Skip to content
Jump to


Dr Philip Reid.
Sir Alister Hardy Foundation for Ocean Science.

Dr Richard Kirby.
University of Plymouth.

Plankton are small free-floating organisms, both plants and animals that form the base of the food web in the sea. The amount of plankton in the oceans, both in terms of where and when they are most abundant, is a major influence on the size of important commercial fish populations such as cod. Even more importantly perhaps, through their varying abundance, timing and distribution plankton can act, rather like the miner's canary in a cage, as an indicator of environmental change - change that is already apparent.

'The Continuous Plankton Recorder (CPR) survey was initiated in 1931 by Sir Alister Hardy to assess the state of the plankton in our oceans', says Philip Reid. 'Since then the survey has, with the exception of the Second World War, operated every month of the year in the North Atlantic, and more recently in the North Pacific, using volunteer merchant ships. These vessels tow a 1 metre long device that samples the plankton on a moving band of silk. Wound onto a reel like a camera film, 6 metres of silk equals 500 nautical miles of tow.'

Back at the Sir Alister Hardy Foundation for Ocean Science (SAHFOS) in Plymouth the lines of silk are analysed using a microscope with up to 450 phyto (plant) and zoo (animal) plankton counted and identified on each 10 nautical mile sample. The SAHFOS archive consists of more than 50 years of uninterrupted monthly sampling and provides important information on changes in the abundance and distribution of a wide range of planktonic organisms during that time.

The climate of the world is changing rapidly due to increased levels of greenhouse gases such as CO2. Plankton regulate the movement of CO2 between the atmosphere and ocean via photosynthesis and sedimentation to the deep-sea carbon reservoir. Without this mechanism the world would be a great deal warmer. In their watery environment plankton also effectively record the forces affecting their growth such as temperature and nutrient levels, providing useful indicators of the changing marine environment.

The ecosystems of the North Atlantic have shown a stepwise change around 1987 with warmer water plankton extending northwards by 10° of latitude in the northeastern Atlantic over the last 40 years. This suggests that marine ecosystems are already responding to global warming.