Isotopic characterisation of nutrient dynamics and UCDW behaviour in the west Antarctic Peninsula sea ice environment

该研究项目旨在研究西南极半岛(WAP)持续的气候变化和海冰减少如何影响整个区域的营养收支和生物地球化学循环。WAP是一个生态上重要的高初级生产力区域，营养循环对浮游植物的生产及其与二氧化碳动态的关系是至关重要的。据了解，上绕极深水(UCDW)是整个WAP地区地表水的主要营养物质来源。UCDW在陆架断裂时产生，并通过大陆架输送，因此营养物质可以通过垂直混合提供给混合层。这种营养物质的来源为沿海地区的高生产力提供了燃料，并对大气中二氧化碳的生物吸收产生了影响。现有证据表明，在夏季生长季，海冰的可变性可以极大地影响表层水域的浮游植物生物量和营养物质的利用，但这种相互作用的机制仍有待完全了解。海冰融化和大气淡水输入可以充分稳定上层海洋，提供有利于生长的光照良好的表面环境，从而促进浮游植物的繁殖。然而，这种分层条件也会限制营养物质的再补给到混合层，从而抑制生产力和水华的规模。叠加在叶绿素、常量营养素和物理环境的巨大年际变化上，有强烈的迹象表明，WAP沿线的海冰长期下降，并伴随着生产力的下降。这项研究项目试图了解WAP海冰区生产力变化背后的营养生物地球化学变化，并解决气候引起的海冰减少和层化将导致初级生产力和表层环境中硝酸盐利用急剧下降的核心假设。这项研究将由三个部分组成：1.在WAP莱德湾的三个南半球夏季生长季的时间序列研究，以研究固定氮收支和循环的时间变化与海冰、水柱结构和生产力年际变化的关系。从陆架断裂到玛格丽特湾的船基样带，研究深水行为及其对高生产力沿海地区营养物质供应的影响。在更广泛的WAP陆架区域进行船舶采样，以检查营养物质动力学、生产力、海冰和物理海洋学的空间变异性，并为时间序列研究提供更广泛的背景。在研究的每个组成部分期间，将进行一套与营养物质收支和循环有关的生物地球化学测量。项目合作伙伴提供了全面的辅助物理和生物数据，以便在进行营养测量的同时检查生产力和环境变量。将每年为WAP大陆架建立一个固定的氮预算，并在持续的海冰损失和浮游植物生产力变化的背景下审查其年际变化。营养生物地球化学是生产力和海洋吸收大气二氧化碳之间关系的核心，因此这项研究对南大洋二氧化碳系统在调节全球气候中的作用具有广泛而深远的影响。这项研究还将有助于通过深入了解气候对地球上变暖最快的海洋环境中的营养循环和生产力的影响，在更广泛的范围内预测海洋对持续气候变化的未来反应。

This research project aims to examine the ways in which ongoing climate change and sea ice decline at the west Antarctic Peninsula (WAP) are impacting upon nutrient budgets and biogeochemical cycling throughout the region. The WAP is an ecologically important region of high primary productivity, and nutrient cycling is known to be crucial to phytoplankton production and its relationship with CO2 dynamics. Upper circumpolar deep water (UCDW) is understood to constitute the principal source of nutrients to surface waters throughout the WAP region. UCDW incurs at the shelf break and is transported across the continental shelf, so that nutrients can be supplied to the mixed layer by vertical mixing. This source of nutrients fuels high productivity in coastal regions, with implications for the biological uptake of atmospheric CO2.Available evidence suggests that sea ice variability can drastically impact phytoplankton biomass and nutrient utilisation in surface waters during the summer growing seasons, but the mechanisms underlying this interaction remain to be fully understood. Sea ice melt and meteoric freshwater inputs can promote phytoplankton blooms by stabilising the upper ocean sufficiently to provide a well-lit surface environment conducive to growth. However, such stratified conditions can also inhibit productivity and the magnitude of the bloom by restricting the resupply of nutrients to the mixed layer. Superimposed onto large interannual variability in chlorophyll, macronutrients and the physical environment, there are strong indications of long-term sea ice decline along the WAP and a concomitant decline in productivity.This research project seeks to understand the changes in nutrient biogeochemistry underlying these changes in productivity in the WAP sea ice zone, and addresses the central hypothesis that a climate-induced reduction in sea ice and stratification will result in drastic declines in primary production and nitrate utilisation in the surface environment. Such conditions would lead to a simultaneous reduction in the capacity of the oceanic CO2 sink during summer and generate an increased pool of unutilised nitrate in surface waters over the shelf, both of which would hold profound implications for global biogeochemical cycles.This study will comprise three components:1. A time-series study over three austral summer growing seasons in Ryder Bay, WAP, to examine temporal changes in fixed nitrogen budgets and cycling, in relation to interannual variability in sea ice, water column structure and productivity.2. A ship-based transect from the shelf break to Marguerite Bay to examine deep water behaviour and its impact on the supply of nutrients to high productivity coastal regions.3. Ship-based sampling across the wider WAP shelf region to examine spatial variability in nutrient dynamics, productivity, sea ice and physical oceanography, and give a broader context to the time-series study.A suite of biogeochemical measurements pertinent to nutrient budgets and cycling will be made during each component of the study. Comprehensive ancillary physical and biological data are available from project partners, to examine productivity and environmental variables alongside these nutrient measurements. A fixed nitrogen budget will be constructed for the WAP shelf on an annual basis and its interannual variability examined in the context of ongoing sea ice losses and changes in phytoplankton productivity. Nutrient biogeochemistry is central to the relationship between productivity and oceanic uptake of atmospheric CO2, so this study has broad and far-reaching implications for the role of the Southern Ocean CO2 system in regulating global climate. This study will also help to predict the future response of the oceans to ongoing climate change at the wider scale by giving insight into the climatic impacts on nutrient cycling and productivity in the fastest-warming marine environment on Earth.