The validity of optical properties as tracers of terrigenous dissolved organic carbon during extensive remineralization in coastal waters

Abstract

Terrestrial dissolved organic carbon (tDOC) is significant for coastal carbon cycling, and spectroscopy of chromophoric and fluorescent dissolved organic matter (CDOM, FDOM) is widely used to study tDOC cycling. However, CDOM and FDOM are often amongst the more labile components of tDOC. Because few studies have compared spectroscopy 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 provide a large tDOC input. A carbon stable isotope mass balance shows that on average 53% of tDOC was remineralized upstream of our site, while 74% of CDOM was bleached. Despite this extensive tDOC remineralization and preferential CDOM loss, optical properties could still 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 direct information about the degree of natural tDOC processing.