Many millions of tiny plants and animals swarm in the world’s oceans, lakes and rivers – this is plankton, and without it life as we know it could not exist. Sudden blooms of plankton are beneficial and necessary, but sometimes they may poison the water, harming the animals and humans who use it.
Full story by Maggy Wassilieff
Main image: Fossilised radiolaria
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What is plankton?
Plankton is the name given to the millions of organisms floating freely in water. They are mostly microscopic, but they can include larger species such as shrimp and jellyfish. There are three main types:
- plant plankton, also called phytoplankton
- animal plankton, or zooplankton
- bacteria, which usually feed on dead organisms.
Plant plankton are important because they release oxygen into the air, regulate carbon dioxide levels in the air and water, and are food for many sea creatures.
There are three important types of plant plankton:
- Diatoms are single cells enclosed in a glassy shell.
- Dinoflagellates move with the aid of whip-like attachments (flagella). There are 230 species in the waters around New Zealand.
- Desmids live in fresh water and are related to green seaweeds.
Many animals in the plankton eat plant plankton, and are in turn eaten by larger fish and sea birds. The most plentiful are called copepods, which look like tiny shrimps and are the size of a pinhead.
When plant plankton grow quickly and reach great numbers, they may become poisonous to other animals in the water. This is called a toxic bloom. Humans can become ill if they eat poisoned seafood.
Most plankton blooms are natural events. Some occur in cold sea water when strong winds stir up the nutrients. Blooms also happen when sewage or fertilisers enter fresh water and increase the level of nutrients.