It is crucial that we understand how our marine environment is changing with time so we can determine the rate of acidification due to human emissions of carbon dioxide to the atmosphere
As carbon dioxide levels in our atmosphere continue to increase, more and more of the gas is drawn down into the world's oceans, causing the pH of water to lower. This means our oceans are acidifying; and as the 21st century rolls on, these changes are expected to have consequences for marine ecosystems, as well as the services they provide, such as fisheries.
It is these ocean changes which a team at Ireland's Marine Institute have been tasked with measuring working aboard the Celtic Explorer research vessel, some 400km from Galway, from where we departed last month.
Looking over the vessel's edge we can see the CTD - an acronym for Conductivity, Temperature, Depth - unit reflecting sunlight from below the sea's surface. From here on deck, within the surrounding darker shades of sea the unit appears as a submerged circle of blue light. Then, as the scientists wait in anticipation, it is raised from the water and pulled aboard.
"When the sampler is brought back on deck a number of different samples are collected" says Dr Tríona McGrath, a marine scientist at the National University of Ireland Galway, and funded by Ireland's Marine Institute. "These samples are used to measure dissolved inorganic carbon and total alkalinity levels,which then combined tell us about ocean acidification - as well as oxygen levels, nutrient concentrations, and salinity."
"The 2017 oceanographic survey across the South Rockall Trough, to the west of Ireland, is a continuation of an existing carbon chemistry time series in the region" says Dr McGrath. "The survey has been repeated annually, weather permitting, since 2009."
Such annually repeated measurements generate what scientists refer to as a time series, and in this case provide important information on rates of ocean acidification and long term trends in environmental and oceanographic behaviour.
The research could not be carried out without the the CTD unit, essentially a piece of kit constituting a number of large sampling bottles arranged in a rosette. Upon being lowered to near the sea floor, each bottle samples sea water at various depths on its travel back up to the surface.
"The nutrient, oxygen and salinity samples are analysed on board using different laboratory instruments, and the carbon is assessed back on land using a custom-built instrument for analysing dissolved inorganic carbon, and total alkalinity," explains Dr McGrath.
Also on board is Dr Evin McGovern, a senior chemist at the Marine Institute. He tells me that "the challenges are to get sufficient data with spatio-temporal coverage to a high enough degree of accuracy and precision.
"Sensors are beginning to come on stream for the deployment and mooring of autonomous vehicles such as gliders, but we still require ship-based monitoring using instruments and old fashioned sampling and analysis."
Like a lot of scientific data, the latest measurements which the team have collected over the course of the week at sea will be made freely available to both scientists and the public alike. Such data allows for international collaboration between oceanographers and environmentalists who are interested in preserving our oceans, and protecting marine biodiversity.
According to Dr McGrath: "It is crucial that we understand how our marine environment is changing with time so we can determine the rate of acidification due to human emissions of carbon dioxide to the atmosphere."
Conor Purcell is a Science & Nature Writer with a PhD in oceanography. He can be found on Twitter @ConorPPurcell and some of his other articles at cppurcell.tumblr.com
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