Zooplankton biomass in the Barents Sea

Closeup of a living plankton.
Photo: Tor Ivan Karlsen / Norwegian Polar Institute

Zooplankton are small, simple organisms that follow ocean currents and essentially have their entire life cycle in the free water masses. Of these, small crustaceans such as the Calanus finmarchicus are amongst the most important. Zooplankton provides the nutritional basis for a number of plankton-eating fish, fish larvae and fry. Zooplankton monitoring contributes to increasing the understanding of changes in the populations of fish, marine mammals, seabirds and benthic communities, as well as how the ecosystem is affected by changes in the marine climate.

What is being monitored?

Average biomass in the Barents Sea

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It appears as if the amount of zooplankton has been relatively stable the past approx. 15 years, despite some mid-year variation. Small changes in the numbers still represent clear changes which may have major impact on the populations that graze on the zooplankton.

Status and trend

The Institute of Marine Research measures the zooplankton biomass in the Barents sea during its annual ecosystem cruise which takes place in August-September.  During the cruise, zooplankton is collected from close to the bottom to the surface using nets with a mesh size of 180 µm. In 2019, the average zooplankton biomass for the Norwegian part of the Barents Sea was 8.0 g dry weight/m2. This was somewhat higher than the 2018 value of 7.2 g dry weight/m2, and was also above the long-term mean for 1988-2018 (7.0 g dry weight/m2). These values represent the total biomass for the entire water column, from the bottom to the surface.

During sampling, the biomass is divided into different size fractions. For the fraction >2000 µm, the average biomass in 2019 was 1.0 g dry weight/m2. This is higher than the previous years, but still slightly lower than the long-term mean for 1988-2018, which is 1.1 g dry weight/m2. Since 2007, the biomass for the largest size fraction has been lower than the long-term average. For the middle fraction of 1000-2000 µm, the biomass in 2019 (at 4.8 g dry weight/m2) was clearly higher than the long-term mean (3.4 g dry weight/m2). The biomass for the smallest size fraction of 180-1000 µm was 2.3 g dry weight/m2 in 2018, thus slightly below the long-term average (2.5 g dry weight/m2).

In August and September, when the Institute of Marine Research is on its cruise and taking measurements, the bulk of the mid-sized zooplankton is migrateing  down to deeper waters in large parts of the Barents Sea, while there are relatively large quantity of small plankton forms still remaining higher in the water column.

The small plankton organisms that cannot be caught in standard nets with a 180 µm mesh will probably occasionally be abundant, but they have far less importance for the standing biomass which is measured.

Characteristic for the distribution of zooplankton biomass in 2019 were high biomasses north and east of Svalbard, and larger areas south of around 75 ° N. Another characteristic feature in 2019 was the partly very low zooplankton biomasses in central areas south and southeast of Svalbard.

Improved data and knowledge about the areas where Arctic and Atlantic water masses meet in the north, as well as on the shelf north of Svalbard, “eggakanten” and the slope towards the Arctic Ocean, are important for understanding the physical processes that take place and what consequences these have for the ecosystem as a whole. This is also important because a warmer sea climate can have a greater impact on production conditions in the sea in these northeastern areas, than areas further south.

Causal factors

Several factors influence the production of zooplankton and the total biomass of zooplankton in the Barents Sea:

  • The inflow of zooplankton from the Norwegian Sea and local production.
  • Temperature influences growth and survival of the various species, and their stages in the lifecycle.
  • There appears to be a close connection, almost an inverse relationship, between the amounts of capelin and zooplankton. When the capelin stock was small in 1994 and 1995, there was a significant peak in the zooplankton biomass. In recent years, this link has been less clear.
  • The Barents Sea is a growth area for several fish species that occasionally  graze intensively on zooplankton. Important examples include juvenile herring and fry of capelin, cod, haddock, pollack and redfish.

The following indicators affect zooplankton levels:

  • Temperature and nutrients
  • Inflow and transport of Atlantic water
  • Time for spring bloom of phytoplankton
  • Phytoplankton quantity and species composition
  • Herring
  • Blue whiting
  • Capelin
  • Seabirds and marine mammals

 

 

Consequences

The autumn Zooplankton biomass appears to have been quite stable over the last 20 years or so, despite some variation between years. Small changes in the data may however have major impact on the stocks that feed on the zooplankton.

Although the average total zooplankton biomass in 2019 was only slightly lower than the long-term mean for 1988-2018, the biomass for the largest size fraction (>2000 mm) remained at a low level, as in previous years.

Years with large capelin stocks represent considerable grazing pressure on the zooplankton. Major changes in the size of the capelin stock must therefore be expected to contribute to changes in zooplankton biomass and geographical distribution patterns.

About the monitoring

The zooplankton biomass indicator in the Barents Sea provides an assessment of the available nutrients for plankton-eating fish species. It also provides a general picture of the result of many factors that affect production in the sea each year and suggests the starting conditions for production the following season. However, it is only an indirect indicator of production, and must be interpreted together with knowledge of the occurrence of plankton-eating fish, fry and jellyfish in the Barents Sea.

The indicator is based on average values calculated from the zooplankton biomass measured in August and September each year. Zooplankton biomass is a term used for the average weight of all zooplankton over a certain size (180 µm) in a given area.

Places and areas

The Institute of Marine Research has regularly monitored zooplankton in the Barents Sea since 1986 using a network of stations throughout the Barents Sea, including stations around the whole of Svalbard from 2009. The Barents Sea network has varied somewhat from year to year. The stations have been chosen to give an aggregated measurement of the zooplankton biomass in the Barents Sea.

Relations to other monitoring

Monitoring programme
International environmental agreements
Voluntary international cooperation
Related monitoring