Distributions and dynamics of dissolved carbohydrate species in Changjiang Estuary and the adjacent East China Sea

Water samples were collected from the Changjiang Estuary (CE) and adjacent East China Sea (ECS) during two cruises in March 2013 and July 2014 to investigate the distributions and dynamics of dissolved carbohydrate species. The concentrations of surface dissolved organic carbon (DOC) and total dissolved carbohydrates (TCHO) were higher in summer compared with those in spring. The dissolved polysaccharides (PCHO) in surface waters accounted for 65 ± 14% and 47 ± 18% of TCHO on average in spring and summer, respectively. The average TCHO/DOC ratio in summer was 17 ± 5%, compared with 11 ± 3% in spring. DOC and dissolved monosaccharides (MCHO) were significantly correlated with salinity in both seasons, indicating that physical mixing was a major controller of the DOC and MCHO distributions. By contrast, PCHO exhibited greater variations with increasing salinity, which suggest that MCHO and PCHO had different features in the study area. According to principal component analysis, PCHO were significantly correlated with the second principal component “biological factors” and they had the opposite loading with nutrients, especially NO2−, which suggest that PCHO were affected greatly by nutrient-related biological processes, such as primary production and bacterial assimilation. By contrast, MCHO were affected mainly by physical factors, especially terrigenous inputs. After comparing the stations with similar salinity along the Changjiang diluted water transport pathways during summer, we observed the obvious production of PCHO and DOC, as well as the consumption of nutrients, which further demonstrated biological impacts on the PCHO concentrations. Furthermore, the increase in PCHO was only half that of DOC, demonstrating the production of other organic matter via the transformation of PCHO, or other biological processes. However, the MCHO concentrations remained constantly high in Changjiang diluted water patches in CE, and appeared to decrease with dilution, suggesting that these molecules persisted rapid microbial turnover. In general, the dynamics of MCHO and PCHO were very different in the study area. MCHO exhibited conservative behavior, whereas PCHO were highly bioactive and influenced greatly by biological factors in the CE and adjacent ECS.