Strong linkages between surface and deep water dissolved organic matter in the East/Japan Sea

Vertical and horizontal distributions of total hydrolysable amino acids (THAA), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) were measured in the East/Japan Sea (EJS). The euphotic zone of this sea is N-limited, and the N : P ratio is ~ 13 below 200 m depth. Elevated THAA concentrations (137 ± 34 nM) and DOC-normalized yields (0.8 ± 0.2 % of DOC) were observed in deep waters (≥ 1000 m) of the EJS compared with those in the deep North Pacific Ocean. Significantly high THAA concentrations and yields were observed in a region of deep-water formation, indicating the convection of margin-derived bioavailable dissolved organic matter (DOM) to deep waters. Declining THAA concentrations (36 ± 12 %) and yields (33 ± 13 %) were observed between 1000–3000 m throughout the EJS, indicating the utilization of bioavailable DOM in deep waters. Concentrations of the D-enantiomers of amino acids (Ala, Glx, Asx, and Ser) were relatively high in deep waters of the EJS, indicating substantial bacterial contributions to DOM from surface and upper mesopelagic waters. These observations suggest that the transport of bioavailable DOM to microbial food webs in deep waters of the EJS is sensitive to changes in deep-water renewal rates.