The occurrence of extreme weather conditions appears on the rise under current climate change conditions, resulting in more frequent and severe floods. The devastating floods in southern China in 2010 and eastern Australia 2010–2011, serve as a solemn testimony to that notion. Accompanying the excess runoffs, elevated amount of terrigenous materials, including nutrients for microalgae, are discharged to the coastal ocean. However, how these floods and the materials they carry affect the coastal ocean ecosystem is still poorly understood. Yangtze River (aka Changjiang), which is the largest river in the Eurasian continent, flows eastward and empties into the East China Sea. Since the early twentieth century, serious overflows of the Changjiang have occurred four times. During the two most recent ones in July 1998 and 2010, we found total primary production in the East China Sea reaching 147 × 103 tons carbon per day, which may support fisheries catch as high as 410 × 103 tons per month, about triple the amount during non‐flooding periods based on direct field oceanographic observations. As the frequencies of floods increase world wide as a result of climate change, the flood‐induced biological production could be a silver lining to the hydrological hazards and human and property losses inflicted by excessive precipitations.

[1] To explore the paradox that the heterotrophic waters of the East China Sea (ECS) shelf act as a significant CO 2 sink in summer, vertical structures of carbon chemistry and hydrography were examined in July 2007. The results show that waters above the pycnocline ($10 to 30 m) in the major CO 2 sink area are supersaturated with oxygen (110 ± 7%; autotrophic) but undersaturated with respect to atmospheric CO 2 (DfCO 2 = À130 ± 58 matm; sink). In contrast, waters below the pycnocline are undersaturated with respect to oxygen (61 ± 16%; heterotrophic) but supersaturated with CO 2 (DfCO 2 = 116 ± 115 matm; source). This demonstrates that summer stratification is the key factor maintaining the CO 2 sink status in the heterotrophic ECS shelf waters. Furthermore, the shallow pycnocline can easily be broken down when strong mixing occurs, potentially allowing the respired CO 2 stored in the subsurface waters to return to the atmosphere.

To understand carbon cycling in marginal seas better, particulate organic carbon (POC) concentrations, POC fluxes and primary production (PP) were measured in the East China Sea (ECS) in summer 2007. Higher concentrations of POC were observed in the inner shelf, and lower POC values were found in the outer shelf. Similar to POC concentrations, elevated uncorrected POC fluxes (720–7300 mg C m⁻² d⁻¹) were found in the inner shelf, and lower POC fluxes (80–150 mg C m⁻² d⁻¹) were in the outer shelf, respectively. PP values (~ 340–3380 mg C m⁻² d⁻¹) had analogous distribution patterns to POC fluxes, while some of PP values were significantly lower than POC fluxes, suggesting that contributions of resuspended particles to POC fluxes need to be appropriately corrected. A vertical mixing model was used to correct effects of bottom sediment resuspension, and the lowest and highest corrected POC fluxes were in the outer shelf (58 ± 33 mg C m⁻² d⁻¹) and the inner shelf (785 ± 438 mg C m⁻² d⁻¹), respectively. The corrected POC fluxes (486 to 785 mg C m⁻² d⁻¹) in the inner shelf could be the minimum value because we could not exactly distinguish the effect of POC flux from Changjiang influence with turbid waters. The results suggest that 27–93% of the POC flux in the ECS might be from the contribution of resuspension of bottom sediments rather than from the actual biogenic carbon sinking flux. While the vertical mixing model is not a perfect model to solve sediment resuspension because it ignores biological degradation of sinking particles, Changjiang plume (or terrestrial) inputs and lateral transport, it makes significant progress in both correcting the resuspension problem and in assessing a reasonable quantitative estimate of POC flux in a marginal sea

Abstract.To assess the impact of rising atmospheric CO 2 and eutrophication on the carbonate chemistry of the East China Sea shelf waters, saturation states ( ) for two important biologically relevant carbonate minerals -calcite ( c ) and aragonite ( a ) -were calculated throughout the water column from dissolved inorganic carbon (DIC) and total alkalinity (TA) data collected in spring and summer of 2009. Results show that the highest c (∼ 9.0) and a (∼ 5.8) values were found in surface water of the Changjiang plume area in summer, whereas the lowest values ( c =∼ 2.7 and a =∼ 1.7) were concurrently observed in the bottom water of the same area. This divergent behavior of saturation states in surface and bottom waters was driven by intensive biological production and strong stratification of the water column. The high rate of phytoplankton production, stimulated by the enormous nutrient discharge from the Changjiang, acts to decrease the ratio of DIC to TA, and thereby increases values. In contrast, remineralization of organic matter in the bottom water acts to increase the DIC to TA ratio, and thus decreases values. The projected result shows that continued increases of atmospheric CO 2 under the IS92a emission scenario will decrease values by 40-50% by the end of this century, but both the surface and bottom waters will remain supersaturated with respect to calcite and aragonite. Nevertheless, superimposed on such decrease is the increasing eutrophication, which would mitigate or enhance the decline caused by anthropogenic CO 2 uptake in surface and bottom waters, respectively. Our simulation reveals that, under the combined impact of eutrophication and augmentation of atmospheric CO 2 , the bottom water of the Changjiang plume area will become undersaturated with respect to aragonite ( a =∼ 0.8) by the end of this century, which would threaten the health of the benthic ecosystem.