AGU Ocean Sciences Meeting, Honolulu, HI, February 2002.

Global estimates of carbon export in the nitrate-depleted tropical and subtropical oceans

K. Lee¹, D. Karl², J.-Z. Zhang³, R. Wanninkhof³

¹Pohang University of Science and Technology School of Environmental Science and Engineering, San 31, Hyoja-dong, Nam-gu, Pohang 790-784, Korea, Republic of Korea

²University of Hawaii Department of Oceanography, 1000 Pope Road, Honolulu, HI 96822, United States

³Atlantic Oceanographic and Meteorological Laboratory, 4301 Rickenbacker Causeway, Miami, FL 33149, United States

Nitrate availability is generally considered to be the limiting factor for oceanic new and export production and this concept is central in our observational and modeling efforts. However, recent time-series observations off Bermuda and Hawaii indicate a significant new production in the absence of measurable nitrate. Here we estimate global new production in nitrate-depleted tropical and subtropical waters with temperatures higher than 20 degree (Celsius) from the decrease in the salinity normalized total dissolved inorganic carbon inventory within the surface mixed layer corrected for changes due to net air-sea carbon exchange. This method yields a global new production of 0.8 giga ton carbon per year, which accounts for a significant fraction of the recent total new production estimates in the tropical and subtropical oceans, with the remainder being supported by upward nutrients into the euphotic zone through eddy diffusion and turbulent mixing processes. Our modeled value is the first global-scale estimate of new production in the absence of measurable nitrate. We hypothesize that it is attributable to nitrogen fixing microorganisms, which can utilize the non-limiting nitrogen and thereby bypass nitrate limitation. This reported new production is significantly higher than published global nitrogen fixation estimates hazed on extrapolation of sparse measurements of nitrogen fixation.