Quantifying the contribution of sympagic versus pelagic diatoms to Arctic food webs and biogeochemical fluxes (SYM-PEL)

在北极食物网的底部，有三种主要的初级生产者：小鞭毛虫、生活在开阔水域的硅藻（远洋）和生长在海冰中的硅藻（同生）。整个研究界对海冰硅藻的作用有不同的看法。对于生态学家来说，它们是极地生态系统的核心，而那些关注全球海洋尺度的人则认为它们不那么重要，并且没有将它们纳入预测气候变化反馈的模型中。这可能反映了它们对北极水域初级生产总量的贡献很小（<10%）。然而，两种新开发的营养标记方法可以在食物网中追踪海冰和开放水域的硅藻，在极地消费者中始终发现强烈的冰藻“信号”。甚至在鲸鱼、海豹和北极熊中，多达80%的身体脂肪储备来自最初由冰藻固定的碳。这怎么可能呢？在变暖的北极，这种情况会发生什么变化？我们的项目旨在回答这个难题，并弥合对冰藻的不同看法之间的差距。我们建议量化冰藻与开放水域硅藻对生活在北极高海拔地区的消费者的相对重要性——考虑到不同的物种、地区和一年中的不同时间。我们还将研究沉入海底的物质，以及在沉积物陷阱中收集的物质。我们的第一个假设是，冰藻对北极食物网的输入和输出通量不成比例地高于它们对初级生产总量的贡献。我们的第二个假设考察了这些能量转移背后的机制，侧重于更微妙的食物效益概念。重要的不仅仅是每年的食物总量；它还必须在正确的时间到达，方便获取，营养丰富。为了验证这些假设，我们开发了一种基于“高支异戊二烯类”（hbi）的方法。这些脂质分子对海冰或开阔水域的一系列硅藻物种都有特异性。利用冰与水柱衍生hbi的比例，我们现在可以追踪这些能量输入对食物网的相对作用。当这些分子通过食物网时，它们的化学稳定性是这种示踪方法的一个关键优势，因为以前很难跟踪冰或水柱衍生的藻类的命运。我们建议参加横跨北冰洋中部的冰漂移（MOSAiC），这将为我们在普利茅斯的实验室进行后续的HBI分析提供机会，从沉积物陷阱中取样食物网和材料。我们还将确定不同消费者的身体状况，作为每一种食物类型在季节中产生的净效益的综合。巡航数据集将与其他北极考察的数据以及我们的项目合作伙伴用第二种独立饮食方法估计的数据相补充。这将给一个泛北极冰藻的重要性概述脂质储存的关键消费者。然后，未来气候预测的模拟模式输出将扩大到整个北极盆地。首先，我们将与项目伙伴合作，对关键浮游动物物种的生命周期进行建模，以估计它们在未来更无冰的北冰洋中部地区定居的潜力。其次，我们将使用NEMO-MEDUSA——英国地球系统模型（UKESM1）的海洋组成部分——来确定预计的远洋初级生产的增加是否可以弥补作为浮游动物食物来源的冰藻的损失。我们的发现，以及MOSAiC其他参与者的发现，将用于启动将冰藻纳入NEMO-MEDUSA的“路线图”。通过帮助在海冰的物理、生物地球化学和生态功能与大规模建模要求之间建立桥梁，我们的目标是提高我们对北极变化及其为人类提供服务的理解。

At the base of the Arctic food web, there are three major primary producers: small flagellates, diatoms living in open water (pelagic) and diatoms growing in sea ice (sympagic). The role of the sea ice diatoms is perceived differently across the research community. For ecologists they are central to the polar ecosystem, while those looking at global ocean scales consider them less important and have not incorporated them into their models projecting climate change feedbacks. This may reflect their minor (<10%) contribution to the total primary production in Arctic waters. However, two newly developed trophic marker approaches that can trace diatoms from sea ice and open water within the food web, consistently find a strong ice algae 'signal' in polar consumers. Even in whales, seals and polar bears, as much as 80% of their body fat reserves are from carbon originally fixed by ice algae. How is this possible? How will this change in a warming Arctic? Our project aims to answer this puzzle and to bridge the gap between the contrasting perceptions of ice algae.We propose to quantify the relative importance of ice algae vs. open water diatoms for consumers living in the high Arctic - considering different species, regions and times of the year. We will also look at material that sinks to the seabed, and is collected in sediment traps. Our first hypothesis is that the input of ice algae to Arctic food webs and to export fluxes is disproportionately higher than their contribution to total primary production. Our second hypothesis examines the mechanisms behind these energy transfers, focussing on the more subtle concept of food benefit. It is not just the total annual amount of food that matters; it also has to arrive at the right time, be accessible and be nutritious. To test these hypotheses, we have developed a method based on "Highly Branched Isoprenoids" (HBIs). These lipid molecules are specific to a series of diatom species specific either to sea ice or open water. Using the ratio of ice-versus water column-derived HBIs, we can now trace the relative roles of these energy inputs to the food web. The chemical stability of these molecules as they pass through the food web is a key advantage of this tracer method, as previously it has been very difficult to follow the fate of ice- or water column derived algae.We propose to take part in an ice drift across the Central Arctic Ocean (MOSAiC) that will give the opportunity to sample the foodweb and material from sediment traps for subsequent HBI analysis in our lab in Plymouth. We will also determine the body condition of various consumers as an integrator of net benefit derived from each food type over the season. The cruise data set will be complemented with data from other Arctic expeditions and those estimated with a second, independent diet method by our Project partners. This will give a pan-Arctic overview of the importance of ice algae to the lipid stores of key consumers. Then, simulation model outputs of future climate projection will allow scaling up to the whole Arctic Basin. First, we will work with Project partners modelling life cycles of key zooplankton species, to estimate their potential to colonise a future, more ice-free central Arctic Ocean. Second, we will use NEMO-MEDUSA - the oceanic component of the UK's Earth system model (UKESM1) - to determine whether projected increases in pelagic primary production could compensate for loss of ice algae as a food source for zooplankton. Our findings, and those of other participants in MOSAiC, will be used to initiate a "roadmap" for the incorporation of ice algae into NEMO-MEDUSA. By helping to bridge between the physical, biogeochemical and ecological functions of sea ice and requirements of large-scale modelling, we aim to improve our understanding of the changing Arctic and its provision of services to mankind.