We know that in the Barents Sea, winters are cold and dark, March has maximum ice cover and algae multiply en masse by April or May. We also know that some copepods go dormant by fall, and many birds and whales leave the area after filling their bellies in summer, while other inhabitants never leave or hibernate. But do we know exactly how much our research results vary depending on the season we take our measurements in? When should we actually measure key ocean variables and ecosystem components that we need to know for sustainable management and conservation?
To find answers, the Nansen Legacy team dedicated four seasonal research cruises, several year-long moorings, and autonomous vehicle missions to the topic of seasonality in the northern Barents Sea and adjacent Nansen Basin. Especially the November and early March cruises were designed to fill seasonal knowledge gaps. In a special issue in the journal Progress in Oceanography, 19 articles by dozens of authors provide a plethora of new information on “Seasonality and interannual variability of physical, chemical and biological states and processes in the northern Barents Sea and adjacent areas”. The editorial of the issue recently got published and summarizes key results. In addition, a beautifully illustrated fact sheet on the topic makes the findings accessible to a broader audience.
Did the work yield anything unexpected? The seasonality is much more pronounced in the pelagic- than the benthic compartment. Seasonality is also modified by external supplies. And the Arctic winter is less of a dormant season than previously believed. While ice algal- and phytoplankton blooms are locally generated in the spring, the Atlantic Water inflow brings additional phytoplankton and zooplankton along with heat and nutrients year-round into parts of the Barents Sea, providing extra fuel to the Barents Sea food webs. Copepods that primarily eat phytoplankton are indeed (mostly) dormant in winter, but other abundant zooplankton consume a wide range of particles year-round and can reproduce year-round. Thereby they provide a stable – albeit less fat-rich – food source for their consumers. Seafloor dwellers such as various worms, bivalves, crustaceans and brittle stars vary way less in their stock size, composition, and food web structure than many pelagic and ice-associated inhabitants. Their often long life spans (years to decades) combined with a basic available food supply appear to dampen links to surface seasonality and processes to some extent. The pelagic offspring of some of these benthic animals, however, vary in occurrence and abundance seasonally, some even peaking in the dark season.
Clearly, differences in the exact timing of all these oceanographic conditions and biological events occur among years in response to varying sea-ice cover. In our sampling year 2018 we had for example much lighter ice conditions in the northern Barents Sea than during the same month (August) in 2019. We conclude that monitoring and management strategies must be closely aligned with the seasonality and interannual variability of the ecosystem component in question and consider that phenology of events is changing under climate change.