Zooplankton species composition in Kongsfjorden

Last updated 4 November 2024

The Kongsfjorden pelagic ecosystem is influenced by both Atlantic and Arctic waters. The relative amount of the Atlantic Calanus finmarchicus and the Arctic Calanus glacialis varies yearly. Calanus finmarchicus has been the dominant species for the last decade, due to increased inflow of Atlantic water and increased temperature. Changes in the zooplankton community will cascade to higher trophic levels, and might change the character of the pelagic ecosystem. Therefore, the composition of the zooplankton community in Kongsfjorden might function as a climate indicator on a local scale.

Status and trend

The Kongsfjorden pelagic ecosystem is influenced by influx of both Atlantic and Arctic waters, consequently the zooplankton community is composed of both Atlantic and Arctic species.  The amount of Atlantic water in the West Spitsbergen Current and hence the inflow into Kongsfjorden depends on larger scale climatic factors. Therefore, the composition of zooplankton community in Kongsfjorden might function as a climate indicator on a local scale. Copepods of the genus Calanus dominates the mesozooplankton community in Kongsfjorden, both in terms of abundance and biomass. The Calanus population consists of three species: the smaller Atlantic Calanus finmarchicus, the medium-sized Arctic Calanus glacialis and the large Arctic Calanus hyperboreus. The relative abundance of C. finmarchicus and C. glacialis in Kongsfjorden depends on the inflow of Atlantic water. The larger C. hyperboreus, which is a deep-water species, is only present in low numbers in Kongsfjorden and does not contribute much to the total abundance of Calanus.

Years dominated by C. glacialis are characterized by lower water temperatures whereas C. finmarchicus dominance usually coincides with periods of increased inflow of Atlantic water. Leading to higherwater temperatures. The years with domination of the cold-water species, C. glacialis, were 1999, 2000, 2008 and 2019. In contrast. the Atlantic C. finmarchicus was the predominat in 1997, from 2001 to 2003 and in 2007 until today, except for 2008. The years 2004, 2006 and 2008 were characterized by the almost equal abundance of both Calanus species.  Since 2021, the relative abundance of C. glacialis has declined, and together with C. hyperboreus these species contributed only 2 % of the total Calanus population at the end of July 2023

The trend is less pronounced when looking at the relative proportion of Atlantic and Arctic Calanus species in terms of biomass. The large Arctic species (C. glacialis and C. hyperboreus) will contribute more biomass than the smaller Atlantic species (C. finmarchicus) at the corresponding development stage. However, there is a large difference in biomass for the different development stages. An adult might have six times as much biomass as a young individual (copepodit stage 2-3). The total biomass is therefore not only dependent on the number of individuals, but also of the stage composition of the different species at the time of sampling. A small increase of larger copepodite stages of C. glacialis and C. hyperboreus will cause relative large increase in their proportion of total biomass.

Causal factors

Changes in the zooplankton community structure in Kongsfjorden can be associated with the advection of water masses from the adjacent shelf. There is a significant variability in the degree to which Atlantic water occupies Kongsfjorden. The large interannual variations in Calanus abundance and species composition observed in the time series data from Kongsfjorden seem to correlate to changes in hydrography, i.e. warm years favour different species than cold years.

Consequences

The variability in the abundance of C. glacialis and C. finmarchicus may have implications for the pelagic ecosystem in Kongsfjorden, since variable abundances of these species lead to a shift in the size structure and the amount of energy as lipids available for the next trophic level.

Planktivorous seabirds and fish tend to be size selective in search of prey. A decrease of the mean size of available prey through a shift in species composition of the zooplankton may have consequences for predators. For example, little auks have a strong preference for larger prey items and years with a dominance of the smaller C. finmarchicus inside the fjord may force the birds to fly longer distances to find suitable food (e.g. C. glacialis), thereby reducing survival and recruitment success. However, for other larger zooplankton, fish and sea birds that don’t have any preference for the larger Calanus species it would be the total amount of Calanus that effect their success independent on the species composition.

About the monitoring

The composition of key zooplankton species in Svalbard waters is being monitored because it may be influenced by climate change and can lead to fundamental changes in the marine ecosystem.

Further reading