Southeast Asia harbors the largest area of the world's tropical peatlands, which are widely distributed in coastal areas of Sumatra and Borneo and store around 69 Pg C of terrestrial organic carbon (Dommain et al., 2014;Page et al., 2011). Consequently, Southeast Asia is also a hotspot of terrestrial organic carbon export to the ocean: the fluvial flux of terrigenous dissolved organic carbon (tDOC) to the Sunda Shelf Sea is ∼21 Tg C yr −1 , which accounts for ∼10% of the annual global land-to-ocean tDOC flux by the world's rivers (Baum et al., 2007;Moore et al., 2011). In addition, most of this peatland area has been anthropogenically disturbed by deforestation and land conversion (Miettinen et al., 2016), which is thought to have increased the peatland tDOC export to the ocean by 32% over the past three decades (Moore et al., 2013;Yupi

Terrestrial dissolved organic carbon (tDOC) is significant for coastal carbon cycling, and absorbance and fluorescence spectroscopy of chromophoric and fluorescent dissolved organic matter (CDOM, FDOM) are widely used to study tDOC cycling. However, CDOM and FDOM are often amongst the more labile components of tDOC. Because few studies have compared CDOM and FDOM to measurements of both bulk tDOC concentration and tDOC remineralization, it remains unclear how accurately CDOM and FDOM actually trace tDOC in coastal waters when tDOC undergoes extensive remineralization. We collected a 4-year coastal timeseries in Southeast Asia, where tropical peatlands supply large tDOC inputs. A carbon stable isotope mass balance shows that on average 56% of tDOC was remineralized upstream of our site, while 77% of CDOM was bleached. Despite this extensive tDOC remineralization and preferential CDOM loss, optical properties could reliably quantify tDOC. CDOM spectral slope properties, such as S275–295, are exponentially related to tDOC; these are highly sensitive tDOC tracers at low, but not at high, tDOC concentrations. Other properties are linearly related to tDOC, and both specific ultraviolet absorbance (SUVA254) and DOC-normalized fluorescence intensity may be suitable to quantify tDOC over a wider range of concentrations. However, the optical properties did not show consistent changes with the extent of tDOC remineralization. Our data support the validity of CDOM and FDOM spectroscopy to trace tDOC across coastal gradients even after the majority of tDOC has been remineralized, but they also show that these measurements may not provide much information about tDOC biogeochemical processing.