A carbon budget for the Amundsen Sea Polynya, Antarctica: Estimating net community production and export in a highly productive polar ecosystem

Patricia L Yager ,Robert M Sherrell ,Rasmus Swalethorp ,Kate E Lowry ,C M Williams ,Ellery D Ingall ,Anton F Post ,Julie Dinasquet ,Stefan Bertilsson ,Rachel E Sipler ,Linquan Mu ,Anne‐Carlijn Alderkamp ,Inga Richert ,Lasse Riemann ,A Jasmin Melara ,Rebekah G Newstead ,Sharon Stammerjohn ,Hugh W Ducklow ,Stephanie E Wilson ,Ramiro Logares ,Oscar Schofield ,Gert L Van Dijken

doi:10.12952/journal.elementa.000140

Abstract

Abstract Polynyas, or recurring areas of seasonally open water surrounded by sea ice, are foci for energy and material transfer between the atmosphere and the polar ocean. They are also climate sensitive, with both sea ice extent and glacial melt influencing their productivity. The Amundsen Sea Polynya (ASP) is the greenest polynya in the Southern Ocean, with summertime chlorophyll a concentrations exceeding 20 µg L−1. During the Amundsen Sea Polynya International Research Expedition (ASPIRE) in austral summer 2010–11, we aimed to determine the fate of this high algal productivity. We collected water column profiles for total dissolved inorganic carbon (DIC) and nutrients, particulate and dissolved organic matter, chlorophyll a, mesozooplankton, and microbial biomass to make a carbon budget for this ecosystem. We also measured primary and secondary production, community respiration rates, vertical particle flux and fecal pellet production and grazing. With observations arranged along a gradient of increasing integrated dissolved inorganic nitrogen drawdown (ΔDIN; 0.027–0.74 mol N m−2), changes in DIC in the upper water column (ranging from 0.2 to 4.7 mol C m−2) and gas exchange (0–1.7 mol C m−2) were combined to estimate early season net community production (sNCP; 0.2–5.9 mol C m−2) and then compared to organic matter inventories to estimate export. From a phytoplankton bloom dominated by Phaeocystis antarctica, a high fraction (up to ∼60%) of sNCP was exported to sub-euphotic depths. Microbial respiration remineralized much of this export in the mid waters. Comparisons to short-term (2–3 days) drifting traps and a year-long moored sediment trap capturing the downward flux confirmed that a relatively high fraction (3–6%) of the export from ∼100 m made it through the mid waters to depth. We discuss the climate-sensitive nature of these carbon fluxes, in light of the changing sea ice cover and melting ice sheets in the region.

Highlights

The Southern Ocean plays a disproportionate role in the global carbon cycle, accounting for approximately 25% of the oceanic uptake of atmospheric CO2 from just 10% of the global ocean surface (Takahashi et al, 2002, 2009)
The expedition could not sample over the entire open-water season, we report here an effort to quantify production and export in the water column at 13 stations sampled during the early phases of the bloom, by distinguishing summertime observations from those expected pre-bloom for Winter Water (WW; potential temperature, θ< −1.79°C, salinity, S > 34.1; see Yager et al, 2012; Wong et al, 1998; Randall-Goodwin et al, 2015)
During Amundsen Sea Polynya International Research Expedition (ASPIRE), in austral summer 2010–11, we aimed to determine physical and biological mechanisms driving the production and fate of this extraordinary algal bloom, with an eye towards predicting how this system will respond to further change

Summary

Introduction

The Southern Ocean (south of 50°S) plays a disproportionate role in the global carbon cycle, accounting for approximately 25% of the oceanic uptake of atmospheric CO2 from just 10% of the global ocean surface (Takahashi et al, 2002, 2009) This CO2 exchange is driven by a balance of contributions from physical (e.g., cooling, deep convection) and biological (e.g., photosynthesis, remineralization) processes. The efficiency of the soft-tissue biological pump, controlling how much organic carbon sinks to depth, depends on the extent to which surface macronutrients are depleted (Sarmiento et al, 2004) among other factors (Frost, 1984; Ducklow et al, 2001; Steinberg et al, 2008; Armstrong et al, 2009; Herndl and Reinthaler, 2013) This efficiency, especially at high latitudes, exerts control on atmospheric CO2 levels over thousands of years and may be linked to availability of micronutrients such as iron (Sarmiento and Toggweiler, 1984; Sigman et al, 2010; Sigman and Hain, 2012)

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