棉兰老涡影响海域DCM层营养盐补充途径及关键控制过程

Extensive phytoplankton blooms have been frequently observed in euphotic layer of oligotrophic North Pacific Subtropical Gyr (NPSG) oceans recently. However, these events have not been fully understood yet, due to their spatial and temporal heterogeneity. In previous studies, several hypotheses for such phenomena were proposed based on long-term investigation and field experiments. Among all, eddy pumping is believed vital to induce proliferation of phytoplankton in eddy controlled waters via enhancing nutrients in euphotic layer from middle layer. In NPWB (North Pacific Western Boundary) in which several eddies exist due to its complexity of currents, similar phenomena was observed in euphotic layer of Mindanao Eddy (ME) controlled waters during comprehensive investigation cruises for western pacific in 2011, 2012 and 2014. Such phenomena were represented by much higher Chla concentration than in other waters of NPWB. Relevant nutrients results suggested different vertical structure and nutrient composition between ME and other waters, either. Studies implemented in such waters would expand our understanding on phytoplankton proliferation in euphotic layer of oligotrophic oceans scientifically. Proposal of this project is coincident with such idea. In this research, while targeting Deep Chlorophyll Maximum (DCM) in ME controlled waters, supplementary and key processes of nutrients would be studied, aiming to describe how nutrients were offered to and consumed by excessive phytoplankton. To fulfill this target, several tech. and methods would be cooperated together, including field investigation, in situ incubation, lab analysis and numerical simulation. As a result, this research would promote current understandings on extensive phytoplankton proliferation in euphotic layer of oligotrophic oceans.

近年研究发现北太平洋副热带环流区真光层内存在浮游植物大量增殖现象，且具有高度的时空异质特征。针对此现象的成因，在前期研究中提出了多种假设，其中涡流变化引起的中层营养盐涌升被认为是一个重要的原因，但目前对这一过程中营养盐补充及消耗机制的科学认识仍不充分。在NPSG西边界流区，2011、2012、2014年现场调查均发现在棉兰老涡影响海域真光层出现类似的高叶绿素现象，该海域营养盐垂直分布与组成结构也与其它海域迥异，存在营养盐等值线抬升现象。在这一区域开展研究，将加深对贫营养大洋真光层浮游植物大量增殖现象的科学认识。为此本研究将针对棉兰老涡影响海域，采用现场调查、现场实验、室内分析和模拟估算相结合的方法，研究该海域深层叶绿素最大层（DCM层）营养盐补充途径及关键控制过程，从营养盐的供给与消耗的角度深入阐释该海域真光层的高叶绿素现象，为探索贫营养大洋真光层内发生的浮游植物大量增殖现象提供科学依据。

西太平洋典型海域南区（8ºN、130ºE断面）存在中层低盐、高营养盐的PIW水团向次表层涌升的现象。涌升最剧烈位置位于130ºE断面上受ME影响的6-9ºN海域。在这一海域，2014年16μmol/L的 NO3-N+NO2-N等值线，0.9μmol/L的 PO4-P等值线和18μmol/L 的SiO3-Si等值线可被抬升至-130m水深的DCM层下方。随着高营养盐水体进入真光层，南区浮游植物生物活动更为丰富，DCM层存在位置较北区高，DCM层内微微型浮游植物生物量较北区高1-2个数量级。在南区，中层涌升主要受Ekman pumping作用影响，在130ºE的ME影响区域，由于存在强烈的涡抽吸，涌升最为剧烈。Ekman pumping和ME涡抽吸强度受ENSO影响存在年际差异，在2014年El Nino事件期间，Ekman pumping和ME存在位置上移、强度增加，导致更多的中层高营养盐水体进入真光层，是该年度营养盐等值线抬升最高的原因。调查期间涌升区内并不存在固氮过程，来自中层的低温、低盐水体涌升是真光层营养盐的主要来源。上述结果将对深入理解典型海域自然驱动下的营养盐输送机制提供了依据。

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