Surfactant control of air-sea gas exchange in coastal waters

表面活性剂是“润湿剂”，可以降低水的表面张力。人们熟悉的日常表面活性剂包括家用洗涤剂，但大量的天然表面活性剂反映了导致其生产的生物过程的多样性。例如，海水中的表面活性剂主要是浮游植物在生长和随后的死亡和分解过程中释放的表面活性剂。这些浮游植物衍生的表面活性物质包括多糖(复合糖)、脂类(脂肪和蜡)、脂肪酸和蛋白质等。这种表面活性剂的“池”聚集在所谓的海洋表面微层(SML)中，它是海洋的最上层，厚度只有十分之一毫米或更小。尽管SML很薄，但它非常重要，原因有很多，其中一个原因是，它的深度以及它的化学和物理性质都会极大地影响海洋和大气之间二氧化碳等气候活性气体的交换。这一点很重要，因为海洋目前吸收了燃烧化石燃料过程中释放到空气中的大约四分之一的二氧化碳(大约一半留在大气中，其余四分之一被陆地植被吸收)。因此，非常重要的是要知道二氧化碳和其他气体在海面上交换的速率，这基本上取决于SML的厚度，而SML的厚度受风、浪和其他因素造成的湍流控制。当湍流(如风速)较大时，SML较薄，气体交换率较高，而当湍流较小时，SML较厚，气体交换率较低。SML中的天然表面活性剂抑制了表面水的湍流，因此可能非常显著地减缓了气-海气体交换的速度。不幸的是，令人惊讶的是，人们对这种表面活性剂对海上气体交换的重要性知之甚少；我们的大部分知识来自于对合成化合物的实验室研究。到目前为止，还没有关于天然表面活性剂在海上的气体交换的详细数据。海洋浮游植物的活动在空间和时间上是可变的，但往往在沿海水域最高；迄今为止开展的极少数研究表明，表面活性剂的浓度急剧下降，组成沿河口向海方向变化，并在近海继续。重要的是，河口和沿海水域是许多气候活跃气体的主要来源区域，因此表面活性剂对气体交换的影响将在那里最为明显。然而，缺乏相关的测量，因此，本研究将研究表面活性剂浓度和组成沿河口-近海梯度的表面活性剂浓度与气体交换之间的关系，以更好地了解这种变异性与海-气气体交换的关系。表面活性剂池的广泛化学成分的各个方面将有助于确定表面活性剂池的总体化学成分是否也在气体交换中发挥作用；这一问题迄今尚未得到解决。因此，这项独特的研究将提供所需的宝贵数据，以促进目前对海-气交换的了解，并有助于更明确地确定在这项工作的基础上开展的后续项目的关键研究目标。这一发展中研究领域的最终目标是为旨在根据大气二氧化碳增长和大气、海洋和陆地耦合系统的气候响应来预测未来气候变化的模型提供数据。

Surfactants are 'wetting agents' that lower the surface tension of water. Familiar everyday surfactants include household detergents but a vast array of natural surfactants reflects a diversity of biological processes responsible for their production. Surfactants in seawater for example are mainly those released by phytoplankton, both during growth and subsequent death and decomposition. These phytoplankton-derived surfactants include, among many other compounds, polysaccharides (complex sugars), lipids (fats and waxes), fatty acids and proteins. This 'pool' of surfactants accumulates in the so-called sea surface microlayer (SML), which is the uppermost 'skin' of the oceans and is of the order of only a tenth of a millimetre thick or less. Despite being so thin the SML is extremely important for a number of reasons, one being that both its depth and its chemical and physical nature can influence greatly the exchange of climate-active gases such as carbon dioxide between the oceans and the atmosphere. This is important because the oceans currently absorb around a quarter of the carbon dioxide that is being released into the air during the burning of fossil fuels (about half stays in the atmosphere and the remaining quarter is absorbed by land vegetation). It is therefore very important to know the rates at which carbon dioxide and other gases exchange across the sea surface and this depends essentially on the thickness of the SML, which is controlled by turbulence due to wind, waves and other agents. When the turbulence (e.g. wind speed) is high the SML is thin and the gas exchange rate is high but when the turbulence is low the SML thickens and the gas exchange rate falls. Natural surfactants in the SML suppress surface water turbulence and so slow down perhaps very significantly, the rate of air-sea gas exchange. Unfortunately surprisingly little is known about the importance of this surfactant effect on gas exchange at sea; most of our knowledge derives from laboratory studies with synthetic compounds. To date there are no detailed gas exchange data for natural surfactants at sea. Marine phytoplankton activity is spatially and temporally variable but tends to be highest in coastal waters; in the very few studies carried out to date surfactants have been shown to strongly decrease in concentration and to change in composition seaward along estuaries, continuing offshore. Importantly, estuaries and coastal waters are major source regions for many climate-active gases and so the surfactant effect on gas exchange will be most pronounced there. However, relevant measurements are lacking and this study will therefore examine the relationship between surfactant concentration and gas exchange along an estuarine-offshore gradient in surfactant concentration and composition to better understand how this variability relates to air-sea gas exchange. Aspects of the broad chemical composition of the surfactant pool will enable determining whether the overall chemical composition of the surfactant pool also plays a role in gas exchange; something that to date has not been addressed. This unique study will thus provide valuable data required to advance current understanding of air-sea gas exchange and help define more clearly key research goals for subsequent projects building on this work. The ultimate goal of this developing research area is to provide data to models designed to predict future climate change based on the growth of atmospheric carbon dioxide and the climate response of the coupled atmospheric, oceanic and terrestrial systems.