions. Increased growth may raise toxicity levels and deplete the water of oxygen. However, phytoplankton usually exhaust their nutrient supply before this happens. Phytoplankton numbers are kept in check by grazing zooplankton.
The three most important types of phytoplankton are:
Animal plankton or zooplankton are the link between plant plankton (phytoplankton, the food producers) and the larger animals of the sea. In the deadly contest for survival in the ocean, size matters. Single-celled zooplankton graze on phytoplankton or ingest each other if they get the chance. They are eaten by larger, many-celled zooplankton such as jellyfish, crustaceans and arrow worms, which are in turn eaten by fish, squid, marine mammals and sea birds.
Foraminifera and radiolaria are predatory, single-celled creatures with shells made of calcium carbonate (foraminifera) and silica (radiolaria). They come in a variety of shapes and sizes, and may form colonies. About 1,000 species of foraminifera and 150 species of radiolaria have been found in New Zealand waters. They feed by extending sticky parts of their body through pores in their shells, engulfing passing victims. These predators are important to the study of fossils, as their shells are often found preserved in ocean sediments.
Vast areas of the seabed around New Zealand consist of plankton ooze, sometimes hundreds of metres thick. It is made from the tiny lime skeletons of foraminifera (calcareous ooze) or the glass skeletons of diatoms and radiolarians (siliceous ooze). It takes about 100 years for a millimetre of ooze to form.
Copepods, the tiny relatives of crabs and crayfish, are the most abundant animals in the plankton. They are important food for young fish, especially hoki (Macruronus novaezelandiae). The adults look like miniature shrimps, and most are about the size of a pinhead. They have a complicated life cycle involving six larval and five juvenile moults; there is little similarity between early larval forms and the adult.
The plant-eating copepods are very energetic, and each day they need to consume their own weight in phytoplankton. Using comb-like feeding organs to sweep food into its mouth, one copepod can eat 130,000 cells of phytoplankton in a day. Bacteria digest copepod faeces, and by doing so, release nutrients back into the water that help sustain the phytoplankton.
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.
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.
Batson, Peter B. Deep New Zealand: blue water, black abyss. Christchurch: Canterbury University Press, 2003.
Breidahl, Harry. Diminutive drifters: microscopic aquatic life. South Yarra: Macmillan, 2001.
Cassie, V. Microalgae: microscopic marvels. Hamilton: Riverside, 1996.
Hall, Julie. The iron hypothesis. Alpha series 106. Wellington: Royal Society of New Zealand, 2000.