SEASONAL VARIABILITY IN THE EFFICIENCY OF UPPER OCEAN CARBON EXPORT

海洋是大气碳的主要储存库。全球碳循环的一个重要组成部分是海洋的生物碳泵（BCP），它主要是由海洋表面的有机颗粒下沉到海洋内部。在表层水中浮游植物初级生产产生的物质中，大部分在上层海洋中被循环利用。一小部分被输出到深海，并在数百年至数千年的时间尺度上与大气隔绝。由于气候变化，预计BCP的规模和效率将在全球范围内下降，可能导致海洋碳储量减少，从而增加大气二氧化碳水平。因此，准确量化全球BCP的大小和效率对于了解地球碳循环和持续的人为二氧化碳输入的影响至关重要。然而，目前对BCP强度的估计范围为4倍，这表明尽管有30多年的研究，但尚未就其大小达成共识。关于是什么控制着BCP的效率及其在季节时间尺度上的可变性，还有更多的不确定性。最近，PI及其合作者开发了一种新的输出比参数化（描述上层海洋BCP的效率）。这表明BCP的效率远低于之前的预期。然而，我们对出口比的参数化在冷海面温度下具有较大的不确定性。据认为，出口比例在很大程度上是由存在于上层海洋的浮游植物的种类决定的，因为大型、密集的浮游植物比小型浮游生物下沉更快，出口效率更高。我们的假设是，低温下出口比例的变化是由于高纬度地区浮游植物华度进化的强烈季节性，由浮游植物群落结构的时间变化驱动。该项目将通过实地和基于卫星数据的研究相结合，评估浮游植物繁殖的季节性变化如何改变亚北极地区的出口比例。我们建议对两个海温适宜、浮游植物华度变异性强的高纬度地区的上层海洋颗粒有机碳通量和同时存在的浮游植物群落结构进行比对。该项目将使用从英国海洋酸化研究计划中收集到的数据，这些数据是在前往北极的游轮上收集到的（这项工作已经得到了NERC的资助），以及在前往拉布拉多海的游轮上收集到的，拉布拉多海正在申请资金。我们建议参加2012年5月的现有巡航（由加拿大海洋与海洋事务部资助），以进一步测量出口通量和浮游植物群落结构。花期的变化和航行中遇到的PCS将被用来确定花期条件的季节梯度对出口效率的影响。然后，我们将把从区域研究中获得的理解应用到一个全球出口测量数据库中，使用卫星获得的关于海面温度、水华期和浮游植物群落结构的数据。然后，我们将开发出口比率的修订参数化，包括相关的季节信息，并在建议工作的最后阶段，在全球范围内应用我们修订的参数化来计算对BCP大小的新估计。该项目旨在了解对出口比例季节性变化的控制，从而减少对全球生物碱规模估计的不确定性。

The oceans are a major repository for atmospheric carbon. An important component of the global carbon cycle is the ocean's biological carbon pump (BCP), which is dominated by the sinking of organic particles from the surface ocean to its interior. Of the material generated via phytoplankton primary production in surface waters, most is recycled in the upper ocean. A small fraction is exported to the deep ocean and sequestered away from further contact with the atmosphere on timescales of hundreds to thousands of years. Both the size and efficiency of the BCP are predicted to decline globally in response to climate change, potentially resulting in reduced ocean carbon storage and hence increased atmospheric carbon dioxide levels. Therefore, accurately quantifying the magnitude and efficiency of the global BCP is essential to understanding the Earth's carbon cycle and the impact of continued anthropogenic inputs of carbon dioxide. However, current estimates of the strength of the BCP range 4-fold, suggesting that despite more than 30 years of study, no consensus on its magnitude has yet been reached. There is even more uncertainty about what controls the efficiency of the BCP and its variability on seasonal timescales.Recently, a new parameterisation of the export ratio (which describes the efficiency of the upper ocean BCP) was developed by the PI and co-authors. This suggested that the BCP efficiency was substantially lower than previously thought. However, our parameterisation of the export ratio has relatively large uncertainty at cold sea surface temperature. The export ratio is thought to be driven in large part by the type of phytoplankton present in the upper ocean, because large, dense phytoplankton sink rapidly and export more efficiently than smaller plankton. Our hypothesis is that the variability in export ratio at low temperatures is due to strong seasonality in phytoplankton bloom evolution at high latitudes, driven by temporal shifts in phytoplankton community structure. This project will assess how seasonal variability of the phytoplankton bloom alters the export ratio in the sub-Arctic through a combination of in situ and satellite data based studies. We propose to collate measurements of upper ocean particulate organic carbon flux and simultaneous phytoplankton community structure from two high latitude regions with suitably cold SST and strong variability in phytoplankton blooms. This project will use data to be collected on a UK Ocean Acidification Research Programme cruise to the Arctic (work already funded by NERC), and on a cruise to the Labrador Sea, for which funds are requested here. We propose to participate in an existing cruise in May 2012 (funded by DFO Canada) to take additional measurements of export flux and phytoplankton community structure. The variability in bloom stage and PCS encountered during the cruises will be used to determine the impact of seasonal gradients in bloom conditions on the export efficiency. We will then apply the understanding gained from the regional studies to a global database of export measurements, using satellite-derived data on sea surface temperature, bloom stage and phytoplankton community structure. We will then develop a revised parameterisation of the export ratio, including relevant seasonal information and in the final stage of the proposed work, apply our revised parameterisation globally to calculate a new estimate of the magnitude of the BCP. The project aims to gain understanding of controls on seasonal variability in the export ratio, and hence reduce uncertainty in the estimate of global BCP magnitude.