The Continuous Plankton Recorder is a plankton sampling instrument designed to be towed from ships at normal cruising speeds. This allows it to be deployed opportunistically from commercial vessels, enabling sampling of plankton across whole oceans.
The first CPR survey started in 1931 and has monitored plankton in the North Atlantic and North Sea ever since. Now run by the Sir Alister Hardy Foundation for Ocean Science (SAHFOS) in Plymouth, UK, it represents one of the longest running marine biological monitoring programmes in the world. This survey has identified major changes in local marine ecosystems.
How the CPR works
The CPR is generally towed behind a ship at a depth of 10 m. Water passes through a narrow aperture at the front of the CPR body, plankton are filtered onto a slow-moving band of silk and covered by a second silk. The silks and plankton are then spooled into a storage tank containing formalin to preserve the plankton for later identification.
Assessing plankton biomass and diversity from the CPR
On return to the laboratory, the silks are removed from the mechanism and divided into sections representing 10 nautical miles (19 km) of tow. Two different techniques are used needed to assess the plankton content:
- a ‘greenness index’ is used to estimate phytoplankton (photosynthesising plankton) biomass by comparing the greenness of the silk against a standard colour chart. This relates to the level of green staining from the chlorophyll pigments in the phytoplankton and allows fragments and small phytoplankton to be included in biomass estimates.
- microscopic analysis is used to count and identify zooplankton (animal plankton) and phytoplankton to species level where possible.
These data can then be related to environmental data collected during the tows, such as location, water temperature, salinity, and climate data.