Abstract
Abstract The Amundsen Sea Polynya is characterized by large phytoplankton blooms, which makes this region disproportionately important relative to its size for the biogeochemistry of the Southern Ocean. In situ data on phytoplankton are limited, which is problematic given recent reports of sustained change in the Amundsen Sea. During two field expeditions to the Amundsen Sea during austral summer 2010–2011 and 2014, we collected physical and bio-optical data from ships and autonomous underwater gliders. Gliders documented large phytoplankton blooms associated with Antarctic Surface Waters with low salinity surface water and shallow upper mixed layers (< 50 m). High biomass was not always associated with a specific water mass, suggesting the importance of upper mixed depth and light in influencing phytoplankton biomass. Spectral optical backscatter and ship pigment data suggested that the composition of phytoplankton was spatially heterogeneous, with the large blooms dominated by Phaeocystis and non-bloom waters dominated by diatoms. Phytoplankton growth rates estimated from field data (≤ 0.10 day−1) were at the lower end of the range measured during ship-based incubations, reflecting both in situ nutrient and light limitations. In the bloom waters, phytoplankton biomass was high throughout the 50-m thick upper mixed layer. Those biomass levels, along with the presence of colored dissolved organic matter and detritus, resulted in a euphotic zone that was often < 10 m deep. The net result was that the majority of phytoplankton were light-limited, suggesting that mixing rates within the upper mixed layer were critical to determining the overall productivity; however, regional productivity will ultimately be controlled by water column stability and the depth of the upper mixed layer, which may be enhanced with continued ice melt in the Amundsen Sea Polynya.
Highlights
The Southern Ocean is disproportionately important to the global biogeochemical system, accounting for up to half of the annual oceanic uptake of anthropogenic carbon dioxide (CO2) from the atmosphere (Arrigo et al, 2008; Gruber et al, 2009)
In this paper we focus on the discrete data from the Amundsen Sea Polynya International Research Expedition (ASPIRE) expedition
Prior to and after deployment, the glider conductivity temperature and depth (CTD) was compared to independent CTDs in a tank test, and the results indicated that the glider CTD did not exhibit any drift
Summary
The Southern Ocean is disproportionately important to the global biogeochemical system, accounting for up to half of the annual oceanic uptake of anthropogenic carbon dioxide (CO2) from the atmosphere (Arrigo et al, 2008; Gruber et al, 2009). Given the large-scale documented changes being observed in many sectors of the Southern Ocean gaining a better understanding of the biogeochemical dynamics is critical (Ducklow et al, 2007; Schofield et al, 2010). One region showing dramatic change is the Amundsen Sea, which is influenced by some of the largest and most rapid glacier melt and ice sheet thinning in the Southern Ocean (Rignot, 2008). The Amundsen Sea located near the Dotson and Crosson Ice shelves shown in blue on the inset map (adapted from Rignot et al, 2013). Colored lines indicate the glider missions: yellow for the glider mission during ASPIRE, red for the glider mission during the KOPRI cruise. Depths are indicated by the blue color scale and contour lines, the Antarctic continent is dark gray, and the ice shelves are light gray
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