Composition and cycling of dissolved organic matter from tropical peatlands of coastal Sarawak, Borneo, revealed by fluorescence spectroscopy and parallel factor analysis

Abstract

Abstract. Southeast Asian peatlands supply ∼10 % of the global flux of dissolved organic carbon (DOC) from land to the ocean, but the biogeochemical cycling of this peat-derived DOC in coastal environments is still poorly understood. Here, we use fluorescence spectroscopy and parallel factor (PARAFAC) analysis to distinguish different fractions of dissolved organic matter (DOM) in peat-draining rivers, estuaries and coastal waters of Sarawak, Borneo. The terrigenous fractions showed high concentrations at freshwater stations within the rivers, and conservative mixing with seawater across the estuaries. The autochthonous DOM fraction, in contrast, showed low concentrations throughout our study area at all salinities. The DOM pool was also characterized by a high degree of humification in all rivers and estuaries up to salinities of 25. These results indicate a predominantly terrestrial origin of the riverine DOM pool. Only at salinities > 25 did we observe an increase in the proportion of autochthonous relative to terrestrial DOM. Natural sunlight exposure experiments with river water and seawater showed high photolability of the terrigenous DOM fractions, suggesting that photodegradation may account for the observed changes in the DOM composition in coastal waters. Nevertheless, based on our fluorescence data, we estimate that at least 20 %–25 % of the DOC at even our most marine stations (salinity > 31) was terrestrial in origin, indicating that peatlands likely play an important role in the carbon biogeochemistry of Southeast Asian shelf seas.

Distribution and flux of dissolved iron in the peatland-draining rivers and estuaries of Sarawak, Malaysian Borneo

“…This finding may be related to the flocculation of the negatively charged colloids with cations during the mixing of fresh-saline water. This phenomenon has been observed in many rivers and simulation experiments (Boyle et al, 1977;Oldham et al, 2017;Zhu et al, 2018). In addition, dFe was negatively correlated with SPM in low-salinity waters ( Fig.…”

“…It has been confirmed that the partition coefficient of dFe decreases with increasing SPM concentration and is inversely proportional to the log of the SPM concentration, termed the particle concentration effect (Benoit, 1995;Jiann et al, 2013;Turner and Millward, 2002). Furthermore, Zhu et al (2018) suggested that desorption from particles was the main reason for the dFe enhancement occurring in the river mouth area of the Changjiang estuary. In the wet season, Serendeng tributary samples were collected during a spring tide.…”

“…3a, a strong increase in the dFe and SPM contents at salinities of 5-15 PSU are shown. Given a similar level of SPM content among the Rajang, river waters of Texas (Jiann et al, 2013) and Changjiang estuary (Zhu et al, 2018), we assumed that the dFe enrichment under this special condition may be related to desorption from the riverine SPM, though we lacked experimental confirmation, e.g., a mixing experiment to validate our assumptions. In addition, the limited number of bottom water samples studied in the Rajang estuary also revealed that the addition of dFe from salinities of 5-15 PSU in the wet season might also be the result of the resuspension of bottom water sediments because the bottom water dFe concentration was much higher than the surface dFe concentration.…”

“…Estuaries, which are the interaction zone between surface loading and coastal oceans, effectively modulate dFe concentrations during mixing and hence change the magnitude of the riverine dFe flux. A wide range of studies on the behaviors of dFe in estuaries have been conducted, and several distribution patterns have been documented (Boyle et al, 1977;Herzog et al, 2017;Oldham et al, 2017;Zhu et al, 2018). Generally, estuaries act as a sink for dFe due to flocculation occurring between cations and high-molecular-weight colloids (Bergquist and Boyle, 2006;Boyle et al, 1977;Stolpe and Hassellov, 2007).…”

Distribution and behaviour of dissolved selenium in tropical peatland-draining rivers and estuaries of Malaysia

“…In the Maludam Estuary, DOSe concentrations were negatively correlated with the CDOM spectral slope from 275 to 295 nm (S 275-295 ) and were positively correlated with the humic-like C3 component and specific UV absorbance at 254 nm (SUVA 254 ) during estuarine mixing in both seasons (data from Martin et al, 2018;Zhou et al, 2019, Fig. 5a-c).…”