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 1998, June 2001, June and August 2003, and June and July 2004. It was found that the spatial patterns of inorganic nutrients (e.g. nitrate: <0.15 to 28 μM), chlorophyll concentrations (chl a: <0.20 to 16 mg m3) and BGR (<0.03 to 1.05 d-1) 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. © Inter-Research 2006.