Atlantification of the marine ecosystem in Kongsfjorden, Svalbard
Climate warming is rapidly altering the physical marine environment in fjords on the west coast of Svalbard towards a more temperate state. Reductions in sea ice cover and increased ocean temperatures are evident, resulting in changes of ice-associated and pelagic ecosystems.
Documenting climate-related changes in a marine ecosystem is detective work. Oceans are in a constant state of change due to weather, currents, tides, and seasonality. Consequently, samples collected from the same spot even just a few minutes apart can vary considerably. Similar to judges in a court, scientists need reliable evidence to ascertain that differences among samples taken over time are part of a long-term trend rather than shorter-term natural variability. Acquiring such evidence requires thoughtful long-term sampling campaigns and thorough statistical analysis of collected data. During the last years, researchers working in Kongsfjorden have published long-term ecological and physical data series. These time series range from diet samples collected from seabirds to direct measurements of temperature, sea ice and plankton abundances. Shifts in the ecosystem reflecting physical changes are now apparent and sometimes surprising.
Phytoplankton investigations in Kongsfjorden date back to the early 1970s with systematic monitoring each summer since 2009. The time series indicates three different spring bloom scenarios. Until the early 2000s, Kongsfjorden often had a long-lasting ice cover, and the phytoplankton bloom was in May. The recent decrease in sea ice cover has resulted in an earlier bloom in April. Both these bloom scenarios are characterised by the dominance of diatoms due to weak thermal gradients allowing resting spores to be mixed up into the water column during winter. The last scenario, encountered during the warmest years in the time series, is associated with a surface inflow of Atlantic Water during winter, preventing deep winter mixing and ice formation. This scenario results in a delayed and diminished bloom dominated by colonies of Phaeocystis pouchetti. Such changes in the timing, magnitude, and composition of the spring bloom can have ripple effects on the entire marine food web.
Zooplankton has been monitored each summer in Kongsfjorden since 1996. The time series shows an increase in the contribution of the Atlantic copepod Calanus finmarchicus, possibly due to warming and loss of sea ice in Kongsfjorden. Surprisingly, the abundance of its Arctic cousin C. glacialis has not declined in Kongsfjorden nor in Isfjorden further south. Earlier onset of the spring bloom, combined with warmer temperatures does not appear to displace C. glacialis but rather to improve its breeding efficiency, shorten its life cycle and reduce its body size to resemble the smaller C. finmarchicus. The transfer of energy, from primary producers through Calanus to higher predators, may then become more efficient due to changes in generation length and population turnover rate that accompany the changes in body size. For size-selective predators, however, smaller copepods will be harder to catch than larger ones, and as the efforts increase, the rewards decrease
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