The large variation in organic carbon consumption in spring in the East China Sea

[摘要] A tremendous amount of organic carbon respired by plankton communities hasbeen found in summer in the East China Sea (ECS), and this rate has beensignificantly correlated with fluvial discharge from the Changjiang River.However, respiration data has rarely been collected in other seasons. Toevaluate and reveal the potential controlling mechanism of organic carbonconsumption in spring in the ECS, two cruises covering almost the entire ECSshelf were conducted in the spring of 2009 and 2010. These results showedthat although the fluvial discharge rates were comparable to the highriverine flow in summer, the plankton community respiration (CR) variedwidely between the two springs. In 2009, the level of CR was double that of2010, with mean (± SD) values of 111.7 (±76.3) and 50.7 (±62.9) mgC m⁻³ d⁻¹, respectively. The CR was positively correlated withconcentrations of particulate organic carbon and/or chlorophyll a (Chl a)in 2009 (all p < 0.01). These results suggest that the high CR rate in2009 can be attributed to high planktonic biomasses. During this period,phytoplankton growth flourished due to allochthonous nutrients dischargedfrom the Changjiang River. Furthermore, higher phytoplankton growth led tothe absorption of an enormous amount of fugacity of CO₂ (fCO₂) inthe surface waters, even with a significant amount of inorganic carbonregenerated via CR. In 2010, even more riverine runoff nutrients weremeasured in the ECS than in 2009. Surprisingly, the growth of phytoplanktonin 2010 was not stimulated by enriched nutrients, and its growth was likelylimited by low water temperature and/or low light intensity. Low temperaturemight also suppress planktonic metabolism, which could explain why the CR waslower in 2010. During this period, lower surface water fCO₂ may havebeen driven mainly by physical process(es). To conclude, these resultsindicate that high organic carbon consumption (i.e. CR) in the spring of 2009could be attributed to high planktonic biomasses, and the lower CR rateduring the cold spring of 2010 might be likely limited by low temperature inthe ECS. This further suggests that the high inter-annual variability oforganic carbon consumption needs to be kept in mind when budgeting the annualcarbon balance.

[学科分类] 地球化学与岩石