Metabolic poise in the North Atlantic Ocean diagnosed from organic matter transports
Limnol. Oceanogr., 49(4), 2004, 1084-1094 | DOI: 10.4319/lo.2004.49.4.1084
ABSTRACT: Recently there has been discussion about the metabolic state of the ocean, with arguments questioning whether the open ocean is net autotrophic or net heterotrophic. Accurately determining the metabolic balance of a marine system depends on fully defining the system being evaluated and on quantifying the inputs and outputs to that system. Here, a net northward transport of dissolved organic carbon (DOC) (across 24.5°N) of 3.3 ± 1.9 Tmol C yr-1 was determined using basin-wide transport estimates of DOC. This flux, coupled with DOC inputs from the Arctic Ocean (2.2 ± 0.8 Tmol C yr-1), the atmosphere (0.6 ± 0.08 Tmol C yr-1), and rivers (3.1 ± 0.6 Tmol C yr-1), indicates net heterotrophy in the North Atlantic (full depth, 24.5-72°N) of 9.2 ± 2.2 Tmol C yr-1. This rate is small (<2%) compared to autochthonous production (~494 Tmol C yr-1) and consumption (production : respiration of 0.98), indicating that the North Atlantic is essentially metabolically balanced and that autochthonous production is remineralized within the basin. The upper layer of the subtropical gyre has previously been reported to exhibit high rates of net heterotrophy, but our analysis does not support those findings. Instead, allochthonous inputs of organic carbon to the upper subtropical gyre are an order of magnitude less than required by the elevated rates of net heterotrophy reported. We find, too, that net mineralization of allochthonous DOC within the basin could account for 10% of the preindustrial inorganic carbon exported from the basin to the south. Two factors, the import of organic matter and the unique thermohaline circulation pattern of the North Atlantic, are primary in ensuring net heterotrophy in the basin.