Jul 18, 2012
Seagrass to the rescue?
Seagrass in the tropics could help counteract the effects of ocean acidification by altering seawater chemistry. The plant's carbon-absorbing photosynthesis could ultimately raise coral calcification rates by up to 18%, according to a team from the UK and Australia.
"Tropical seagrass meadows often produce so much photosynthetically-derived oxygen that you can see them practically bubbling away," Richard Unsworth of Swansea University, UK, told environmentalresearchweb. "We wanted to understand whether this could be a major local influence on the seawater and send the carbon chemistry in the opposite direction to the problems of ocean acidification."
Unsworth and colleagues from James Cook University, Australia, University of Oxford, UK, and the Queensland Government's Northern Fisheries Centre, Australia, analysed data from 11 studies of seagrass meadows in the Indo-Pacific. They found the ecosystems had an 83% chance of being net autotrophic (i.e. all organisms in the system producing less carbon by respiration than is absorbed by seagrass photosynthesis). On average the seagrass meadows provided a net sink of 155 g of carbon per square metre per year.
"Our results are of wide implication to the management of coral reefs under the threat of global ocean acidification," said Unsworth. "Seagrass may be able to locally offset the impacts of coral-reef acidification if it is appropriately managed to remain productive and resilient into the future."
The team found that the productivity of tropical seagrass could change the pH of seawater and the saturation state of aragonite (Ωarag), both of which affect the capacity of corals to calcify. Aragonite is a form of calcium carbonate that is vital for shell building.
"Our models indicate that increases in pH of up to 0.38 units and aragonite saturation state increases of 2.9 are possible in the presence of seagrass meadows," said Unsworth, "with the precise values of these increases dependent on water-residence time of the water – tidal flushing – and the water depth."
The team reckons that in shallow-water reef environments, scleractinian (stony) coral calcification downstream of seagrass "has the potential to be around 18% greater than in an environment without seagrass".
According to Unsworth, the results build on previous short measures of pH and detailed measures of seawater carbon chemistry from seagrass meadows in the Mediterranean, but this is the first work to "consider these processes in a tropical context and in relation to their potential effect on coral reefs".
Next the researchers will test the validity of their models using detailed analyses of seawater chemistry from fieldwork this summer in Green Island, Australia and Wakatobi, Indonesia.
"We also intend to establish the ecological drivers for what determines the capacity of the seagrass system to become net autotrophic, and consequently change the seawater carbon chemistry," said Unsworth. "By understanding the system-wide drivers, seagrass can be managed better in the future to enhance this capacity and in the long-term help to locally mitigate against ocean acidification."
The team reported the results in Environmental Research Letters (ERL).
About the author
Liz Kalaugher is editor of environmentalresearchweb.