Carbon to Nitrogen Uptake Ratios Observed Across the Southern Ocean by the SOCCOM Profiling Float Array

Abstract

Measurements of pH and nitrate from the Southern Ocean Carbon and Climate Observations and Modeling array of profiling floats were used to assess the ratios of dissolved inorganic carbon (DIC) and nitrate (NO3) uptake during the spring to summer bloom period throughout the Southern Ocean. Two hundred and forty‐three bloom periods were observed by 115 floats from 30°S to 70°S. Similar calculations were made using the Takahashi surface DIC and nitrate climatology. To separate the effects of atmospheric CO2 exchange and mixing from phytoplankton uptake, the ratios of changes in DIC to nitrate of surface waters (ΔDIC/ΔNO3) were computed in the Biogeochemical Southern Ocean State Estimate (B‐SOSE) model. Phytoplankton uptake of DIC and nitrate are fixed in B‐SOSE at the Redfield Ratio (RR; 6.6 mol C/mol N). Deviations in the B‐SOSE ΔDIC/ΔNO3 must be due to non‐biological effects of CO2 gas exchange and mixing. ΔDIC/ΔNO3 values observed by floats and in the Takahashi climatology were corrected for the non‐biological effects using B‐SOSE. The corrected, in situ biological uptake ratio (C:N) occurs at values similar to the RR, with two major exceptions. North of 40°S biological DIC uptake is observed with little or no change in nitrate giving high C:N. In the latitude band at 55°S, the Takahashi data give a low C:N value, while floats are high. This may be due to a change in CO2 air‐sea exchange in this region from uptake during the Takahashi reference year of 2005 to outgassing of CO2 during the years sampled by floats.

Biogenic carbon pool production maintains the Southern Ocean carbon sink

“…2 A ), the dominance of N-poor DOM production in the STZ drives the C:N of TOM production to 10-12 ( Fig. 2 A ), deviating significantly from the classical Redfield Ratio [~6.6; Redfield ( 28 ) and Johnson et al ( 29 )]. This helps to explain the conundrum of pronounced seasonal DIC consumption and oxygen production observed in the nutrient-devoid subtropical, sunlit oceans ( 30 , 31 ).…”

Nanoplankton: The dominant vector for carbon export across the Atlantic Southern Ocean in spring

“…Across our transect, the mixed-layer seawater Fe:NO 3 − ratios ranged from 3 to 27 μmol mol −1 , indicating that the supply of iron at some stations (i.e., in the OAZ and MIZ) was insufficient to sustain NO 3 − uptake. Assuming balanced phytoplankton growth (i.e., assigning a C:N uptake ratio of 6.7, 7.5, and 7.2 for the SAZ, PFZ and AZ, respectively, as has been recently inferred for phytoplankton biomass in the Atlantic sector) ( 52 ), we can assess whether the mixed-layer iron concentrations could sustain the Fe:C uptake ratios of the different phytoplankton size classes. Multiplying our measured seawater Fe:NO 3 − by C:N yields theoretical Fe:C uptake ratios of 0.4 to 4.2 μmol mol −1 (Fig.…”

“…5C). Under balanced growth conditions, the Fe:C uptake ratio of phytoplankton should equal their Fe:N uptake ratio × C:N ratio ( 52 ). However, the AZ microplankton Fe:C uptake ratios were roughly four times higher than expected from the Fe:N uptake ratios (i.e., data falling above the dashed line in Fig.…”

“…We computed NCP using three different estimates of the C:N ratio—the Redfield C:N ratio of 6.6:1, the C:N ratio estimated by ( 52 ) from high-resolution float-based measurements for each zone of the Atlantic Southern Ocean, and the bulk biomass C:N ratio that we measured at each station. [PON], [NH 4 + ], and [urea] are the average measured mixed-layer concentrations of the various species, and NO 3 − depletion was estimated asNO3−0.25emdepletion=false[NO3−false]source−false[NO3−false]measuredwhere [NO 3 − ] source is the average NO 3 − concentration directly below the mixed layer at the time of sampling and [NO 3 − ] measured is the average measured mixed-layer concentration ( 84 , 92 ).…”

“…We computed NCP using three different estimates of the C:N ratio-the Redfield C:N ratio of 6.6:1, the C:N ratio estimated by (52) from high-resolution float-based measurements for each zone of the Atlantic Southern Ocean, and the bulk biomass C:N ratio that we measured at each station. [PON], [NH 4 + ], and [urea] are the average measured mixed-layer concentrations of the various species, and NO 3 − depletion was estimated as…”