Pan-Arctic Acoustic Archives: Quantifying zooplankton behaviours in a changing Arctic

北极正在迅速变化。最明显的变化之一是夏季海冰的范围和厚度减少。据预测，由于气候变暖，未来几年冰的损失将会增加。到2030年，北极可能不会有夏季海冰。最重要的是，冰起到了遮挡阳光的作用，当它退缩时，它使开阔的水域暴露在光照下，导致海洋植物(浮游植物)的生长迅速增加。这些植物会消耗大气中的二氧化碳，因此是地球气候系统的重要组成部分。一旦形成，浮游植物就成为食草性浮游动物的食物，这些浮游动物能够将这种碳源输送到更深的水域，在那里它被排泄出来并埋在沉积物中。这一过程被称为“生物泵”，它从大气中转移碳并将其锁住。重要的是，我们要了解冰、浮游植物、浮游动物和碳之间的关系，这些关系可以用生物地球化学循环模型来模拟。这条链中的关键一环是食草性浮游动物。它们有一种特殊的行为，称为“垂直迁移”(DVM)，这是许多海洋生态系统的一个显著特征。这些动物在黎明和黄昏前后迅速垂直移动数十到数百米，这是地球上生物量最大的周期性变化。经典观点认为，DVM是个体在获取食物和躲避捕食者之间的一种权衡。浮游动物在夜间向上移动，进食到发生初级生产的近表面。在这里，在黑暗的掩护下，视觉捕食者的风险被降至最低。这种向上/向下的迁移将通过光合作用固定在地表附近的碳重新分配到更深的水域，可能会比其他情况下从大气中去除更多的二氧化碳，从而减缓大气中二氧化碳的积累速度。由于北极的通道和冰盖，研究北极全年的浮游动物是困难的。一种成功的记录DVM行为的技术使用了一种名为声学多普勒海流剖面仪(ADCP)的仪器。在过去的十年里，北极部署了许多ADCP来测量洋流，但声音信号也记录了浮游动物的迁徙。通常，对这些数据的分析只是为了了解仪器部署的局部区域内的洋流。我们正处于北极研究的关键时刻，我们必须对海洋过程有一个更广泛的“泛北极”视角。我们建议与国际组织合作，对ADCP数据进行整理、处理和存档，为研究DVM创造独特的资源。规律性的、有节奏的行为意味着我们可以使用数字技术(昼夜节律分析)来量化迁徙行为的强度和规律性，并将其与存在的生物群落、光照水平和海冰覆盖量联系起来。我们将利用这一知识来改进浮游动物如何通过它们的粪便物质向深处运输碳的模型。由于许多原因，了解浮游动物的DVM很重要。量化DVM的行为将使我们能够提高我们预测海冰变化可能如何改变碳在生产力丰富的北极海域捕获和储存方式的能力。这将使我们更深入地了解动物如何以及为什么进行这种定期的迁徙，以及这些迁徙的时间是如何控制的。通过将声学数据与物种数据联系起来，我们将能够了解浮游动物在北极生态系统中的作用，如果要预测依赖浮游生物的鱼类物种的影响，这一点尤其重要。

The Arctic is changing rapidly. One of the clearest changes is a reduction in the extent and thickness of summer sea ice. The loss of ice is predicted to increase in the coming years as a consequence of climatic warming. There may be no summer sea ice in the Arctic by 2030. Critically, the ice acts as a shade to sunlight and as it retreats it exposes open water to illumination causing a rapid increase in the growth of marine plants (phytoplankton). These plants use up carbon dioxide (CO2) from the atmosphere and are therefore an important component of Earth's climate system. Once formed, the phytoplankton become food for herbivorous zooplankton who are able to transport this source of carbon to deeper waters where it is excreted and buried in the sediments. This process, called the 'biological pump', transfers carbon from the atmosphere and locks it away. It is important that we understand the relationships between ice, phytoplankton, zooplankton and carbon and these relationships can be simulated in models of biogeochemical cycles. The critical link in this chain is the herbivorous zooplankton. They have a particular behaviour called 'diel vertical migration' (DVM) which is a prominent feature of many marine ecosystems. The animals move quickly tens to hundreds of meters vertically around dawn and dusk in migrations that comprise the most massive periodic shifts in biomass on Earth. The classical view is that DVM occurs as a trade off by individuals between food acquisition and predator avoidance. Zooplankton move upwards to feed at night into the nearsurface where primary production occurs. Here, under the cover of darkness, the risk from visual predators is minimised. This upward/downward migration redistributes carbon fixed by photosynthesis near the surface to deeper waters, and may remove larger quantities of CO2 from the atmosphere than would otherwise be the case, reducing the rate of CO2 accumulation in the atmosphere. Studying zooplankton in the Arctic year round is difficult because of access and ice cover. One successful technique for recording DVM behaviour uses an instrument called an acoustic Doppler current profiler (ADCP). Many ADCPs have been deployed in the Arctic over the last decade to measure currents but the acoustic signals also record zooplankton migrations. Usually these data are only analysed to understand the ocean currents within the localised region where the instrument was deployed. We are at a critical time in Arctic research where we must take a wider, 'pan-Arctic' view of marine processes. We propose to work with international groups to collate, process and archive the ADCP data, creating a unique resource for studying DVM. The regular, rhythmic behaviour means that we can use numerical techniques (circadian rhythm analysis) to quantify how strong and regular the migration behaviour is and relate this to the biological communities that are present, the level of illumination and the amount of sea ice cover. We will use this knowledge to improve models of how zooplankton transport carbon, through their faecal material, to depth. Understanding zooplankton DVM is important for many reasons. Quantifying DVM behaviour will allow us to improve our ability to predict how changes in sea ice might alter changes in the way carbon is captured and stored in the productive Arctic seas. It will give us a greater insight into how and why animals undertake such regular migrations and how the timing of these migrations is controlled. By relating the acoustic data with species data we will be able to understand the role of zooplankton in Arctic ecosystems and this is of particular importance if predictions on the effect of plankton-dependent fish species are to be made.