Majority of Southern Ocean Seasonal Sea Ice Zone Bloom Net Community Production Precedes Total Ice Retreat

Abstract

The Southern Ocean seasonal sea ice zone (SIZ) spring is characterized by sea ice retreat and the development of phytoplankton blooms. Until recently, assessing SIZ blooms and associated net community production (bNCP) has been limited by a lack of under-ice observations. We relate the timing of phytoplankton growth to the drawdown of surface nitrate and sea ice cover and estimate bNCP from biogeochemical profiling float observations. The onset of biological production closely follows initial sea ice breakup and, on average, 64% of bNCP occurs before total sea ice retreat. This indicates that satellite-derived estimates largely miss under-ice production and underestimate SIZ bNCP. Float bNCP estimates range from <1 to >4 mol C m−2 bloom−1, with higher bNCP when sea ice breakup occurs early in the year, and the highest bNCP near topographic features that may increase micronutrient supply. Our results suggest changes in Southern Ocean sea ice will influence future bNCP.