The Primary Study On Amino Acids In Different Molecular Fractions Of DOM In The Changjiang River Estuary And Its Adjacent Sea

The high molecular weight dissolved organic matters and their organic compositions, collected from the Changjinag estuary and its adjacent sea, were examined and compared to the aging seawater which preserved in the dark condition in the lab for years. Dissolved organic matters (DOM) were fractionated by ultrafiltration into 30kDa-0.45μm,1k—30kD and <1k(DOM_LMW) molecular weight fractions. The DOC and amino acids (AAs) concentration in bulk DOM and three molecular weight fractions were also determined in order to proving the complementary information on DOM source, the impact of biological processing and the transport of organic material through the estuarine interface, The results were showed as following:The DOC concentrations in the Changjiang estuary and its adjacent sea were rang from 1.19 to 4.85 μmol/L, accounting for 13.8～67.5% of 30 kDa-0.45μm, 12.5～40.4% of 1k - 30kDa and 35.5 ～ 80.0% of DOM_LMW, in the summer. Compared from the DOC concentration of 3 fractions, the DOC decreases rapidly in the Changjiang estuary where salinity < 3, is mainly attributed to that the DOM which molecular > 1 kDa were flocculated to Particulate form while the DOM_LMW were diluted in this region. In the turbidity maximum (TM) of the Changjiang estuary, DOC concentrations increased along with the salinity increasing, and much rapidly near the salinity 15 isoline where 30 kDa—0.45μm molecular fraction account for 67.5% of bulk DOC, indicated that the DOC from desorption of Particulate contributed the most part of the bulk DOC in this region. However, the DOC compositions showed a strong changes in fractions, even the DOC concentrations were consistent after the flocculation and desorption processes in the estuary.The concentrations of total dissolved amino acids (TDAAs ) in the surface water along the Changjiang PN section were rang from 1.19～4.85μmol/L, accounting for 1.8～5.4% of 30 kDa-0.45μm, 2.0～18.0% of 1k-30kDa (DOM_LMW-AA) and 79.8 ～ 93.2% of DOM_LMW, in summer. Meanwhile, the concentrations of TDAAs and DOM_LMW-AA increased along the salinity isoline increasing obviously. The dominant amino acids were Ala, Gly and Ser, which accounting for 2.26 ± 6.08%, 16.27 ± 3.54% and 12.13 ± 3.59% in the fresh seawater, respectively. The Mole percentage of Asp, Glu, Thr and Arg were higher in the fresh seawater than in the aging seawater, respectively, but the Ala, Gly and Phe were contrary.The degradation index (DI) of amino acids in DOM increased with salinaty in the Changjiang estuary and its adjacent sea. The DI of DOM_LMW was lower than that ofthe fraction of >1 kDa DOM in the both fresh seawater and aging seawater, which indicated that the degradation degree of DOM was stonger when its particular size became small.In the plume region of the Changjiang estuary, the DI of DOM was higher than in the Changjiang River water and the aging seawater, but closed to plankton DI. Obviously, and the TDAAs originated from the high primary productivity in this region. Furthermore, the concentrations of TDAAs and DOM_LMW -AAs had a good correlations to Chl-a content, picoplankton, nanophyplankton and viruses biomasses, suggested that microbialfood web may control the key processes of the production and consumption of TDAAs' in the ocean. In all of the plankton refered above, the picoplankton might dominate those processes, which were proved by that the correlation indexes between its biomass to TDAAs and DOM_LMW -AAs were 0.96 to 0.97, respectively. As the dominant population of picoplankton in the Changjiang estuary and its adjacent sea, Prochlorococcus spp may pay a key role in the biogeochemical processes of DOM cycle.