Plankton dynamics and carbon cycling in the equatorial Pacific Ocean: Control by Fe, Si and grazing

知识价值：在海洋-大气-生物圈系统中生物活性元素循环之间的许多联系中，通过向地表水提供氮、磷、硅和铁的过程对海洋碳循环的调节可能是最重要的。浮游植物的光合作用、表层有机碳的输出以及深海中有机碳的再矿化构成了一个生物泵，它以全球显著的速率将二氧化碳从大气输送到深海。冰芯记录表明，在冰期/间冰期时间尺度上，该泵全球尺度效率的振荡可能在控制大气CO2浓度方面起主要作用。由于已知氮、磷、硅和铁的有效性都限制了浮游植物的有机物生产或其向海洋深处的出口，这些元素的限制可能是调节大气CO2水平的主要生物地球化学机制。在一些大型海洋区域，硝酸盐和磷酸盐的高表面浓度全年持续存在，表明浮游植物生长的铁限制和/或硅藻生长的硅限制。在那些高营养、低叶绿素（HNLC）的地区，输送到地表水的溶解无机碳（DIC）被吸收的相对较少，这使得该系统成为大气二氧化碳的更大来源或更小的汇，而不是所有的氮和磷都被利用。两个HNLC系统在这些过程中具有最重要的全球意义：在南大洋，冰期/间冰期铁供应的变化可能会刺激浮游植物在冰期的光合作用，并在间冰期限制其光合作用，从而驱动有充分记录的大气CO2的变化。在太平洋，赤道风驱动的上升流和浮游植物对上升流N、P和DIC的低效利用，使东赤道太平洋（EEP）成为目前条件下大气中CO2的最大海洋来源。现在有直接的实验证据表明，铁限制和硅限制在HNLC地表水。此外，硅藻（唯一需要Si的主要浮游植物群）通常比其他种类的硅藻更受铁限制释放的刺激。铁和硅的有效性还可以相互作用，控制高碳低碳地区有机质的生产和输出，因为低铁增加了硅藻的硅/碳和硅/氮吸收比率。本项目将开展实地研究、生物地球化学建模和教育的协调计划，重点研究铁限制、硅限制和浮游动物放牧在调节高海拔地区碳循环中的作用。实验工作和建模都将强调这些控制机制对浮游植物中三个功能群的影响——硅藻、球石藻和微浮游生物——它们的营养需求有很大的不同，它们产生的有机物在海洋中有明显不同的命运。该项目的教育阶段将涉及南大洋和赤道太平洋，研究阶段将在EEP的上升流区（135 - 140 W）进行。本研究将结合野外观测、可操作的野外实验和生物地球化学模拟，所有的野外观测和实验都旨在支持新一代的上层海洋模型，这些模型可以明确区分硅藻、颗石藻和微浮游生物在海洋碳循环中的作用。这些实验和模型的目的是研究在不同的营养制度下这些群体之间的竞争，浮游动物对它们的选择性去除，以及改变光照和营养条件对它们元素组成的影响。在以前的模型中，这些相互作用项还没有得到充分的探讨，无法解决它们对任何海洋系统中碳循环的影响。更广泛的影响：该项目的一个组成部分是针对小学和高中教师的教育计划，重点是海洋生物地球化学，全球碳循环及其与全球气候的联系。该计划的目标是帮助教师以一种准确而引人入胜的方式向他们的班级介绍这些主题，从而导致真正的理解。为教师举办的三个讲习班将说明海洋碳循环的物理和生物控制，解释该循环变化的全球范围后果，并编写供教师使用的教学材料。这些教育工具将包括一个互动网站，在那里教师和学生可以运行EEP的生物地球化学模型，对系统如何工作做出自己的假设，并使用该模型来探索这些假设的后果。此外，每艘邮轮将有一名教师亲身体验航海研究，并为学生解释研究结果。在该项目的海上航行和建模阶段，还将与法国布雷斯特的欧洲海洋大学（IUEM）的科学家进行重要的国际合作。

Intellectual merit: Of the many linkages among the cycles of biologically active elements in the ocean-atmosphere-biosphere system, regulation of the oceanic carbon cycle by the processes that supply nitrogen, phosphorus, silicon and iron to surface waters may be the most important. Phytoplankton photosynthesis, export of organic carbon from the surface layer and remineralization of that carbon in the deep sea comprise a biological pump, which transports CO2 from the atmosphere to the deep ocean at globally significant rates. Ice-core records suggest that oscillations in the global-scale efficiency of this pump may play a major role in controlling atmospheric CO2 concentrations on glacial/interglacial time scales. Because N, P, Si and Fe availability are all known to limit organic matter production by phytoplankton or its export to depth in present-day ocean habitats, limitation by those elements may be the main biogeochemical mechanism regulating atmospheric CO2 levels In several large oceanic areas high surface concentrations of nitrate and phosphate persist throughout the year, suggesting Fe limitation of phytoplankton growth and/or Si limitation of diatom growth. In those high-nutrient, low-chlorophyll (HNLC) areas, relatively little of the dissolved inorganic carbon (DIC) delivered to surface waters is taken up, making the system either a greater sourceor smaller sink for atmospheric CO2 than it would be if all N and P were used. Two HNLC systems are of the greatest global importance with respect to these processes: In the Southern Ocean, glacial/interglacial changes in Fe supply may stimulate phytoplankton photosynthesis during glacial periods and limit it during interglacials, driving the well-documented changes in atmospheric CO2. In the Pacific Ocean, wind-driven upwelling at the equator and inefficient use of the upwelled N, P and DIC by phytoplankton combine to make the Eastern Equatorial Pacific (EEP) the largest oceanicsource of CO2 to the atmosphere under present conditions. There is now direct experimental evidence of both Fe limitation and Si limitation in HNLC surface waters. In addition, diatoms (the only major phytoplankton group that requires Si) are usually stimulated more than other groups by release from Fe limitation. Fe and Si availability can also interact to control the production and export of organic matter in HNLC areas because low [Fe] increases the Si/C and Si/N uptake ratios of diatoms. This project will undertake a coordinated program of field research, biogeochemical modeling and education focused on the roles of Fe limitation, Si limitation and zooplankton grazingin regulating the carbon cycle in HNLC areas. The experimental work and modeling will both stress effects of these control mechanisms on three functional groups within the phytoplankton -- diatoms, coccolithophores and picoplankton -- whose nutrient requirements differ significantly and which produce organic matter that has distinctly different fates in the ocean. The educational phase of the project will address both the Southern Ocean and the equatorial Pacific, the research phase will be conducted in the upwelling zone of the EEP (135 - 140 W). This research will combine field observations, manipulative field experiments and biogeochemical modeling, with all field observations and experiments designed to support a new generation of upper-ocean models that distinguish explicitly among the roles of diatoms, coccolithophores and picoplankton in the oceanic carbon cycle. The experiments and models are designed to examine competition among these groups under different nutrient regimes, their selective removal by zooplankton and the effects of changing light and nutrient conditions on their elemental composition. These interaction terms have not been explored well enough in previous models to address their effects on carbon cycling in any ocean system.Broader impacts: An integral part of this project is an educational program for elementary and high school teachers, focused on ocean biogeochemistry, the global carbon cycle and their connections with global climate. The program's goal is to help teachers present these topics to their classes in an accurate and engaging way, leading to real understanding. Three workshops for teachers will illustrate physical and biological controls on the oceanic carbon cycle, explain the global-scale consequences of changes in that cycle and develop instructional materials for the teachers to use. Those educational tools will include an interactive web site where teachers and students can run biogeochemical models of the EEP, make their own assumptions about how the system might work and use the model to explore the consequences of those assumptions. In addition, one teacher will go on each cruise to experience seagoing research first-hand and interpret the results for students. There will also be a significant international collaboration with scientists at l'Institut Universitaire Europen de la Mer (IUEM) in Brest, France in both the seagoing and modeling phases of this project.