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

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 20°C 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.