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The importance of zooplankton

Euphausia superba

Euphausia superba is a variety of krill found in Antarctic

Photo Credit: "" CC-BY-NC-ND 2.0, some rights reserved.
Lead author: Sonia Batten

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.

Zooplankton are the link between oceanic primary production and valued marine resources such as fish. Many fish and mammals feed directly on zooplankton and other food chains have intermediate steps

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.

Zooplankton data

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.

Zooplankton are showing unexpected changes in composition and abundance in response to climate variability and subsequent changes in marine water temperature and chemistry

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.


The Northeast Atlantic has the longest record of CPR data, with some sub-regions consistently sampled for over 50 years. Over this period there has been a progressive increase in abundance of warm-water/sub-tropical copepod species (which are typically smaller) in the more temperate areas of the North-East Atlantic and a decline of (typically larger) colder-water species. This is consistent with the general increase in Northern Hemisphere temperatures over this period. However, several of the 41 sub-regions of the Northeast Atlantic have shown an increase over time in Average Copepod Community Size, so that there are also important local effects to consider.
These results are not yet understood but as ultimately all marine ecosystems are underpinned by plankton there are implications for all ecosystems

The Southern Ocean has also experienced substantial warming since the 1930s, which has affected the volume and extent of sea-ice in this region. The CPR record in the East Antarctic region spans more than 20 years, and all of its sub-regions have shown an unexpected increase over time in Mesozooplankton Abundance and in Average Copepod Community Size. The adjacent Ross Sea sub-regions have seen generally higher abundance than the East Antarctic regions, likely a result of measured higher chlorophyll-a levels and assumed higher primary productivity.

The Northeast Pacific has only 14 years, but during this time there has been significant climate variability, with some of the warmest and coldest years since the 1950s. Zooplankton variables in many sub-regions have responded to these changes with generally fewer, but larger, organisms in the cool years. The technical report for this theme gives more detail (ref) and also describes the status of zooplankton in the other sampled regions; the northwest Atlantic, northwest Pacific, the Benguela Current system and Australian waters.


Although fully global data are not available, the time series included here show trends and patterns of variability that indicate the responsiveness of zooplankton to ocean and climate. Given the importance of zooplankton in ecosystem food supply, other organisms will also be affected, either by the need to follow their food supply, or by experiencing local changes in their stocks as the nutritional value of their food source varies. However, our ability to fully document and understand changing zooplankton distributions is hampered by the lack of fully synoptic sampling, a gap that needs to be addressed. This issue is especially relevant to transboundary waters, where sampling intensity may differ either side of boundaries because of national priorities, confounding assessments of species movements and density changes.
Sampling gaps need to be addressed to enable future assessments to better understand the changing distributions and trends