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

Chung-Chi Chen,Gwo-Ching Gong,Chih-Ching Chung,Chih-Hao Hsieh,Fuh-Kwo Shiah,Kuo-Ping Chiang,Wen-Chen Chou

doi:10.5194/bg-14-2597-2017

Abstract

Abstract. This study was designed to determine the effects of flooding on a pelagic ecosystem in the East China Sea (ECS) with a focus on plankton activity and plankton community respiration (CR). In July 2010, a flood occurred in the Changjiang River. As a comparison, a variety of abiotic and biotic parameters were monitored during this flooding event and during a non-flooding period (July 2009). During the flood, the Changjiang diluted water (CDW) zone covered almost two-thirds of the ECS, which was approximately 6 times the area covered during the non-flooding period. The mean nitrate concentration was 3-fold higher during the 2010 flood (6.2 vs. 2.0 µM in 2009). CR was also higher in the 2010 flood: 105.6 mg C m−3 d−1 vs. only 73.2 mg C m−3 d−1 in 2009. The higher CR in 2010 could be attributed to phytoplankton respiration, especially at stations in the CDW zone that were not previously characterized by low sea surface salinity in 2009. In addition, zooplankton (> 330 µm) were another important component contributing to the high CR rate observed during the 2010 flood; this was a period also associated with a significant degree of fCO2 drawdown. These results collectively suggest that the 2010 flood had a significant effect on the carbon balance in the ECS. This effect might become more pronounced in the future, as extreme rainfall and flooding events are predicted to increase in both frequency and magnitude due to climate change.

Highlights

Riverine runoff has a profound effect on the production and consumption of organic carbon in coastal ecosystems (e.g., Dagg et al, 2004; Hedges et al, 1997 and the references therein)
As previously stated, compared to the 2009 non-flooding period, the increase in the community respiration (CR) rate in the 2010 flood might be associated with the increase in phytoplankton biomass (Fig. 4a). These results indicate that the significant amount of fugacity of CO2 (f CO2) absorption in the 2010 flood was related to the strength of plankton activity, phytoplankton at stations that were not characterized by low sea surface salinity (SSS) in the 2009 non-flooding period
During the 2010 flooding of the Changjiang River, a large quantity of freshwater was discharged into the East China Sea (ECS), and the Changjiang diluted water (CDW) zone covered almost two-thirds of the continental shelf; this represents a 6-fold larger area than during a more typical non-flooding period (2009)

Summary

Introduction

Riverine runoff has a profound effect on the production and consumption of organic carbon in coastal ecosystems (e.g., Dagg et al, 2004; Hedges et al, 1997 and the references therein). Accompanying freshwater discharge, a substantial amount of dissolved inorganic nutrients (DINs) is routinely dispensed into coastal regions, enhancing primary productivity (PP; e.g., Dagg et al, 2004; Nixon et al, 1996). 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). Under projected climate change scenarios, such heavy freshwater discharge events are predicted to become even more pronounced in the near future because of the dramatic frequency and magnitude increases in extreme rainfall events and floods predicted to occur throughout the world in the coming decades (Christensen and Christensen, 2003; Knox, 1993; Milly et al, 2002; Palmer and Ralsanen, 2002)

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