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

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. Plain Language Summary: Phytoplankton take up dissolved inorganic carbon (DIC) and nitrate as they grow. This results in a decrease in DIC and nitrate during the spring through summer bloom periods each year. The ratio of DIC to nitrate uptake is typically near 6.6 mol C/mol N, a value termed the Redfield Ratio (RR). Here, we used sensor data from an array of profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling program in the Southern Ocean to examine the ratio of C:N uptake by phytoplankton during 243 bloom periods from October through February. We find uptake occurred at values near the RR throughout the Southern Ocean, with two exceptions. North of 40°S, C:N ratios exceed the Redfield value, most likely due to phytoplankton production of a gel‐like organic matter deficient in nitrogen. Near 55°S in the Antarctic Southern Zone, an apparent increase in the C:N ratio over the past decade may reflect a change from an air‐sea flux of CO2 into the ocean to a flux out of the ocean. Key Points: Carbon:nitrate uptake ratios by phytoplankton are near the Redfield Ratio in Southern Ocean waters south of 40°SCarbon:nitrate uptake ratios north of 40°S are much higher, likely due to production of dissolved organic matter with little nitrogenA change in air‐sea CO2 flux during October to February from a sink in 2005 to a source in recent years may have occurred near 55°S [ABSTRACT FROM AUTHOR]