Zooplankton biomass in the Barents Sea

Last updated 20 February 2023

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. The zooplankton biomass in the Barents Sea have since 1997 been fairly stable, however, in 2021, it was approx. 23% lower than the long-term average for 1997-2021.

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. For this indicator we have defined a geographic reference area as shown in the map below.

The orange dots on the map are included in the reference area, which essentially is monitored every year, while the area directly east of Svalbard is not included. This is among other things due to ice conditions in the area making it difficult to get good data coverage. The data set is also constricted to the years from 1997 to get a more even coverage with regards to the amount of observations in the different sub-areas. In addition, we present a separate indicator for the area west and north of Svalbard as of 2009. Here, the observations are shown as green dots on the map. The purpose of the area and year restrictions is to achieve increased consistency and comparability within the time series.

In 2021, the average zooplankton biomass for the defined reference area in the Norwegian part of the Barents Sea (orange dots on the map) was 5.2 g dry weight/m². This was approx. 12% lower than the 2020 value of 5.9 g dry weight/m², and was approx.. 23 % below the long-term mean for the past 25 years (1997-2021), which was 6.7 g dry weight/ m². These values represents the total biomass of zooplankton for the entire water column.

During sampling, the biomass is divided into different size fractions, by using sieves of different sizes, 2 mm, 1 mm and 0.18 mm. For the fraction >2 mm, the average biomass in 2021 was 1.0 g dry weight/m². This is the same as in 2020,, and on the same level as the long-term mean for 1997-2021, which is 1.0 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 1-2 mm, the biomass in 2021 (at 2.1 g dry weight/m2) was very similar to the value for 2020, but approx. 34% lower than the  long-term mean for 1997-2021 (3.2 g dry weight/m²). The biomass for the smallest size fraction of 0.18-1 mm was 2.1 g dry weight/m² in 2021, and approx. 24% lower than in 2020 (2.8 g dry weight/m2, as well as 14 % lower than the long-term average for 1997-2021 (2.4 g dry weight/m²).

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 are high 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, the majority of the mid-sized zooplankton is migrating  down to deeper waters in large part 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 0.18 mm mesh will probably occasionally be abundant, but they have far less importance for the biomass which 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 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

The following indicators affect zooplankton production and 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

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

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.

Since 1997 the amount of zooplankton during the autumn have been relatively stable in the defined indicator area in the Barents Sea. However, in 2020 and 2021 the biomass has been lower than the long-term average for 1997-2021. This applies particularly to the middle size fraction, where relatively large copepods are an important component. This can mean that there will be less food 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 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.

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