LAUSR

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.