Temporal and spatial distribution of dissolved amino acids in the surface microlayer and subsurface water of the Bohai Sea and the Yellow Sea

Concentrations of total hydrolyzed amino acids (THAA) and chlorophyll a (Chl-a) in the Bohai Sea (BS) and Yellow Sea (YS) were determined from August to September 2015 (summer) and from January to February 2016 (winter). The average concentrations of THAA in the surface microlayer and subsurface water in the study area were 2.92 (1.14 to 7.31) and 1.64 (0.49 to 3.32) μmol/L in summer, respectively, and 1.01 (0.69 to 1.70) μmol/L in the subsurface water during winter. The spatial distributions of THAA were obviously influenced by phytoplankton biomass and terrestrial input, while vertical distributions of THAA were mainly affected by circulation and phytoplankton. The main constituents of THAA were aspartic acid (Asp), glycine (Gly), serine (Ser), glutamic acid (Glu) and alanine (Ala) during summer and winter. THAA and Chl-a were more abundant in the surface microlayer than in the subsurface water, with average enrichment factors (EFs) of 2.01 and 2.06, respectively. Enrichment of THAA and Chl-a in the surface microlayer is the result of a combination effect, such as the influence of phytoplankton, wind action and mixing processes. This research may have implications for the distribution, enrichment and source of THAA in the BS and YS.