东海束毛藻的时空分布及其固氮、固碳作用研究

Trichodesmium spp. is widely distributed in the oligotrophic tropical and subtropical waters. Due to its strong ability for nitrogen fixation and carbon assimilation, Trichodesmium spp. has been recently paid more attention by many oceanographers. The Kuroshio and Taiwan Warm Current bring the East China Sea (ECS) abundant Trichodesmium spp. which frequently causes red tide. However, little is know about its distribution pattern, nitrogen and carbon fixation. Seven cruises are planned to carried out for a wide investigation of Trichodesmium spp. ecological situation in the year of 2009 (June and Augest), 2011 (four seasons), and 2013 (summer). in the continental shelf and Kuroshio of ECS. The objectives of present project are to (1) find out its species composition, community structure, and spatial-temporal change in horizontal and vertical distribution; (2) identify the main influence factors to its distribution combined with physical, chemical, and meteorologic parameters, and explore the relationship between the abundance distribution and the change of water mass and circulation; (3) uncover its long-time development and controlled factors in the Changjiang Estuary and its adjacent waters during summer, according to the historical data; (4) estimate its nitrogen and carbon fixation in the ECS through the depth-integrated abundance, rates of nitrogen and carbon fixation, and trichome elements (chlorophyll a, nitrogen, and carbon content). Then its contribution to the phytoplankton standing crop, primary and new production in the ECS can be analyzed as well. The study results contribute to a better understanding of the Trichodesmium spp. function in the nitrogen and carbon biogeochemical cycles in the ECS, and provide scientific basis for the red tide occurrence mechanism triggered by Trichodesmium spp. in this area.

束毛藻广泛分布于热带和亚热带的寡营养海域，因其较强的固氮和固碳能力，近来备受关注。东海区的束毛藻由黑潮和台湾暖流携带而来，且大量繁殖时易形成赤潮，但对其在东海的分布特征、固氮和固碳作用知之甚少。本项目拟通过2009年6月与8月、2011年四季和2013年夏季共7个航次的东海束毛藻生态现状调查，探明其在东海的种类组成、种群结构和分布特征。并结合同步理化参数和气象要素，确定束毛藻分布的主要影响因子，理清其分布与水团消长和环流变化的关系。同时，参考历史资料，研究长江口及其邻近海域夏季束毛藻的长期演变规律和主要受控因子。此外，利用其水柱积分丰度值与藻丝的固氮、固碳速率及碳、氮和叶绿素a含量，估算东海束毛藻的固氮、固碳量及其对浮游植物现存量、初级生产和新生产的贡献。项目成果可更深入了解的束毛藻在东海碳、氮生物地球化学循环中的作用，并可为揭示东海束毛藻爆发赤潮的驱动机制提供依据。

束毛藻广泛分布于热带和亚热带的寡营养海域，具有较强的固氮和固碳能力，但对束毛藻在东海的分布特征、固氮和固碳作用知之甚少。本项目通过2006-2014年共13个航次的调查发现，东海束毛藻有铁氏束毛藻、红海束毛藻、汉氏束毛藻和扭束毛藻，后者仅在陆架边缘的黑潮区出现，丰度也较低，且该种在东海陆架还未见报道，铁氏束毛藻占绝对优势，各季均占总丰度的85%以上。束毛藻种群基本由游离藻丝组成，群体状较少，主要在外海区（黑潮影响区）出现，且季节变化明显，在水温较高的夏、秋季，群体状束毛藻丰度分别占4.84%和7.79%，而在水温较低的冬春季，群体鲜有出现。拖网采集的束毛藻水柱丰度略低于CTD采水器采集，两者呈极显著的正相关，且并无显著差异，但网采样品中群体状的束毛藻丰度高于水采样品。2011年东海束毛藻水柱积分丰度季节变化呈夏季(10947.50×103簇/m2)＞秋季(7192.41×103簇/m2)＞春季(699.12×103簇/m2)＞冬季(659.21×103簇/m2)，全年平均值为4874.56×103簇/m2，较1970s以来的丰度有所升高。束毛藻丰度与温度、盐度和水深呈正相关，而与氮磷营养盐呈负相关，在贫营养的黑潮和台湾暖流控制区丰度较高，但在富营养的长江冲淡水区丰度较低，其垂直分布则表现为高值一般位于次表层或中层的30m和50m水深。根据2011年四季束毛藻的水柱积分丰度和叶绿素a，估算出春、夏、秋、冬季东海束毛藻对浮游植物现存量的贡献分别占0.31%、3.60%、3.15%、0.37%。根据夏、秋季不同藻丝形态束毛藻的真光层水柱丰度，参考国际上报道邻近东海黑潮区的束毛藻藻丝固氮速率，同时不计黑潮区发生束毛藻水华的站位，近似估算出夏、秋季东海束毛藻固氮速率分别为17.08和41.69 μmol N m-2d-1，进而估算出夏、秋季东海束毛藻的固氮量占总生物固氮量的81%和57%。若按Redfield比值（C:N=106:16）换算成束毛藻的生产力，则束毛藻对夏、秋季东海初级生产力的贡献分别仅为0.06%和0.48%，而对新生产力的贡献分别为2.52%和5.51%。该研究结果表明束毛藻对东海生物固氮作用影响较大，但对初级生产力和新生产力贡献较小。项目成果可为研究东海束毛藻的赤潮发生机制及其在碳、氮生物地球化学循环中的作用提供支撑。