What are plankton?
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:
- Plant plankton, also known as phytoplankton – single-celled photosynthetic organisms which manufacture food using energy from sunlight.
- Zooplankton – single- and many-celled animals that feed on live plankton.
- Bacteria – some are photosynthetic, but the majority feed on dead organisms.
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
Plant plankton or phytoplankton perform three main functions, crucial to life on earth:
- They provide nearly half of the earth’s atmospheric oxygen.
- They regulate carbon dioxide levels in the water and atmosphere.
- They are the founding organisms of aquatic food webs.
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:
- Diatoms. These consist of single cells enclosed in silica (glass) cases. Each case is made of two interlocking parts with fine holes, through which nutrients and wastes pass. These photosynthetic organisms live in the ocean, with some species common in fresh waters. About 620 species of marine diatom are known to live around New Zealand. Of the freshwater species, only 2% are unique to the country.
- Dinoflagellates. This name refers to two whip-like attachments (flagella) used for forward movement. They include photosynthetic members as well as predatory species. About 230 marine species have been described in New Zealand. When masses of red-brown dinoflagellates gather in surface waters they create what is known as a red tide.
- Desmids. These freshwater photosynthesisers are closely related to green seaweeds. They resemble little green cylinders or miniature barbells, and are common in lakes and rivers.
