Global net community production estimated from the annual cycle of surface water total dissolved inorganic carbon

Abstract

Global net community production is determined, for the first time, from the decrease in salinity (S)-normalized total dissolved inorganic carbon (NCT = CT × 35/S) inventory in the surface mixed layer corrected for changes due to net air-sea CO2 exchange and diffusive carbon flux from the upper thermocline. Changes in the mixed layer NCT inventory are estimated using a derived annual cycle of NCr and global records of the mixed layer depth. The annual NCT cycle is deduced from regional algorithms relating NCT to sea surface temperature (SST) and nitrate (NO3-), along with global records of seasonal mean SST and NO3-, and from the monthly mean surface partial pressure of CO2 and total alkalinity fields using thermodynamic models. The two methods show similar regional trends and yield global net community production estimates of 6.7 and 8.0 Gt C (1 Gt C = 1 × 1012 kg carbon), respectively. The two global estimates are not significantly different and represent an 8-month period of 1990 (warming period) during which the mixed layer NCr concentration decreases. However, the estimates do not account for net community production during a 4-month cooling period. Ratios of net community production during the warming and cooling periods are estimated from multiyear sediment trap data at the Hawaii Ocean Time-series (22°N, 158°W) and Ocean Weather Station P (50°N, 145°W) sites. Global extrapolation of these ratios yields annual rates of net community production of 9.1 ± 2.7 and 10.8 ± 2.7 Gt C yr-1.