Effects of ChangJiang River summer discharge on bottom-up control of coastal bacterial growth

Abstract

The East China Sea (ECS) has one of the largest shelf ecosystems in the world, and among the major external forces that affect physical and biogeochemical processes over the ECS shelf is discharge from the ChangJiang (CJ) River. This is particularly true during summer when prevailing flooding leads to the annual maxima of CJ River discharge. To explore the effects of interannual variations in CJ River summer discharge on bacterial growth rate (BGR), 6 cruises were conducted over the entire width of the ECS shelf in July ) were all negatively correlated with salinity (22.4 to 34.9 psu) during each cruise, a clear sign that river discharge had a significant effect on chemical and biological structures over the ECS shelf. Variations in BGR in the mixing zone (salinity < 33 psu) and oceanic zone (salinity > 33 psu) were positively correlated with chl a. However, the intercepts and slopes of the BGR-chl a relationship in the mixing zone were significantly higher than those in the oceanic zone. Noteworthy too is that on monthly and/or inter-annual scales, BGR-chl a coupling (i.e. the slope) in both zones changed positively with river discharge, and this coupling of the oceanic zone seems to have been more sensitive to changes in discharge. This suggests that bacterial growth in the oligotrophic zone might be more substrate-limited than that in the mixing zone. Our study is one of the few to suggest that, in summer, monthly and/or inter-annual variations in CJ River discharge might significantly alter the supply rates of inorganic nutrients and dissolved organic matter, which in turn affects the relationship between auto-and hetero-trophic processes in the ecosystem of the ECS shelf.

Spatial variations in time‐integrated plankton metabolic rates in Sagami Bay using triple oxygen isotopes and O2:Ar ratios

“…N : G ratios were exceptionally low in the northern basin (0.07 to 0.1) where the Sakawa River discharged. Shiah et al (2006) found that bacterial growth rate increases with increase in river discharge from the Chang-Jiang River to the East China Sea. The low ratios in the northern basin could thus be due to enhanced bacterial respiration associated with freshwater discharge.…”

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

“…Thus, our understanding of global carbon cycling in the coastal oceans depends on our knowledge of how large river discharges influence ecological processes in the adjacent sea. One such large river system, on which there is limited information, is the Changjiang River, which discharges into the East China Sea (ECS) [e.g., Gong et al , 2006; Shiah et al , 2006].…”

Hydrology and Biota Interactions as Driving Forces for Ecosystem Functioning

“…Yet, it is worth noting that direct relationships between bacterial growth and river flow may be explained by different causal mechanisms: a bacterial growth stimulation by riverine DOM, by phytoplankton-derived DOM, or both (see Figure 6). The former explanation probably applies to the East China Sea ecosystem (see Figure 6(b); Shiah et al, 2006).…”

“…Episodic meteorological events (e.g., wind storms and high rainflow episodes), for example, stimulate heterotrophic bacterioplankton due to increased availability of organic matter and/or inorganic nutrients associated to increased con tinental runoff or benthic resuspension (Cotner et al, 2000;Grémare et al, 2003;Alonso-Saéz et al, 2008;Solic et al, 2009). Variability in river discharge rates, usually considered a proxy for organic carbon flux into estuarine systems (Jassby et al, 1993), is also positively related to bacterial production and growth over seasonal (Murrell et al, 1999;Schultz et al, 2003) and interannual time scales (Murrell, 2003;Shiah et al, 2006;Barrera-Alba et al, 2009). In some estuaries, a shift in ecosystem metabolism toward heterotrophic dominance is observed under increased freshwater discharge rates (see Hitchock et al, 2010).…”

“…The establishment and evaluation of flood-management practices should take into account that river-flow patterns affect not only phytoplankton (primary producers) but also heterotrophic bacteria (consumers). Indeed, the construction of the Three Gorges Dam and related river-discharge reduction was recently associated to a signifi cant decrease of bacterial growth rates in adjacent coastal and oceanic ecosystems waters (East China Sea; see Figure 6(b); Shiah et al, 2006). Regardless of the source of autochthonous (e.g., phytoplank ton, microphytobenthos, and macrophytes) and allochthonous (e.g., continental runoff, riverine inputs, and atmospheric deposition) organic carbon assimilated by heterotrophic bacter ioplankton, climate, and hydrodynamic processes are, inevitably, fundamental determinants of DOM fluxes into estuarine and coastal ecosystems.…”