Preprints
https://doi.org/10.5194/bg-2021-122
https://doi.org/10.5194/bg-2021-122

  17 May 2021

17 May 2021

Review status: this preprint is currently under review for the journal BG.

Summertime productivity and carbon export potential in the Weddell Sea, with a focus on the waters adjacent to Larsen C Ice Shelf

Raquel Flynn1, Thomas Bornman2,3, Jessica Burger1, Shantelle Smith1, Kurt Spence1, and Sarah Fawcett1,4 Raquel Flynn et al.
  • 1Department of Oceanography, University of Cape Town, Cape Town, South Africa
  • 2South African Environmental Observation Network, Elwandle Coastal Node, Port Elizabeth, South Africa
  • 3Institute for Coastal and Marine Research, Nelson Mandela University, Port Elizabeth, South Africa
  • 4Marine and Antarctic Research centre for Innovation and Sustainability (MARIS), University of Cape Town, Cape Town, South Africa

Abstract. The Weddell Sea (WS) represents a point of origin in the Southern Ocean where globally-important water masses form. Biological activities in WS surface waters thus affect large-scale ocean biogeochemistry. During summer 2018/2019, we measured net primary production (NPP), nitrogen (nitrate, ammonium, urea) uptake, and nitrification in the western WS at the Antarctic Peninsula (AP) and Larsen C Ice Shelf (LCIS), in the southwestern Weddell Gyre (WG), and at Fimbul Ice Shelf (FIS) in the south-eastern WS. The highest average rates of NPP and greatest nutrient drawdown occurred at LCIS. Here, the phytoplankton community was dominated by colonial Phaeocystis antarctica, with diatoms increasing in abundance later in the season as sea-ice melt increased. At the other stations, NPP was variable, and diatoms known to enhance carbon export (e.g., Thalassiosira spp.) were dominant. Euphotic zone nitrification was always below detection, such that nitrate uptake could be used as a proxy for carbon export potential, which was highest in absolute terms at LCIS and the AP. Surprisingly, the highest f-ratios occurred near FIS rather than LCIS (average of 0.73 ± 0.09 versus 0.47 ± 0.08). We attribute this to ammonium inhibition of nitrate uptake at LCIS (where ammonium concentrations were 0.6 ± 0.4 μM, versus 0.05 ± 0.1 μM at FIS) driven by increased heterotrophy following the accumulation of nitrate-fuelled phytoplankton biomass in early summer. Across the WS, carbon export appears to be driven by a combination of physical, chemical, and biological factors, with the highest export flux occurring at the ice shelves and lowest in the central WG.

Raquel Flynn et al.

Status: open (until 17 Jul 2021)

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Raquel Flynn et al.

Raquel Flynn et al.

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Short summary
Biological activity in the shallow Weddell Sea affects the biogeochemistry of recently-formed deep waters. To investigate the drivers of carbon and nutrient export, we measured rates of primary production and nitrogen uptake, characterised the phytoplankton community, and estimated nutrient depletion ratios across the under-sampled western Weddell Sea in mid-summer. Carbon export was highest at the ice shelves, and was determined by a combination of physical, chemical, and biological factors.
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