Estimating variability and long-term change in sea ice primary productivity using a satellite-based light penetration index

Sea ice algae in the Southern Ocean have strong ecological and biogeochemical significance, providing a lipid rich food-resource to keystone species such as Antarctic krill. Despite their ecological importance, estimating seasonal or interannual changes in ice algal production at the Antarctic circumpolar scale is not presently possible. We show that the product of ice concentration and irradiance penetrating into sea ice (E_ice) explains 69% of the variability in ice algal production at the sector and seasonal scale estimated from two respected sea ice algae models. After allowing for a greater quantum yield of sea ice algae in summer than in other seasons we estimated a simple light-based index of potential ice algal production (P_ice) and this explained 91% of the variability in modelled ice algal production. Our results suggest that between 1987 and 2017, ice algal production has generally increased with the largest increases seen off East Antarctica, the Weddell Sea, and the western Ross Sea.