厦门湾水体中颗粒有机碳的垂向输出通量:234Th/238U不平衡的应用

对厦门湾塔角附近海域某站位叶绿素 a、POC、初级生产力、234Th/238U不平衡进行的周日变化研究表明,POC含量介于14.4～34.6 mmol/m3之间,其中碎屑有机碳与活体有机碳所占份额分别为74%～92%和8%～26%.POC垂直分布呈现由表及底降低的趋势,且白昼期间POC含量高于晚间,说明研究海域POC含量与生物过程具有密切联系.初级生产力水平在1d之中变化达5倍,垂直分布亦随深度增加而降低,与叶绿素a的变化相对应.短时间(2h)培养获得的初级生产力水平明显高于长时间培养(24 h)的结果,证实部分新固定的碳被优先呼吸排出.结合234Th/238U不平衡法获得的颗粒态234Th输出通量及输出界面颗粒物中的POC/P_Th比值,可计算出真光层 POC的垂向输出通量为16.0mmol/(m2·d),其中碎屑有机碳与活体有机碳贡献的数量分别为13.3和2.7mmol/(m2·d).POC输出通量与初级生产力的比值(ThE比值)平均为0.31,真光层POC停留时间平均为11d.上述结果与Aksnes和Wassmann1]的模型计算结果相吻合,但与其他大多数模型的结果仍存在一定的差异.

Particulate organic carbon export fluxes estimated from 234Th/238U disequilibria in the Xiamen Bay

The daily variations of Chl.a,POC,primary production and ²³⁴Th/²³⁸U disequilibria in the Xiamen Bay were studied.POC concentration in the study area ranged from 14.4 to 34.6 mmol/m³,with an average concentration of 21.6 mmol/m³.The contributions of living organic carbon(phytoplankton) and organic detritus were estimated to be about 8%-26% and 74%-92% of the total POC,respectively.The profiles of POC showed a gradually decrease with the depth,and its temporal variation showed that the POC concentrations in daytime are higher than those in nighttime.Both features suggested that POC is closely related to biological processes in the Xiamen Bay.The primary production in the study area varied up to 5-folds in 1 d,which was consistent with that of the biomass.Primary production also decreased with the depth,was coincident with the patterns of Chl.a and POC.In the meantime,we studied the effect of incubation time on the determination of primary production.The primary production calculated from short-time incubation(2 h) is higher than that from long-time incubation(24 h),indicating some of the new fixed carbon are preferentially respired and then excreted.Based on the particulate ²³⁴Th export fluxes from ²³⁴Th/²³⁸U disequilibria and the POC/P_Th,ratios in particles,the Pox-export flux from euphotic zone is estimated to be 16.0 mmol/(m²·d),in which the export fluxes of living organic carbon and detrital organic carbon are 2.7 and 13,3 mmol(m²·d),respectively.The ratio between POC export flux and primary production,referred as ThE ratio,is 0.31.Both POC export fluxes and ThE ratios are consistent with the prediction value from relation formula presented by Aksnes and Wassmann in 1993,but not with other models.The residence time of POC in the euphotic zone was estimated to be 11 d,indicating a rapid regeneration rate of POC in study sea area.