The fate and variability of terrestrial dissolved organic carbon in the Sunda Shelf Sea

Abstract

Terrestrial dissolved organic carbon (tDOC) plays an important role in the global carbon cycle, accounting for 0.25 Pg C delivered from land to ocean annually. Yet despite this substantial input to the ocean, only limited amounts of tDOC are detected in the open ocean. This indicates that tDOC experiences relatively rapid and active biogeochemical degradation along coastal margins. A significant proportion of tDOC has been found to be remineralized in the major shelf seas globally, affecting ocean pH and air-sea CO2 exchange. However, the rates and relative contribution of different pathways, as well as the impact from the climate have not been well addressed. Although there are numerous chemical techniques to distinguish tDOC from DOC produced in aquatic ecosystems, all of these techniques have significant uncertainties and their accuracy may be affected when biogeochemical processes alter the tDOC. Moreover, studies about the biogeochemical fate of tDOC have been predominantly focused on subtropical and high-latitude regions, leaving a knowledge gap in the tropics. Southeast Asia harbors the largest area and carbon stock of tropical peatlands, accounting for 11–14% of global peat carbon. These peatlands deliver ~10% of the global land-to-ocean DOC flux annually, the majority of which enters the Sunda Shelf Sea. In the present thesis, I evaluated the validity of optical properties in quantifying tDOC concentration and remineralization, studied the degradation processes of tDOC remineralization and their relative importance, as well as the possible climatic influence on the variability of tDOC in the Sunda Shelf Sea. To study the validity of optical tDOC tracers, I used a 4-year time series of DOC, optical properties and carbon stable isotope ratios in the Singapore Strait, which is situated in the central Sunda Shelf. A carbon stable isotope mass balance shows that on average 56% of tDOC is remineralized to CO2 upstream of the study site, while 77% of chromophoric dissolved organic matter (CDOM) is lost, demonstrating that CDOM is preferentially removed compared to the bulk tDOC. Nevertheless, the data show that CDOM optical properties can be used to quantify the proportion of tDOC in the total coastal DOC pool accurately. However, these optical properties cannot reveal the degree of prior remineralization. To estimate the rates and relative importance of the main biogeochemical processes driving the remineralization, I performed laboratory incubations to measure photodegradation, biodegradation and photo-influenced biodegradation of tDOC in coastal water and river samples. My results show that tDOC exported into the Sunda Shelf Sea is highly photo-labile but has low biodegradability, and that prior irradiation enhances the following biodegradation. The apparent quantum yield and microbial decay rates were applied to a 1-dimensional model to simulate tDOC degradation from these 3 processes over the water residence time (2 years) in the Sunda Shelf Sea. The simulation shows that 64% of the initial tDOC input is remineralized within 2 years, of which pure photodegradation, pure biodegradation and photo-influenced biodegradation account for 41%, 36% and 23%, respectively. However, the effect of photo-influenced biodegradation may be underestimated, and we recommend that improved experiments should be designed in which photochemical and microbial degradation can interact simultaneously in a more realistic way. To study the seasonal and interannual variability of biogeochemistry, I performed correlation analysis between our time series of salinity, DOC and CDOM concentrations in the Singapore Strait and hydroclimatic and physical oceanographic factors. The strong correlations indicate that water volume transport, precipitation, El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) drive the variation in biogeochemical parameters. Given the important ecological consequences of tDOC, the complex combination of hydroclimatic and physical oceanographic drivers of the variation suggests that climate change can have significant influence on key aspects of coastal water quality in Southeast Asia, which needs to be addressed in the future.