Recycling versus export of bioavailable dissolved organic matter in the coastal ocean and efficiency of the continental shelf pump

Abstract

At least 15% of the 2 Pg y−1 of dissolved organic carbon (DOC) that accumulates in the surface layer of the open ocean has been exported from the ocean margins. The C: N: P stoichiometry of the production and microbial degradation of dissolved organic matter (DOM) in the coastal ocean conditions the quality of the exported substrates. In this work, DOC, dissolved organic nitrogen (DON) and phosphorus (DOP) bioavailability measurements from published bottle incubation experiments have been compiled and reanalyzed to examine the role of bioavailable DOM (BDOM) in the coastal ocean. DOM bioavailability decreased significantly (p< 0.001) in the sequence DOP > DON > DOC, with bioavailable DOC (BDOC) representing 22 ± 12% (mean ± SD) of the total DOC, bioavailable DON (BDON) 35 ± 13% of the total DON and bioavailable DOP (BDOP) 70 ± 18% of the total DOP. This suggests that the role of DOM on the recycled and export production of the coastal ocean is more relevant for the P than for the N and for the N than for the C biogeochemical cycles. First‐order microbial degradation rate constants (κ) of BDOM (normalized to 15°C) increased significantly (p < 0.05) in the same sequence, with κC being 0.066 ± 0.065 day−1, κN 0.111 ± 0.096 and κP 0.154 ± 0.137 day−1. Significant (p < 0.001) power relationships were found among κC, κN and κP (R2 = 0.84–0.87). The C: N: P molar ratio of the DOM that resists microbial degradation was extremely depleted in N and P, 2835 (±3383): 159 (±187): 1, compared with the BDOM fraction, 197 (±111): 25 (±16): 1. The flushing time (τ) of the coastal ocean in relation to the turnover time of BDOM (1/κ), i.e., τ · κ, dictates the fate ―degradation versus accumulation― of the large scale export of BDOM, which could fuel parts of the oceanic new production and influence the N/P limitation of the open ocean.