Zooplankton include many different types and sizes of organism – from single-celled protozoa to larger crustaceans such as krill. Zooplankton support all marine ecosystems by supplying the energy from primary production (where phytoplankton use sunlight to grow and reproduce) to fish, marine birds and mammals.

Because zooplankton are not normally harvested by people, have limited control over their movement and have short generation times, they respond to changes in their environment in rapid and unambiguous ways. This makes zooplankton useful indicators of ecosystem change.

Climate variation affects zooplankton in many ways. For example, because they are cold-blooded organisms their metabolism is affected by temperature. Warmer waters can decrease their life-cycle duration, altering the timing of annual seasonal abundance peaks. This in turn affects their predators. Another example can be seen in ocean chemistry, where shelled zooplankton such as pelagic marine snails may be negatively affected by ocean acidification. Different species of zooplankton have different optimal conditions for growth and reproduction, and different tolerances. As local conditions change so too will the success of each species, resulting in locally variable zooplankton communities and shifting distributions. Zooplankton predators may need to move to find their preferred food source, or if they are unable to move may need to switch to a less suitable prey. If numbers of one or more zooplankton groups fluctuate this will change ecosystem interactions.

This report is based on zooplankton sampled by Continuous Plankton Recorders (CPRs). The CPR is a device towed behind ships (usually commercial vessels but also research ships) that filters plankton from the water along the ship’s path onto a length of mesh. After sampling, the mesh is divided into 18.5km samples, and the plankton are identified with a microscope and counted.

Several CPR surveys have been conducted around the globe. Though they have used a consistent methodology, these surveys have varied in duration from multiple decades to just a few years. Although there are still many places not sampled, this approach affords the greatest open ocean coverage of the status and trends of zooplankton.

Within each region we have divided the data into sub-regions according to hydro-meteorological factors, such as shelf versus deep ocean, or prominent fronts or gyres. Coverage is naturally limited in each case to where the ships have travelled and data may not be available for the whole of that sub-region.

Map showing most of the historic Continuous Plankton Recorder sampling lines and the start year of each regional survey

Each line shows where a ship towed a CPR (many are overlapping), with the colours reflecting the different regional surveys responsible for that sampling. The start year of each regional survey is also shown.

The variables included here are based on the groups best sampled by the CPR and help to understand the quantity and composition of zooplankton. The variable “Mesozooplankton Abundance” records the abundance of zooplankton in the size range most frequently sampled by the CPR. This includes organisms from about 200 µm to a few mm in length, but excludes gelatinous plankton as these organisms are not well preserved by the sampler. The variable “Average Copepod Community Size” records the community size of copepods, arguably the most important and numerous group of crustacean zooplankton. As most copepods have been identified to the species level, this allows changes in community composition to be detected.