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

[1] Nitrate availability is generally considered to be the limiting factor for oceanic new production and this concept is central in our observational and modeling efforts. However, recent time-series observations off Bermuda and Hawaii indicate a significant removal of total dissolved inorganic carbon (C-T) in the absence of measurable nitrate. Here we estimate net carbon production in nitrate-depleted tropical and subtropical waters with temperatures higher than 20degreesC from the decrease in the salinity normalized C-T inventory within the surface mixed layer. This method yields a global value of 0.8 +/- 0.3 petagrams of carbon per year (Pg C yr(-1),Pg = 10(15) grams), which equates to a significant fraction (20-40%) of the recent estimates (2.0-4.2 Pg C yr(-1)) of total new production in the tropical and subtropical oceans [Emerson et al., 1997; Lee, 2001]. The remainder is presumably supported by upward flux of nutrients into the euphotic zone via eddy diffusion and turbulent mixing processes or lateral exchange. Our calculation provides the first global-scale estimate of net carbon production in the absence of measurable nitrate. We hypothesize that it is attributable to dinitrogen (N-2) fixing microorganisms, which can utilize the inexhaustible dissolved N-2 pool and thereby bypass nitrate limitation.