The impact of coastal updwellings on air-sea exchange of climatically important gases.

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The world's climate affects us all, and we depend upon accurate scientific measurements and models to predict how climate will change and how we might reduce or cope with the changes. changes in global temperature, light and weather patterns are due to increases or changes in atmospheric constituents and gases such as carbon dioxide, methane, nitrous oxide and dimethylsulphide. The concentration of each of these gases in seawater can be changed by the activity of the microbes (= viruses, bacteria, plants (phytoplankton) and animals (zooplankton)) which live in the surface of the ocean. in addition, the concentrations of some of these gases and the activity of the microbes which alter their concentrations can be changed by the amount of light which reaches the sea surface. Therefore, in order to understand how the concentrations of these climatically important gases are maintained in the atmosphere, scientists must understand how they are produced in the ocean (by microbiological and photochemical reactions) and how the gases exchange between the sea surface and the atmosphere. There are certain regions in the ocean where fluxes of these gases are known to be very high. These include some coastal areas of the eastern Atlantic where, due to consistently strong winds parallel to the shore, deep cold water is caused to rise to the surface at the coast to replace surface waters being driven offshore by the wind. These 'upwelling' deep waters tend to have been away from the sunlit oxygenated upper ocean for tens to hundreds of years, and so the predominant microbial activity is of the bacteria which produce carbon dioxide, methane, nitrous oxide and nutrients such as the 'garden fertilizers' nitrate and phosphate. These waters also contain dissolved nutrient compounds which have resisted bacterial breakdown due to their complex structure. When the deep waters reach the surface, the high concentrations of nutrients stimulate blooms of algae which can produce dimethylsulphide and high levels of organic nutrients which stimulate further (but different) bacterial activity. The complex nutrient compounds which avoided bacterial breakdown in the dark deep ocean, can be converted into climatically important gases such as carbon dioxide and carbon monoxide and nutrients more readily consumed by bacteria, by the relatively high light availability once at the surface. These interacting processes (air-sea exchange of dissolved gases, photochemical production of gases and microbial nutrients, and microbially mediated production of gases) are very difficult to measure in the ocean, and particularly in an upwelling region, as the water is continually moving and mixing. One way to make sure that the measurements are taken from the same water mass, so one can follow the progression of microbial and photochemical processes, is to add an inert compound to the upwelling water as a 'label' of that particular patch of water. This inert compound (called sulphur hexafluoride) normally exists in the ocean in vanishingly small amounts and so the instruments which measure it have to be extremely sensitive. By continually measuring this compound in surface seawater we can maintain the position of the research vessel in the same water body. This unique project will label upwelling water off the coast of Northwest Africa, allowing the experienced chemical and biological scientists on the research ship to measure the interplay between photochemistry, microbiology and physical chemistry in producing climatically important gases in the ocean and delivering them to the atmosphere. Only by understanding how these processes occur now can we predict how they might change in a changing environment. Results from the project will be made available to scientists throughout the world who are working on this globally important question.