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

Abstract

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 (Fv/Fm) 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.

Wind Speed and Sea State Dependencies of Air‐Sea Gas Transfer: Results From the High Wind Speed Gas Exchange Study (HiWinGS)

“…SO GasEx was conducted during the Austral autumn, following the summer bloom, but targeted on an area of enhanced productivity to maximize CO 2 fluxes. Based on MODIS data, the SO GasEx study area experienced a large bloom just prior to the start of the project (see Figure , Lance et al, ). In contrast, HiWinGS occurred during North Atlantic autumn, well after the summer bloom, during a time of rapidly declining productivity and deepening mixed layer.…”

Lagrangian evolution of DMS during the Southern Ocean gas exchange experiment: The effects of vertical mixing and biological community shift

“…For Patch 1 (Figures and ), DMS concentration (mean of 2.2 nM) was variable but showed a slight increase with time. DMSP concentration (mean of 63 nM) also built up over the 4 days, while chlorophyll a (Chl a ) from high performance liquid chromatography (from CTD) remained relatively constant [ Lance et al ., ]. The DMS:DMSP ratio was around 0.03, much lower than in Patch 2.…”

“…Lance et al . [] attributed the decrease in FUCO in this phase to preferential consumption of diatoms by grazers, in accord with enhanced ammonium concentrations (mean of 2.2 µM for Patch 2, compared to 1.1 µM for Patch 1). The f‐ratio (nitrate based production/primary production) was also lower in Patch 2 (0.15) compared to Patch 1 (0.24), indicating greater regenerated production during the second patch.…”

“…From Lance et al . [], the main diagnostic pigments for phytoplankton during SO GasEx were fucoxanthin (FUCO; e.g., diatoms, which produce low amounts of DMSP) and 19′‐hexanoyloxyfucoxanthin (HEXA, e.g., prymnesiophytes, which produce high amounts of DMSP). For Patch 1, FUCO decreased slightly while HEXA increased, causing the HEXA:FUCO ratio to increase from 0.75 to 0.90.…”

“…Over the 2 weeks of Patch 2, the ML concentrations of nitrate and phosphate increased at rates of 8.3 and 0.75 mmol m −2 d −1 , respectively, partly due to vertical mixing (Appendix section A2; see figures from Lance et al . []). At these concentrations, nitrate and phosphate were not directly limiting the growth of phytoplankton.…”