Process studies on the ecological coupling between sea ice algae and phytoplankton

The seasonal dynamics of pelagic and sea ice communities are closely related in ice-covered waters, however, modelling works that analyse such interactions are scarce. We use the Biogeochemical Flux Model in Sea Ice (BFM-SI) coupled to the pelagic Biogeochemical Flux Model (BFM) in a study area in Greenland to quantitatively investigate: (1) the significance of photoacclimation/photoadaptation strategies of autotrophs, (2) the fate of the sea ice biomass in case of algae seeding, algae aggregation and at different mixed layer depths and (3) the changes in community production under a climate change scenario. The results show that sea ice algae need to be both photoacclimated and photoadapted to the sea ice environment in order to grow, while phytoplankton may adopt different strategies for optimising their growth. The seeding of the phytoplankton bloom shows to be driven, both in timing and magnitude, by the viability of sea ice algae and by the degree of aggregation of algae released from the ice, which also affects the sinking rate to the sea floor. Under a mild climate change scenario (SRES B2, 2071–2090) the sea ice community is projected to be generally more productive, whereas phytoplankton growth will be reduced because the melt of sea ice will occur earlier in the season when light is less favourable to sustain the growth. While it is generally anticipated that the melting of multi-year ice in the Arctic Ocean will cause an increase in marine production, this study shows that seasonal ice-covered seas in the Northern hemisphere may actually be less productive and may shift to more oligotrophic conditions within the next 100 years.

► A comprehensive ecosystem model is used to study primary production in ice-covered seas.
► Sea ice algae need to be both photoacclimated and photoadapted to the sea ice environment in order to bloom.
► The viability of ice algae in seawater anticipates the phytoplankton bloom, but does not influence the magnitude of it.
► The aggregation of ice diatoms in seawater favours the dominance of flagellates and, at a less extent, also the magnitude of their bloom.
► Without considering a dynamic Chl:C ratio in rapidly changing light environment such as seasonal ice-covered oceans, carbon biomass estimates are shown to be severely biased.
► Within the limit of 1D model simulations, under a mild climate change scenario overall (sea ice and seawater) primary production will decrease.