Flexible-composition biogeochemical model results for BATS and the Southern Ocean (KERFIX), 2005 (U.S. JGOFS Synthesis & Modeling Phase project results)

<p>Several recent studies, in systems as different from one another as the Sargasso Sea, Equatorial Pacific and Southern Ocean, have shown that diatoms are responsible for much of the new and export production in surface waters. Those observations, combined with the diatoms' absolute growth requirement for Si, suggest that the availability of dissolved Si may regulate new and export production in much of the sea. Models of carbon and nitrogen cycling in the upper ocean must therefore incorporate Si control of diatom productivity and organic-matter export if their goal is to predict the biological response of the oceans to natural and anthropogenic forcing. That task is currently impossible for most of the ocean due to the scarcity of data regarding factors regulating Si cycling (e.g. silica production rates, silica dissolution rates and Si limitation of diatom productivity).</p>

<p>It is now clear that the marine Si cycle is strongly bimodal in character. The Southern Ocean lies at one extreme, where a relatively high fraction (on the order of 10%) of the silica produced by diatoms in the surface waters is preserved in the sediments. At the opposite extreme are the mid-ocean gyres, where annual rates of silica production in surface waters are surprisingly close to those in the Southern Ocean but almost none of the opal produced accumulates in the sediments. The mechanisms that produce and regulate this bimodal Si cycle must be understood, as they play a major role in controlling the availability of dissolved Si in surface waters. This availability in turn regulates diatom productivity and the ability of diatoms to contribute to new and export production. Thus we must achieve a more realistic understanding of the linkages between the Si cycle and the cycles of carbon and nitrogen, both to determine when Si is - and is not - a major regulator of organic carbon export and to model carbon export accurately when it is strongly influenced by Si availability.</p>

<p>We propose to combine the synthesis of several large data sets obtained during the U.S. and French JGOFS programs with a new generation of physical/biogeochemical models which explicitly include Si regulation of diatom productivity, to make the first data-based determination of the factors controlling the cycling of Si in the upper 200 - 500 m of the ocean. We propose further to investigate how changes in the character of the Si cycle affect the ability of diatoms to contribute to carbon and nitrogen export from surface waters. A team of U.S. and French investigators (Dave Nelson, Mark Brzezinski, Paul Tréguer and Philippe Pondaven) will synthesize the information on Si cycling and Si regulation of diatom productivity from extensive JGOFS data sets obtained during the U.S. Bermuda Atlantic Times Series (BATS) program in the Sargasso Sea, the U.S. Antarctic Environment Southern Ocean Process Study (AESOPS) in the Pacific sector of the Southern Ocean and the French ANTARES and KERFIX programs in the Indian sector of the Southern Ocean. BATS, ANTARES and AESOPS are the only three projects yet conducted, anywhere in the open sea, where studies of Si cycling and Si limitation have been carried out in coordination with studies of primary production and nitrogen cycling, with seasonal coverage. It is a great advantage that these projects also investigated the two end members of the bimodal marine Si cycle.</p>

<p>While those field programs were underway, new physical/biogeochemical models were developed which explicitly include Si cycling and Si limitation terms regulating diatom growth and productivity. One of us (Pondaven) has been instrumental in developing those models, and we now propose to apply them to the BATS, ANTARES and AESOPS study areas and the large data sets on Si, C and N cycling obtained there. Through a combination of data synthesis and numerical modeling we will: 1) identify those processes that are the strongest determinants of the character of the Si cycle in the upper ocean, and 2) assess how the resulting differences in the Si cycle affect the ability of diatoms to contribute to new and export production.</p>

<p>The results will establish a foundation for the next generation of global biogeochemical models of marine carbon cycling, which must explicitly incorporate Si regulation of carbon and nitrogen export in systems where diatom productivity is limited by Si.</p>

近期多项针对马尾藻海、赤道太平洋与南大洋等迥异海洋系统的研究表明，硅藻（diatoms）主导了表层水体中绝大部分的新生产（new production）与输出生产（export production）。结合硅藻生长绝对依赖硅（Si）这一特性，上述观测结果暗示：溶解硅（dissolved Si）的可用性可调控全球多数海域的新生产与输出生产。若要精准预测海洋对自然及人为强迫（anthropogenic forcing）的生物响应，上层海洋碳氮循环模型必须纳入硅对硅藻生产力与有机物质输出的调控机制。但目前，由于调控硅循环的关键数据（如硅生产速率、硅溶解速率及硅对硅藻生产力的限制作用）极度匮乏，全球大部分海域的该类研究仍难以开展。

目前学界已明确，海洋硅循环具有显著的双峰特征。南大洋处于该特征的一端：其表层水体中硅藻生成的硅质有相对较高比例（约10%）会在沉积物中保存下来。而大洋环流中心区域则处于另一端——尽管其表层水体的年硅生产速率与南大洋近乎持平，但生成的蛋白石（opal）几乎无任何留存于沉积物中。阐明产生并调控这种双峰硅循环的机制至关重要，因为这些机制直接控制着表层水体中溶解硅的可用性，而溶解硅的可用性又进一步调控硅藻生产力，以及硅藻参与新生产与输出生产的能力。因此，我们需要更深入地理解硅循环与碳、氮循环之间的关联：既要明确硅何时会、何时不会成为有机碳输出的主要调控因子，也要在硅可用性对碳输出存在显著影响时，精准构建碳输出模型。

本研究计划将美法两国全球海洋通量联合研究（Joint Global Ocean Flux Study，JGOFS）计划获取的多组大型数据集，与明确纳入硅对硅藻生产力调控机制的新一代物理-生物地球化学模型（biogeochemical models）相结合，首次基于实测数据确定调控海洋上层200~500米硅循环的关键因子。此外，我们还将探究硅循环特征的变化如何影响硅藻从表层水体输出碳、氮的能力。由美法两国研究者（Dave Nelson、Mark Brzezinski、Paul Tréguer及Philippe Pondaven）组成的团队，将整合三大JGOFS项目的海量数据集：包括美国在马尾藻海开展的百慕大大西洋时间序列（Bermuda Atlantic Time-series Study，BATS）计划、美国在南大洋太平洋海域开展的南极环境南大洋过程研究（Antarctic Environment Southern Ocean Process Study，AESOPS），以及法国在南大洋印度洋海域开展的ANTARES与KERFIX计划，以此获取硅循环及硅对硅藻生产力调控的相关信息。截至目前，全球公海范围内仅有BATS、ANTARES与AESOPS这三项研究，实现了硅循环、硅限制作用与初级生产、氮循环研究的协同开展，并覆盖完整季节周期。尤为关键的是，这三项研究恰好覆盖了双峰海洋硅循环的两个极端端元。

在上述野外计划实施期间，学界已开发出明确纳入硅循环及硅限制作用项的新一代物理-生物地球化学模型，用于调控硅藻生长与生产力。本团队成员之一Pondaven在该类模型的开发中发挥了关键作用，因此我们计划将这些模型应用于BATS、ANTARES与AESOPS的研究区域，以及上述区域获取的硅、碳、氮循环大型数据集。通过数据整合与数值模拟相结合的研究路径，我们将达成两大目标：1）明确上层海洋硅循环特征的核心调控过程；2）评估硅循环差异对硅藻参与新生产与输出生产能力的影响。

本研究成果将为新一代海洋碳循环全球生物地球化学模型奠定基础——这类模型必须在硅藻生产力受硅限制的海洋系统中，明确纳入硅对碳、氮输出的调控机制。