Degradability and remineralization of peat-derived terrestrial dissolved organic carbon in the Sunda Shelf Sea

The remineralization of terrestrial dissolved organic carbon (tDOC) plays an important role in coastal carbon and nutrient cycling, and can affect primary productivity and seawater pH. However, the fate of tDOC in the ocean remains poorly understood. Southeast Asia’s Sunda Shelf Sea receives around 10% of global tDOC input from peatland-draining rivers. Here, we performed photodegradation and long-term (2 months to 1.5 years) biodegradation experiments with samples from peatland-draining rivers and from peat tDOC-rich coastal water. We used the resulting photochemical and microbial decay rates to parameterize a 1-dimensional model simulation. This indicates that 24% and 23% of the initial tDOC entering the Sunda Shelf can be remineralized by pure photo- and pure biodegradation, respectively, after 2 years (which represents an upper limit of seawater residence time on the Sunda Shelf). We also show for the first time that the biodegradation rate of Southeast Asian peat tDOC is enhanced by prior photodegradation. Adding photo-enhanced biodegradation to our model simulation causes remineralization of an additional 16% of the initial tDOC. However, the contribution of photo-enhanced biodegradation was likely underestimated because the photo- and biodegradation steps were conducted successively in our experiments. Overall, our results suggest a notably higher contribution of photodegradation compared with other regions, owing to the combination of slow biodegradation, high solar irradiance, long water residence time on the shelf, and the photo-enhancement of the biodegradation rate. Our results are important for informing tDOC modeling studies, and highlight a need for further research on interactive photo–biodegradation of tDOC.