Effects of the Changjiang (Yangtze) River discharge on planktonic community respiration in the East China Sea

Abstract

[1] Planktonic communities tend to flourish on the western margins of the East China Sea (ECS) fueled by substrates delivered largely from the Changjiang River, the fifth largest river in the world. To study the effects of the Changjiang River discharge on planktonic community respiration (CR), physical-chemical variables and key processes were measured in three consecutive summers in the ECS. Results showed that concentrations of nitrate and Chl a, protozoan biomass, bacterial production, as well as CR in the surface water were all negatively correlated with sea surface salinity, reflecting the strong influence of river discharge on the ECS shelf ecosystem. Moreover, mean values of nitrate, Chl a concentrations, and CR rates were proportionally related to the area of Changjiang diluted water (CDW; salinity 31.0 practical salinity units (psu)), an index of river discharge rate. Presumably, higher river flow delivers higher nutrient concentrations which stimulate phytoplankton growth, which in turn fuels CR. CR exhibited significant monthly and interannual variability, and rates appear to be dominated by bacteria and phytoplankton. Although the plankton community was relatively productive (mean = 0.8 mg C m À2 d À1) in the CDW, the mean ratio of production to respiration was low (0.42). This suggests that the heterotrophic processes regulating CR were supported by riverine organic carbon input in addition to in situ autotrophic production.

The influence of episodic flooding on a pelagic ecosystem in the East China Sea

“…Plankton CR was also measured onboard from seawater subsamples. The methods used to collect the hydrographic data and analyze the aforementioned response variables followed Chen et al (2006Chen et al ( , 2013Chen et al ( , 2009). Descriptions of the methods used are presented briefly in the following sections.…”

“…Generally, the transparency of the coastal ocean might be low during flooding periods due to the riverine discharge of terrestrial matter. A low transparency value was documented in June 2003 in the ECS, during which the CDW area was 43.1 × 10 3 km 2 (∼ 40 % of the CDW area of the 2010 flood; Chen et al, 2009), and the average transparency values for the ECS and the CDW were 70.9 and 66.0 %, respectively (C.C. Chen, unpublished data).…”

“…In general, a large quantity of dissolved inorganic nutrients is delivered from the Chinese coast to the ECS during the wet season (May to September; Chen et al, 2013Chen et al, , 2009Gong et al, 1996). A high concentration of nitrates in the fluvial discharge of the Changjiang River was documented in the ECS during the 2010 flood.…”

“…Overall, the effects of river plumes on coastal ecosystems are strongly related to the volume of the freshwater discharged (e.g., Chen et al, 2009;Dagg et al, 2004;Tian et al, 1993). Thus, understanding how freshwater discharge influences coastal ecological processes is an important factor in modeling global carbon cycling in the ocean.…”

Unique Dispersal of the Changjiang-Diluted Water Plume in the East China Sea Revealed from Satellite Monitoring of Colored Dissolved Organic Matter (CDOM)

“…Freshwater from the Changjiang discharged into the ECS usually mixes with warm, saline waters (the Taiwan Warm Current and the Kuroshio and its branch currents, which flow northeastward over the outer shelf) to form a low salinity water mass called the Changjiangdiluted water (CDW) (Beardsley et al 1985;Ichikawa and Beardsley 2002;Lie et al 2003). The CDW generally has a sea-surface salinity (SSS) value less than 31 or 32 on the ECS shelf (Chang and Isobe 2003;Chen et al 2009;Moon et al 2012). The Asian monsoon, which is southerly during summer, enhances the northeastward flow of the CDW plume.…”