东海浅陆架区溶解有机氮的时空分布特征及生物可利用性

通过2014年5~6月(春末夏初季)和9~10月(秋季)在东海浅陆架区的现场调查，分析该海域DON来源和时空分布特征，结合同步进行的船基受控生物降解培养实验，评价了该海域两个季节DON的生物可利用性。春末夏初季和秋季东海浅陆架区DON为总溶解态氮(TDN)的主要存在形式，在TDN中占比均值分别为57.88%±21.42%和66.09%±19.74%。受陆源输入和黑潮水涌升及浮游生物分泌和微生物降解作用共同影响，表层海水DON春末夏初季呈现近岸高、远岸低的变化特征，高值区与长江冲淡水影响范围基本一致；秋季高值区则位于该海域的东北部和东南部；而受长江冲淡水影响显著的调查海域北部DON浓度高于南部；从水体表层到底层DON浓度及其在TDN中的占比都呈现逐步降低的变化趋势。该海域DON的生物可利用性具有较大时空差异性，具有高生物可利用性的DON(L-DON)含量基本呈现近岸高、远岸低的分布特征，春末夏初季和秋季受陆源输入影响显著的海域中L-DON在DON中占比分别达到28.3%和23.6%，高于受陆源输入影响小的海域的1.3%和18.9%。

Spatial-temporal distribution and bioavailability of dissolved organic nitrogen in the shallow continental shelf of the East China Sea

In order to investigate the temporal-spatial distributions of DON in the shallow shelf of the East China Sea (SSESC), two cruises were implemented in late spring to early summer (25 May to 5 June) and in fall (18 October to 29 October) 2014, respectively. At the same time, the microcosm experiments on DON biodegradation were conducted on the ship board synchronously to evaluate the bioavailability of DON in the two seasons. The investigation results showed that DON comprised the majority of TDN in SSECS. The average proportion of DON to TDN in late spring to early summer and in fall were 57.88%±21.42% and 66.09%±19.74%, respectively. The spatial distributions of DON were the consequence of joint contribution of the terrigenous input, the Kuroshio upwelling as well phytoplankton secretion and bacterial degradation. During late spring to early summer, DON concentration decreased from inshore to offshore, and the regions with high value of DON were consistent with the direction of the Yangtze River diluted water (YZRDW). While in fall, the regions with high value of DON were located in the northeast and southeast of the SSESC. On a whole, the average DON concentration in the north of SSESC was higher than that in the south, due to the influence of the YZRDW. From the surface layer to the bottom of the water column, the supply of DON by the phytoplankton photosynthesis decreases while degradation of DON by bacteria increased, resulting that the concentration of DON and its proportion to total dissolved nitrogen (TDN) decreased gradually. The temporal-spatial distribution of bioavailability of DON also varied significantly. The labile DON (L-DON)in the surface layer showed a decreasing pattern from inshore to offshore. The proportion of L-DON to DON in the region influenced obviously by terrigenous input accounted for 28.3%in late spring to early summer and 18.9% in fall, which was significantly higher than that in the region influenced weakly by terrigenous input.