The role of krill grazing in Southern Ocean nutrient cycles

人类在本世纪面临的主要问题之一是自然灾害的数量和强度不断增加（如飓风、洪水、冰盖融化）。这些灾难中的许多都是与人类向大气中排放的二氧化碳不断增加有关的全球气候变化的指标。到目前为止，这些二氧化碳排放的很大一部分已经被海洋吸收并隐藏了几个世纪，但为了预测未来，我们需要了解其中的机制。这一提议解决了南大洋内部的一种可能机制。我们假设一种小型甲壳类动物——南极磷虾——有助于将大气中的碳(C)减少到深海中。关于磷虾的三个事实导致了这种说法：首先，磷虾非常丰富，其总质量超过了人口的总质量。其次，磷虾的取食率非常高，主要以浮游植物和藻类为食，浮游植物从溶解在水中的二氧化碳中生成自己的有机碳。第三，磷虾粪便以致密颗粒的形式沉入海底。因此，磷虾介导了碳从小型漂浮藻类到大型下沉粪便颗粒的转变，这一过程被称为“生物碳泵”。不幸的是，实际情况比这稍微复杂一些。随着下沉的颗粒，磷虾也可能从表层输出其他元素，例如铁（Fe）和硅（Si），这些元素对藻类生长至关重要，而且通常浓度有限。尽管磷虾是南大洋食物网和商业捕捞的关键物种，但人们对它们在生物地球化学循环中的作用知之甚少。在此之前，没有一个重要的过程被详细地测量过，因此，我们需要更多的信息来检验我们的假设：磷虾粪丸中含有多少碳、硅和铁？2. 这些元素在沉到深处之前会从小球中溶解出来吗？3. 磷虾会在体内积累铁吗？4. 磷虾在摄食时释放出多少溶解态的铁和硅？最后一个问题尤为重要，因为磷虾将颗粒铁快速再生为溶解形式，可能会刺激藻类生长，从而进一步吸收二氧化碳。我们的计划是在南大洋巡航期间解决这些问题。我们将收集磷虾并在船上孵育，以测量颗粒生产和溶解营养物质释放的速度。我们将从不同的水深对它们的颗粒进行取样，比较它们的总数和碳、铁、硅的含量。这些测量将与铁和硅的水柱剖面有关，无论是作为营养物的溶解形式还是在藻类细胞中的颗粒形式。我们将在不同环境下的一系列站点取样，有些站点有很多藻类，有些站点很少，有些站点有足够的铁和硅，有些站点太少。这将使我们能够建立简单的方程式，将磷虾的各种比率（见上面的4个问题）与它们的可用食物和营养状况联系起来。在这些方程的帮助下，我们可以扩大采样点的结果来回答我们的总体问题：磷虾是通过输出C和循环养分来支持生物碳泵，还是通过从地表水中去除铁和硅来阻止碳泵？我们的船期报价是2009/2010年在斯科舍海11天。为了增加我们的季节和区域覆盖范围，我们将用以前巡航的一系列冷冻样本补充数据集。测量铁和处理磷虾都是不平凡的任务；因此，该提案结合了英国南极调查局和南安普顿国家海洋学中心这两个研究所的专业知识。参与其中的科学家们提供了定位、捕捉和试验磷虾（阿特金森、菲尔丁、施密特）、微量金属清理工作（阿赫特伯格、里肯伯格）、海洋学（维纳布尔斯）和海洋化学（怀特豪斯）的所有基本技能。

One of the major problems mankind is facing in this century is an increasing number and intensity of natural disasters (e.g. hurricanes, floods, melting ice sheets). Many of these disasters are indicators of global climatic changes related to the ever-increasing amount of CO2 humans release into the atmosphere. So far, a large proportion of these CO2 emissions has been absorbed by the oceans and tucked away for centuries, but to predict the future, we need to understand the mechanisms involved. This proposal tackles one possible mechanism within the Southern Ocean. We hypothesize that a small crustacean - Antarctic krill / helps the drawdown of carbon (C) from the atmosphere into the deep ocean. Three facts about krill lead to this suggestion: Firstly, krill are very abundant, with a total mass greater than that of the human population. Secondly, krill have very high feeding rates and feed mainly on phytoplankton, the algae, which build their own organic C from the CO2 dissolved in the water. Thirdly, krill faeces sink as compact pellets towards the seafloor. Thus, krill mediate the carbon transition from small floating algae to large sinking fecal pellets, a process known as the 'biological C pump'. Unfortunately, it is slightly more complicated than this. With the sinking pellets, krill might also export other elements from the surface layer, e.g. iron (Fe) and silicon (Si) that are essential for the algae to grow and often in limiting concentrations. Even though krill are a key species in the Southern Ocean food web and commercially fished for, little is known about their role in biogeochemical cycles. None of the essential processes has been measured before in detail, thus, we need more information to test our hypothesis: 1. How much C, Si and Fe are in the krill fecal pellets? 2. Do the elements dissolve out of the pellets before sinking to depth? 3. Do krill accumulate Fe into their bodies? 4. How much Fe and Si do krill release in dissolved form when feeding? The last question is especially important, because a fast regeneration of particulate Fe into the dissolved form via krill might stimulate algal growth and therefore a further uptake of CO2. Our plan is to tackle these questions during a cruise in the Southern Ocean. We will collect krill and incubate them on board to measure the rates of pellet production and release of dissolved nutrients. We will sample their pellets from different water depths, to compare the total numbers and the content of C, Fe and Si. These measurements will be related to water column profiles of Fe and Si, both in dissolved form as nutrients and in particulate form in algal cells. We will sample at a range of stations within different environments / some with lots of algae, others with few, some with sufficient Fe and Si, others with too little. This will enable us to make simple equations that relate the various rates in krill (see 4 questions above) to their available food and nutrient situation. With help of these equations, we can scale up the results from our sampling sites to answer our overall question: Do krill support the biological C pump by exporting C and recycling nutrients, or do they stop the pump by removing Fe and Si from surface water? Our ship-time bid is for 11 days in the Scotia Sea during 2009/2010. To increase our seasonal and regional coverage, we will supplement the data set with a range of frozen samples from previous cruises. Both measuring Fe and handling krill are non-trivial tasks; therefore the proposal combines expertise across two institutes, the British Antarctic Survey and the National Oceanography Centre Southampton. The scientists involved supply all the essential skills in locating, catching and experimenting with krill (Atkinson, Fielding, Schmidt), in trace metal clean work (Achterberg, Rijkenberg), in oceanography (Venables) and in marine chemistry (M. Whitehouse).