Seasonal cycle of CO2 from the sea ice edge to island blooms in the Scotia Sea, Southern Ocean

Abstract

The Scotia Sea region contains some of the most productive waters of the Southern Ocean. It is also a dynamic region through the interaction of deep water masses with the atmosphere. We present a first seasonally-resolved time series of the fugacity of CO2 (fCO2) from spring 2006, summer 2008, autumn 2009 and winter (potential temperature minimum) along a 1000km transect from the pack ice to the Polar Front to quantify the effects of biology and temperature on oceanic fCO2. Substantial spring and summer decreases in sea surface fCO2 occurred in phytoplankton blooms that developed in the naturally iron-fertilised waters downstream (north) of South Georgia island (54–55°S, 36–38°W) and following sea ice melt (in the seasonal ice zone). The largest seasonal fCO2 amplitude (ΔfCO2) of −159μatm was found in the South Georgia bloom. In this region, biological carbon uptake dominated the seasonal signal, reducing the winter maxima in oceanic fCO2 by 257μatm by the summer. In the Weddell–Scotia Confluence, the southern fringe of the Scotia Sea, the shift from wintertime CO2-rich conditions in ice covered waters to CO2 undersaturation in the spring blooms during and upon sea ice melt created strong seasonality in oceanic fCO2. Temperature effects on oceanic fCO2 ranged from ΔfCO2 sst of ~55μatm in the seasonal ice zone to almost double that downstream of South Georgia (98μatm). The seasonal cycle of surface water fCO2 in the high-nutrient low-chlorophyll region of the central Scotia Sea had the weakest biological control and lowest seasonality. Basin-wide biological processes dominated the seasonal control on oceanic fCO2 (ΔfCO2 bio of 159μatm), partially compensated (43%) by moderate temperature control (ΔfCO2 sst of 68μatm). The patchwork of productivity across the Scotia Sea creates regions of seasonally strong biological uptake of CO2 in the Southern Ocean.