Global net community production and the putative net heterotrophy of the oligotrophic oceans

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Abstract

[1] Reconciling rates of organic carbon export from the euphotic zone with the consumption of organic material in the dark ocean remains one of the major quantitative uncertainties of the ocean carbon cycle. Euphotic zone net community production (NCP) provides one broad constraint on export flux and potential carbon drawdown. However, in vitro measurements of NCP consistently suggest that oligotrophic oceans are net heterotrophic, which is inconsistent with evidence of their carbon export to depth. Further, we have been unable to identify organic inputs on a scale to supplement the purported net heterotrophy. Here, we calculate global NCP rates using empirical relationships between in vitrophotosynthesis (P) and respiration (R) and a satellite-based productivity model. A low value for global NCP (∼139 ± 325 Tmol C a−1) is found when a single P versus R (PvR) relation is derived from all in vitro data, with areas of net heterotrophy occupying 52% of the surface ocean. If a set of PvR relationships are instead derived by segregating the in vitro data into broad latitudinal zones associated with differing nutrient dynamics, we find a global NCP distribution in better agreement with independent model estimates of particulate carbon export, except in the 10°–40° latitudinal band where negative NCP values remain. Consistency between NCP and particulate export across all latitudes is achieved by applying a single PvR relationship derived using all in vitro data collected outside the 10°–40° latitudinal band. With this model, global NCP is estimated at ∼781 ± 393 Tmol C a−1and modeled values at well-characterized field sites are in good agreement with non-incubation based in situ measurements. We infer from our results thatin vitro NCP data from oligotrophic sites are too low, and suggest that this error is more likely the result of underestimated photosynthesis than overestimated respiration, although the precise physiological nature of the problem remains to be demonstrated.

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