Surfactant control of air-sea gas exchange in coastal waters

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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.