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

Lead Research Organisation: University of East Anglia
Department Name: Environmental Sciences

Abstract

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.

Publications

10 25 50
 
Description 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.



As part of the NERC UK SOLAS programme, we used sulphur hexafluoride and drifting buoys for the first time to unequivocally follow filaments of upwelled water as they moved away from the coast of NW Africa. The specific objectives of the project were 1) To determine the role of upwelling on the supply, loss and air-sea exchange of trace and biogenic gases; 2) To determine the photochemical and biological fate of upwelled, and recently produced dissolved organic matter, and its role in air-sea exchange of climatically important trace gases; and 3) To determine the impact of nutrient enriched upwelled water on the spatial and temporal variability of plankton community structure and activity, and resultant influence on biogenic gas flux.



The fieldwork took place in May 2009, and all three objectives were successfully achieved. Large scale physical and chemical surveys indicated that unstable conditions and internal wave activity were prominent features of the upwelling region. Water upwelled from ~ 150m onto the shelf, where it had a surface water signature characterised by carbon dioxide concentrations > 600 µatm, methane concentrations > 200% and dissolved oxygen saturations < 85%. High spatial resolution mapping of surface waters within and outside the SF6 labelled waters showed high variability in dimethylsulphide concentrations. Eight day surveys within each of two filaments, allowed the determination of the temporal succession of bacterial, phytoplankton, microzooplankton and mesozooplankton community structure and activity, alongside measurements of seven different biogenic gases and gas transfer velocities. The high nutrient upwelled water enabled production of the phytoplankton to reach rates around twenty fold higher than typical open ocean values. There was a transition from nitrate to ammonium utilisation by phytoplankton, and high surface nitrification rates added 'regenerated' nitrate to the upwelled 'new' nitrate pool. Bacterial uptake rates of the amino acids leucine, methionine and tyrosine ranged from 0.1 to 0.3 nM h-1. In vivo DMSP production and consumption were measured to assess coupling between DMSP producing phytoplankton and their microzooplankton grazers. The DMSP pool was attributed to specific phytoplankton taxa using flow cytometric sorting. Photobleaching of upwelled and recently produced coloured dissolved organic matter (CDOM) caused the photo-production of ammonia, carbon dioxide and carbon monoxide, and the photo-consumption of oxygen. The annual flux of carbon monoxide to the atmosphere from the upwelling region was derived from measurements of photochemical production and microbial oxidation and a 1-D steady state model. Light data collected during the cruise have been used to develop a coupled ocean-atmosphere model to use satellite data to derive the propagation of UV through the water column and hence photochemical process rates from space. The cruise also allowed the first measurements to be made of ethanol and propanol in seawater and acetone and acetaldehyde oxidation rates and uptake onto particles.
Exploitation Route Measurements and parameterisations for biogeochemical models
Sectors Environment

 
Description Alcohols in Seawater & Their Exchange With The Atmosphere GW4 DTP
Amount £80,000 (GBP)
Funding ID 1503209 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 09/2014 
End 03/2018
 
Description In-situ concentrations and air-sea exchange of acetone 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Presentation at the Soft Ionisation Mass Spectrometry (SIMS) Meeting at the University of Birmingham

N/A
Year(s) Of Engagement Activity 2014
 
Description In-situ concentrations and air-sea exchange of acetone 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Poster presentation at Challenger conference, discussion with participants

N/A
Year(s) Of Engagement Activity 2014
 
Description Keynote presentation Royal Society Deoxygenation 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Keynote presentation at Royal Society Ocean deoxygenation workshop
Year(s) Of Engagement Activity 2016
URL https://royalsociety.org/science-events-and-lectures/2016/09/ocean-ventilation/