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.
Highlights
Tropical peatlands store around 100 Pg of carbon, of which 55 % is found in Southeast Asia (Page et al, 2011; Dargie et al, 2017), mostly on the islands of Sumatra and Borneo (Dommain et al, 2014)
Terrigenous dissolved organic matter can play significant roles in aquatic environments: tDOM is susceptible to decomposition processes that can remineralize a considerable proportion (40 %–50 %) of it in estuaries and shelf seas (Fichot and Benner, 2014; Kaiser et al, 2017)
We report our values in Raman units (RU), which can be roughly converted to quinine sulfate units (QSU) by multiplying by 48.9 (Stedmon and Markager, 2005a)
Summary
Tropical peatlands store around 100 Pg of carbon, of which 55 % is found in Southeast Asia (Page et al, 2011; Dargie et al, 2017), mostly on the islands of Sumatra and Borneo (Dommain et al, 2014). The rivers draining Southeast Asia’s peatlands export large quantities of terrigenous dissolved organic carbon (tDOC), accounting for ∼ 10 % of the global land-to-ocean DOC flux of 0.2–0.25 Pg C yr−1 (Meybeck, 1982; Baum et al, 2007; Moore et al, 2011; Dai et al, 2012). Terrigenous dissolved organic matter (tDOM) can play significant roles in aquatic environments: tDOM is susceptible to decomposition processes that can remineralize a considerable proportion (40 %–50 %) of it in estuaries and shelf seas (Fichot and Benner, 2014; Kaiser et al, 2017). In the Congo River, which drains extensive tropical peatlands, > 95 % of the lignin phenols and 45 % of the total DOC pool are labile to photodegra-
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