Acidification impacts on the surface ocean
How will ocean acidification affect surface ocean biology, biogeochemistry and climate?
The burning of fossil fuels is releasing vast quantities of extra carbon dioxide (CO2) to the Earth’s atmosphere. A proportion of this stays in the atmosphere, raising atmospheric CO2 levels, but much leaves the atmosphere after a time, either to become sequestered in trees and plants or to become absorbed in the ocean. When it dissolves in the ocean, CO2 changes the chemistry of the water, making it more acidic. As a result the pH of surface water has fallen by 0.1 units since the industrial revolution, the period in which human activities become high producers of CO2.
What we know is that if we keep emitting CO2 at today rates, by 2100 at today’s rates, by 2100 average surface ocean pH will have fallen from 8.1 to around 7.8 – average levels the Earth has probably never experienced for more than 20 million years. What is much less well known and more strongly debated is the impact that the changing chemistry will have on marine organisms and ecosystems, on biogeochemical cycling in the sea, and on how the sea interacts with the atmosphere to influence global climate.
The group’s research is focused on explaining how different degrees of ocean acidification will affect the surface ocean. We are investigating how changes in the carbonate chemistry of the ocean may have a strong impact on bacteria (an important part of the marine ecosystem), on phytoplankton and on zooplankton species that together form the lower levels of the marine food chain. Furthermore, it investigates how the sea interacts with the atmosphere to influence climate
Most work to date has consisted of experimental studies in which organisms have been subjected to elevated CO2 (and the associated lower pH/increased acidity) in tanks or flasks. An advantage of these laboratory experiments is control; everything is kept constant except for the changes to CO2 and pH, so if a response is observed the cause is clear. However, there are also limitations to such studies. For instance, there is no time for organisms to adapt evolutionarily or for there to be shifts in species composition, away from more affected forms towards more acid-tolerant forms, as might occur in nature.
We aim instead to study the response of real-world plankton to acidification and provide an increased mechanistic and quantitative understanding of the effects of ocean acidification on organism physiology, variation in shell size and shape, biogeochemical rates, plankton biodiversity and community structure, food webs and climate-affecting processes.
In addition to some laboratory experiments, the project focuses on a complementary approach, primarily based on observations of how naturally-occurring ecosystems vary between places where the chemistry of seawater is naturally more acidic, and those where it is normally more alkaline. We are also conducting a large number of experiments in which we bring volumes of natural seawater from the ocean surface and place them in customised laboratory container on deck, and there subject the organisms within to higher CO2 and other stressors such as altered temperature and/or nutrients. We will monitor the changes that take place within these natural communities as the seawater acidity is increased to controlled levels that may be expected in the future oceans at current CO2 emission rates. A major strength of such studies is the inclusion of natural environmental variability and complexity that is difficult or impossible to capture in laboratory experiments. Thus, the responses measured during these controlled experiments on the naturally-occurring community may represent more accurately the future response of the surface ocean to acidification
By contrasting the two sets of observations, we will gain an improved understanding of how acidification affects organisms living in their natural environment, after natural selection and evolutionary adaptation have had time to play out.
Most of the planned work will be carried out on three cruises to locations with strong gradients in seawater carbon chemistry and pH; the Arctic Ocean, around the British Isles and the Southern Ocean.
A large grouping of UK researchers will work together in order to carry out this research. Together they possess many years of experience at conducting sea-going oceanographic research.
The research is part of the UK Ocean Acidification Research Programme (UKOA). funded by the Natural Environment Research Council (NERC), the Department of Environment, Food and Rural Affairs (Defra) and the Department of Energy and Climate Change (DECC).