Large seasonal variations in distributions, sources, and fluxes of dissolved amino acids in the continental shelf water

Abstract

We measured seasonal variations in dissolved organic carbon (DOC) and total dissolved amino acids (TDAA)—a major component of bioavailable dissolved organic matter (DOM)—in the continental shelf water of the northwestern Pacific marginal seas, including the Yellow Sea and East China Sea. We show that the TDAA concentrations, especially L-amino acids (L-AA), are predominantly dependent on phytoplankton production, showing elevated values in spring and autumn. Conversely, based on a ²²⁸Ra tracer, we found that the concentrations of D-amino acids (D-AA), as well as DOC, are mainly dependent on benthic inputs, with elevated values in autumn and winter. AA-derived indices also show higher DOM bioavailability in spring relative to winter. The TDAA benthic flux (∼3.5 Gmol y^-1), which was comparable with phytoplankton production, was about 2-fold higher than its flux from the Changjiang River discharge (∼1.8 Gmol y^-1). These findings highlight the crucial role of benthic inputs in controlling the composition and bioavailability of DOM in continental shelf waters.