Zooplankton biomass in the Barents Sea

Last updated 8 May 2024

Zooplankton are small animals 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. In the Barents Sea, the biomass of zooplankton in autumn has been fairly stable, and in 2023 it was approximately qual to the average for 1997-2022.

Status and trend

In 2023, the average zooplankton biomass for the reference area in the Norwegian part of the Barents Sea – represented by the orange points in the map figure – was 6.5 grams dry weight/m². This was approximately 13.8 % higher than the value in 2022 of 5.7 g dry weight/m², and approximately 0.9 % below the long-term average for the last 25 years (1998–2022), which was 6.6 g dry weight/m²). These values ​​represent the total biomass of the zooplankton in the entire water column.

The biomass is divided into different size fractions during sampling by sieving through sieves with pore sizes of 2, 1, and 0.18 mm, respectively. For the fraction >2 mm, there was an average biomass of approximately 0.9 grams dry weight/m2 in 2023. This is 73% higher than the value for 2022 (approximately 0.5 grams dry weight/m2) but 12% below the long-term average for the years 1998-2022 (approximately 1 gram dry weight/m2). Since 2009, the biomass for the largest size fraction has mostly been lower than the long-term average. For the intermediate fraction 1-2 mm, the biomass in 2023 (2.0 grams dry weight/m2) was higher than in 2022 (1.5 grams dry weight/m2) and approximately 36% lower than the long-term average for 1998-2022 (approximately 3.1 grams dry weight/m2). The biomass for the smallest size fraction (0.18-1.0 mm) was 3.7 grams dry weight/m2 in 2023, which is approximately 2.4% lower than in 2022 (3.8 grams dry weight/m2), and approximately 48% above the long-term average for 1998-2022 (2.5 grams dry weight/m2). The biomass for the smallest fraction has been significantly higher in the last two years – 2022 and 2023 – than in previous years.

In the area west and north of Svalbard, shown as green dots on the map, only data from 2009 is available. The biomass in this area can be relatively high and at the same time the spread is also often significant. In 2014 and 2015, the result shows a relatively high total biomass, at the same time the variation for the different size fractions is significant in those years, and few observations are noted in 2015. We cannot see any clear trends for the period 2009-2021 in the data set for the area west and north of Svalbard.

In August and September, when the Institute of Marine Research is on its cruise and doing measurements in the Barents Sea, most of the mid-sized zooplankton is migrating towards deeper waters, while there are relatively large quantities of small plankton forms still remaining higher in the water column. The small plankton organisms that cannot be caught in standard nets with a 0.18 mm mesh will probably occasionally be numerous, but they have far less significance for the standing biomass that is measured.

Improved data and knowledge about the areas in the north where Arctic and Atlantic water masses meet, 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 whole ecosystem. 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

The following factors can affect the production and amount of zooplankton:

• temperature and nutrients
• phytoplankton quantity and species composition
• the time of the spring bloom of phytoplankton
• transport of zooplankton with inflowing Atlantic waters from the Norwegian Sea.
• predation from fish, seabirds and marine mammals.

There seems to be a close link, almost an inverse relationship, between capelin and zooplankton. When the capelin population was very low in 1994 and 1995 there were a marked peak in the zooplankton abundance. However, in later years this link has been less clear.

The Barents Sea is an important upbringing area for many fish species that in some periods forage intensively on zooplankton. Important examples are young herring, capelin fry, cod fry, haddock fry, saithe fry and redfish fry.

Consequences

In the defined indicator area in the Barents Sea, the amount of zooplankton in autumn has been relatively stable since 1997. In the two most recent years of the time series, in 2022 and 2023 we have observed a marked increase in the biomass for the smallest size fraction. In the very last few years, however, the biomass for the middle size fraction where relatively large copepods are an important component, has been lower than normal. The decrease in the biomass of the intermediate fraction will probably mean a reduced food supply for other animals in the ecosystem, including many fish species.

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 shows the calculated biomass of zooplankton and provides an assessment of the available nutrients for plankton-eating fish species. Zooplankton biomass is a term used for the average weight of all zooplankton above a certain size (0.18 mm) in a given area.

The indicator is based on average values calculated from the zooplankton biomass measured in August and September each year.

Collection and calculations of the data are done by the Institute of Marine Research, who has regularly monitored zooplankton in the Barents Sea since 1986. The monitoring is important for understanding the ecosystem and the fluctuations in fish populations, and it can contribute to better understanding of fluctuations of marine mammal, seabird and benthic communities.

Further reading