| We analyzed the degradation and transformation of seawardly-transported dissolved organic matter (DOM) under different hydrodynamic conditions in order to investigate the factors associated with both the Yangtze Estuary (YE) and the East China Sea (ECS). The distribution and composition of Dissolved Organic Carbon (DOC), Monosaccharides (MCHO), Polysaccharides (PCHO) and Total Carbohydrate (TCHO) were measured from several cruises and experiments about degradation process of DOM from the dead phytoplanktons were also conducted. We measured the concentration of DOM off the YE in August 2013 and of field samples (DOC, MCHO, PCHO and TCHO) in the YE and the ECS shelf in May, August and October 2013. At the same time, we also collected groundwater samples from the near shore areas.The main conclusions are as follows:1. Early degradation process of DOM released by dead phytoplanktons the YE was studied.Based on the incubation experiments fresh organic matters released by dead phytoplanktons (dominated by diatoms) which were collected from the YE during August 2013. We found that concentrations of DOC and dissolved oxygen (DO) decreased exponentially and TCHO was simultaneously degraded. The ratios of TCHO/DO、PCHO/TCHO, MCHO/TCHO can be used to assess the freshness of DOM. We also discovered that higher the initial concentration of DOC larger is the rate constants k(DOC), k(DO) of degradation. Moreover, both the reactivity of DOC and the concentration of DO also has an impacts on k(DOC). There was a significant increase in bacterial abundance during the incubation experiments. Control experiments where HgCl2 was added showed that the bacteria played an important role in the degradation process. The results showed that the rapid degradation of DOM followed by the consumption of DO had a significant influence on the formation of hypoxic environment in YE.2. The distribution characteristics of DOM and its influencing factors in the YE and the ECS.DOC and carbohydrates analysed from samples collected at Xuliujing station (XLJ) showed obvious seasonal variation. MCHO was the main component of carbohydrates, and MCHO/TCHO was always more than 60%. TCHO reached its peak value during spring, which is an important contributor to the DOC.The distribution of DOC was found to be higher towards the landward side but lower in the deeper waters of ECS shelf during spring, summer and autumn. The influencing factors of DOC distribution were seasonally different. DOC had a good correlation with salinity, density and DO in the southern study area while no relationship is observed in the northern study area of May 2013. While in August 2013, the concentration of DOC decreased with increasing salinity, but within the salinity range of 28 to 30, the concentrations of DOC in some stations increased, which indicated the gradual removal of terrestrial organic matter during seaward transport and the supplementary addition from marine organic matter. The vertical distribution of DOC at stations with depths more than 1000 km was mainly controlled by the temperature, density and DO while salinity was irrelevant. Moreover, there was no apparent relationship between the distribution of DOC and other factors (temperature, salinity, density and DO) in October 2013.We selected section C as our typical section to focus on the distribution of DOM in the ECS shelf. We found that the concentrations of DOC in May and October were similar, while lower values were observed in August 2013. TCHO and MCHO reached their highest concentration May, while similar concentration were observed in August and October 2013. TCHO/DOC ratio was in the range of 19%～22% for all the seasons, which suggests that the composition of DOM is stable in the ECS. In August 2013, MCHO, TCHO and DOC all showed good conservative mixing behavior while non-conservative mixing behavior was observed for PCHO in sections O and C. Also, the concentration of DOC and carbohydrates, the ratio of PCHO/DOC and TCHO/DOC in the hypoxic zone were significantly higher than those of the non-hypoxic zone, which suggested that differences in organic matters composition was an important factor contributing to the formation of hypoxic zone in the northern part of ECS.3. The groundwater has an influence on the distribution of DOM in its surrounding water bodies.Concentration of DOC was higher from surface to bottom and from North to South in the ECS in October 2013 and MCHO is the dominant carbohydrate. The concentration of groundwater DOC was in the range of 22.1 to 193.5 μM C. In the shores near the ECS and the ratio of TCHO/DOC varied from 15% to 39%. The flux of DOC from the groundwater into the ECS was 1.4 × 1010 mol contributing to 10% of the Yangtze River DOC input flux every year, which definitely cannot be ignored. The concentration of DOM was higher in the northern area than that in the southern area. The factors controlling the distribution of DOM are suggested to be the yellow water masses, the particle size of sediment and groundwater discharge. |
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