Abstract
Abstract. A tremendous amount of organic carbon respired by plankton communities has been found in summer in the East China Sea (ECS), and this rate has been significantly correlated with fluvial discharge from the Changjiang River. However, respiration data has rarely been collected in other seasons. To evaluate and reveal the potential controlling mechanism of organic carbon consumption in spring in the ECS, two cruises covering almost the entire ECS shelf were conducted in the spring of 2009 and 2010. These results showed that although the fluvial discharge rates were comparable to the high riverine flow in summer, the plankton community respiration (CR) varied widely between the two springs. In 2009, the level of CR was double that of 2010, with mean (± SD) values of 111.7 (±76.3) and 50.7 (±62.9) mg C m−3 d−1, respectively. The CR was positively correlated with concentrations of particulate organic carbon and/or chlorophyll a (Chl a) in 2009 (all p < 0.01). These results suggest that the high CR rate in 2009 can be attributed to high planktonic biomasses. During this period, phytoplankton growth flourished due to allochthonous nutrients discharged from the Changjiang River. Furthermore, higher phytoplankton growth led to the absorption of an enormous amount of fugacity of CO2 (fCO2) in the surface waters, even with a significant amount of inorganic carbon regenerated via CR. In 2010, even more riverine runoff nutrients were measured in the ECS than in 2009. Surprisingly, the growth of phytoplankton in 2010 was not stimulated by enriched nutrients, and its growth was likely limited by low water temperature and/or low light intensity. Low temperature might also suppress planktonic metabolism, which could explain why the CR was lower in 2010. During this period, lower surface water fCO2 may have been driven mainly by physical process(es). To conclude, these results indicate that high organic carbon consumption (i.e. CR) in the spring of 2009 could be attributed to high planktonic biomasses, and the lower CR rate during the cold spring of 2010 might be likely limited by low temperature in the ECS. This further suggests that the high inter-annual variability of organic carbon consumption needs to be kept in mind when budgeting the annual carbon balance.
Highlights
The annual carbon buTdhgeet isCcrruycoiasl ipnhdeeterremining whether a system is a carbon sink or source
High fluvial discharge was observed in both spring seasons included in this study, and this can be demonstrated in the large area of the Changjiang Diluted Water (CDW; SSS 31.0), which is assumed to be an index of riverine runoff
The increasing trend of surface seawater temperature (SST) and SSS from the inner shelf to the slope was found in both spring seasons included this study, with the lowest values observed in the Changjiang plume region (Fig. 2)
Summary
The annual carbon buTdhgeet isCcrruycoiasl ipnhdeeterremining whether a system is a carbon sink or source. This issue is especially important in high productivity coastal and shelf ecosystems, which account for one-fifth to one-third of global marine primary productivity 2002; Duarte and Agustı, 1998; Walsh et al, 1981). Estimation of the annual carbon budget will depend on the production and consumption of organic carbon, it will be affected by other processes, e.g. transporting from terrestrial sources, mixing with other water bodies, benthic processes, etc. Highly seasonal and annual variability of organic carbon consumption make it even more difficult to budget the carbon balance (e.g. Chen et al, 2009; Smith and Kemp, 1995)
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