Primary productivity, new productivity, and their relation to carbon flux during two Southern Ocean Gas Exchange tracer experiments

[1] Biological uptake rates of inorganic carbon and nitrate were measured during two sequential tracer release gas exchange experiments, together known as the Southern Ocean Gas Exchange Experiment (SO GasEx) in the southwest Atlantic sector of the Southern Ocean Antarctic Zone (51°N, 38°W). Primary productivity estimated from ¹⁴C incubations ranged from 26.7 to 47.2 mmol C m⁻² d⁻¹ in the first experiment (Patch 1) and 13.7 to 39.4 mmol C m⁻² d⁻¹ in the second experiment (Patch 2). Nitrate-based productivity estimated from ¹⁵NO₃ incubations ranged from 5.8 to 13.1 mmol C m⁻² d⁻¹ in Patch 1 and 1.9 to 7.1 mmol C m⁻² d⁻¹ in Patch 2. The average ratio of nitrate-based productivity to primary productivity (approximating the f ratio) was 0.24 in Patch 1 and 0.15 in Patch 2. Chlorophyll concentrations for both patches were less than 1 mg m⁻³. Photochemical efficiency (F_v/F_m) was low (∼0.3) in Patch 1 and moderate (∼0.45) in Patch 2. Si(OH)₄ concentrations were potentially limiting (<1 mmol m⁻³ for Patch 1 and ∼3 mmol m⁻³ for Patch 2), while NH₄⁺ concentrations were elevated (∼1 mmol m⁻³ for Patch 1 and ∼2.2 mmol m⁻³ for Patch 2) compared with typical open ocean Antarctic Zone water. We hypothesize that Patch 1 productivity was regulated by the availability of Si(OH)₄, while Patch 2 productivity was regulated by grazers. Primary production and nitrate-based production (as a proxy for C export) determined here provide components for a mixed layer carbon budget from which the air-sea flux of CO₂ will be quantified.