Decomposition and environmental impacts of terrestrial organic carbon in the Sunda Shelf Sea, Southeast Asia

The flux of dissolved organic carbon (DOC) from soils on land through rivers to the ocean remains a poorly understood aspect of the global carbon cycle. This flux is quantitatively significant and has increased in many parts of the world as a result of human activities, including the conversion of natural ecosystems to other land uses. However, it is still unclear how much of this terrestrial DOC decomposes to carbon dioxide in the ocean, how rapid this decomposition is, and how it is controlled in different parts of the world. Global carbon cycle models do not represent this flux at all. Consequently, we have a poor understanding of how changes in land–ocean DOC fluxes impact the global carbon cycle and marine ecosystems. The proposed project will track the fate of DOC from Southeast Asian peatlands, which contribute around 10% of the global land–ocean DOC flux, across the Sunda Shelf Sea. We will test the hypothesis that most of this DOC decomposes to carbon dioxide (CO2) within the shelf sea. We will additionally test the hypotheses that (1) undecomposed DOC absorbs environmentally significant quantities of sunlight underwater, and (2) that the decomposition of this DOC causes environmentally significant levels of ocean acidification. We will measure dissolved organic and inorganic carbon at the three locations where peatland DOC can be transported off the Sunda Shelf into the Indian Ocean, and in the Singapore Strait, which is close to major peatland DOC sources. We will use optical spectroscopy and stable carbon and hydrogen isotopes to quantify terrestrial DOC and its decomposition to CO2. We will measure the seawater carbonate system to quantify ocean acidification, and measure underwater light attenuation and inherent optical properties to quantify the absorption of sunlight by terrestrial DOC. We will measure the decomposition rates of terrestrial DOC by both photo-oxidation and microbial decomposition in laboratory experiments. We will then use a numerical ocean model to simulate the input and decomposition of terrestrial DOC across the Sunda Shelf. The model will predict in detail how much terrestrial DOC is decomposed within the Sunda Shelf, and which areas are impacted to what extent by sunlight absorption and by ocean acidification. The study is feasible, because many of the techniques are individually well established, the PI and collaborators have substantial experience of conducting the necessary field work, and a suitable model has already been applied to other aspects of the carbon biogeochemistry of this region. The novelty lies in combining several established and novel field and laboratory techniques with biogeochemical modelling to resolve the fate and environmental impacts of terrestrial DOC in an important, but little-studied region of the world. This study will significantly advance our understanding of terrestrial DOC in the ocean, as well as our ability to measure it and model its biogeochemistry.