Zooplankton biomass in the Barents Sea

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

The zooplankton biomass in the Barents Sea has fluctuated over the years. In 2017, the average biomass in the Norwegian sector of the Barents Sea was slightly lower than the long-term mean for 1988-2017.

What is being monitored?

Average biomass in the Barents Sea

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It appears as though the amount of zooplankton has been relatively stable the past ca 15 years, despite some fluctuations between years. Small changes in the numbers still represent clear changes which may be of great importance for the populations which graze upon the zooplankton.

Status and trend

The Institute of Marine Research measures the zooplankton biomass 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 2017, average zooplankton biomass based on Norwegian data from the cruise was 6.4 g dry weight/m2. This was somewhat lower than the 2016 value of 7.7 g dry weight/m2, and also slightly below the long-term mean for 1988-2017 (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 was 0.7 g dry weight/m2. As in previous years, this is clearly lower than the long-term mean for 1988-2017, which is 1.1 g dry weight/m2. For the middle fraction of 1000-2000 µm, the biomass in 2017 (at 3.2 g dry weight/m2) was just slightly lower than the long-term mean (3.4 g dry weight/m2). The biomass for the smallest size fraction of 180-1000 µm was 2.5 g dry weight/m2 in 2017, the same as the long-term mean.

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 starting to migrate to deeper water over much of the Barents Sea, but a relatively large quantity of small plankton forms still remain high 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.

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 zooplankton growth and survival during the different stages of their lifecycle stages.
•    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.
•    The Barents Sea is a nursery habitat for several fish species that sometimes graze intensively on zooplankton. Key examples include young herring and capelin, cod, haddock, pollack and redfish fry.

Consequences

Zooplankton biomass appears to have been quite stable over the last 20 or so years, despite some variation between years. Small changes in the data may however be very significant for the stocks that feeds on the zooplankton.

After zooplankton biomass was mainly in decline after 2006, we have registered rises again since 2013. Although the average total zooplankton biomass in 2017 was only slightly lower than the long-term mean for 1988-2017, the biomass for the largest size fraction (>2000 mm) remained at a low level, as in previous years.

The average capelin stock was approx. 3.8 million tonnes in the years 2008-2013, while the estimates for 2014-2016 were much lower and varied between 1.9 and 0.3 million tonnes. For 2017 however, the capelin stock is estimated to have risen to 2.5 million tonnes. 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 role of zooplankton as the link between the primary production and the higher trophic levels, is a strong incitement for monitoring zooplankton biomass.

Another reason is that a change in the ocean climate affects the lower trophic levels.

Places and areas

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 so as 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