Autonomous Biogeochemical Floats Link Huge Southern Ocean Phytoplankton Bloom to Maud Rise Polynya

An array of more than 150 autonomous biogeochemical profiling floats deployed over the past five years in the Southern Ocean has provided unprecedented spatial coverage of subsurface fluorescence and backscatter, which yield estimates of chlorophyll-a concentrations and phytoplankton biomass. This unique dataset can be used to examine seasonal to interannual variability in phytoplankton bloom timing, vertical structure, and magnitude. The largest annually integrated chlorophyll measured by any float in the array was associated with a phytoplankton bloom in the 2017-18 season over Maud Rise seamount in the Weddell Sea. Maud Rise is also known to be the site of rare polynya events, the most recent of which occurred in austral winter of 2016 and 2017. Starting in 2015, two floats were trapped in a topographically locked circulation over the seamount, capturing the full annual cycle of chlorophyll in the Maud Rise region for the two years with polynyas as well as two years without. We find that the 2017 polynya had large and far-reaching effects on phytoplankton blooms in the region. The opening of the ice increased light availability earlier in the season, and the bloom was subsequently triggered months earlier than in other years. Moreover, deep mixing associated with the polynya likely increased the input of limiting micronutrients to the surface layer, allowing for a deep extended bloom late into the season. This longer-lasting bloom, also observed at locations north of Maud Rise, led to higher annual net community production as estimated from nitrate concentrations. The 2016 polynya, however, likely did not have this same effect on the biology because it occurred too early in winter and was too small to strongly affect access to light or nutrients.