Selenium (Se) is an essential micronutrient for aquatic organisms. Despite its importance, our current knowledge of the biogeochemical cycling of
dissolved Se in tropical estuaries is limited, especially in Southeast Asia. To gain insights into Se cycling in tropical peat-draining rivers and
estuaries, samples were collected from the Rajang, Maludam, Sebuyau, Simunjan, Sematan, Samunsam and Lunda rivers and estuaries in western Sarawak,
Malaysia, in March and September 2017 and analysed for various forms of Se (dissolved inorganic and organic). Mean total dissolved Se (TDSe),
dissolved inorganic Se (DISe) and dissolved organic Se concentrations (DOSe) were 2.2
Selenium (Se) is an essential trace element for aquatic organisms (Bodnar et al., 2014). Low levels of Se in the food chain lead to disease or death, whereas high levels are toxic (Lobanov et al., 2009; Winkel et al., 2015). Selenium depletion in the Phanerozoic oceans may have contributed to three major mass extinction scenarios (Long et al., 2016). Thus, there has been great interest in Se biogeochemical cycling in aquatic systems for many decades (e.g. Cutter and Bruland, 1984; Cutter and Cutter, 1995, 2001; Mason et al., 2018).
The bioavailability of Se is determined by its concentrations and species (Fernández and Charlet, 2009). The behaviour of Se in natural waters is
complicated, as it exists in several oxidation states (
The chemical behaviour of Se in estuarine mixing plays an important role in overall geochemical cycling. From their investigation into dissolved Se
species in various estuaries, Chang et al. (2016) found that Se speciation was controlled by biological, physical and redox processes in the
estuaries; non-conservative processes resulting from phytoplankton uptake; absorption by suspended particles; and generation of particulate organic
selenide in the water. Thus far, the behaviour of Se in estuaries has been studied mainly in the temperate zone of the Northern Hemisphere (between
20 and 60
In the high-latitude peatland-draining rivers, dissolved Se concentrations are spatially variable, with concentrations of up to
13
To the best of our knowledge, the present study is the first analysis of the distribution and behaviour of dissolved species of Se in seven rivers and estuaries in western Borneo (Sarawak, Malaysia, Southeast Asia). We hypothesize that DOSe is the major species in those peatland-draining rivers that come mainly from peat soils and that sizable Se from peatland is delivered to the coastal areas. The main objectives of the study were to (1) evaluate the fate of dissolved Se species in peatland-draining estuaries, (2) characterize the DOSe fractions and (3) estimate the magnitude of Se fluxes delivered from to coastal ocean. The results of this study should contribute to an improved understanding of how Se behaves in tropical peat-draining rivers and estuaries.
Sarawak, Malaysia's largest state, is in the northwest of the island of Borneo, Malaysia (Müller et al., 2016). Sarawak has a tropical climate,
with a mean annual air temperature at the capital Kuching of 26.1
The physico-geographical parameters of sampled rivers.
Two sampling campaigns were conducted in peat-draining rivers and estuaries in Sarawak in 2017. The first was at the end of the northeastern monsoon
(from 4 to 12 March 2017, just after the wet season), and the second was shortly before the beginning of the southwestern monsoon (from 4 to 17 September 2017, in the dry season) (Fig. 1). Six rivers, namely the Rajang, Maludam, Simunjan, Sebuyau, Sematan and Samunsam, were sampled in March
and September, and the Lundu River was sampled only in September (Fig. 1). The physico-geographical parameters of sampled river basins are summarized
in Table 1. Four of the rivers (the Maludam, Simunjan, Sebuyau and Samunsam) drain catchments with high peatland coverages and are known as
blackwater rivers, whereas the Sematan and Lundu drain catchments with high proportions of mineral soils (Table 1). The Rajang River drains mineral
soils in its upper reaches (Staub et al., 2000) but branches into multiple distributary channels at Sibu (the Igan, Paloh and Rajang) that flow
from north to south through land covered with thick peat (Staub et al., 2000) (Fig. 1). Water samples were collected from a boat. As the boat moved
forward, surface water was collected upstream and to the side of the boat into an acid-cleaned polyethylene bottle attached to the end of a plastic
pole sampler (3–4
The
TDSe fluxes (
The Statistical Package for Social Sciences (SPSS) version 23.0 was used to perform a Student's
The water chemistry in the freshwater reach of the Maludam, Simunjan, Sebuyau and Samunsam rivers is typical of blackwater rivers draining peatlands with acidic pH and low DO concentrations, and the mixing with coastal water increased the pH and DO (Table S1, Fig. S1 in the Supplement). Values of pH and DO concentrations in the Sematan and Lundu, which drain mostly mineral soils, were higher than those in the blackwater rivers (Fig. S1 in the Supplement). In the Rajang estuary, values of pH and DO were lower in the riverine side, especially in the distributaries covered by the peat (Fig. S2 in the Supplement).
TDSe concentrations in the studied rivers and estuaries ranged from 1.0 to 5.7
The box plot of TDSe, DISe and DOSe concentration and DOSe
In the Rajang estuary,
Variation in Se species concentrations along a salinity gradient in the three tributaries (the Igan, Lassa and Rajang) of the Rajang Estuary in March
and September are shown in Fig. 3. Theoretical mixing lines (TMLs) were developed using two endmembers, namely, a freshwater endmember in the
freshwater reach of the Rajang River and a marine endmember with a salinity of
Relationships between DISe
Plots of DISe concentration against salinity show a positive linear regression between DISe and salinity in the Maludam, Sebuyau and Samunsam
estuaries (
Generally, relationships between the Se species and salinity fell into three groups. In the blackwater estuaries (the Maludam, Sebuyau and Samunsam), DISe concentrations were positively correlated with salinity; DOSe and TDSe concentrations were negatively correlated with salinity (Fig. 3). In the Rajang estuary, DISe increased with salinity but behaved non-conservatively and was removed in the brackish water region; whereas DOSe and TDSe decreased with salinity, behaved non-conservatively and were added during estuarine mixing (Fig. 3). In the Sematan estuary, TDSe, DOSe and DISe behaved non-conservatively and showed little change during estuarine mixing (Fig. 3).
For the freshwaters (
Relationships between
In the Maludam Estuary, DOSe concentrations were negatively correlated with the CDOM spectral slope from 275 to 295
Relationships between DOSe concentrations and
On a global perspective, TDSe concentrations in the sampled rivers were comparable with those in other reported rivers (between 0.2 and
30
Overview of the TDSe concentrations and DOSe
Species of Se are very sensitive to redox conditions and pH values (Sharma et al., 2015).
During estuarine mixing, reversed DISe concentration–salinity relationships were observed in the Rajang, Maludam, Sebuyau and Samunsam estuaries
(Figs. 3, S5 in the Supplement), which contrasts with those reported for other estuaries (Measures and Burton, 1978; Takayanagi and Wong, 1984;
Van der Sloot et al., 1985; Cutter, 1989a; Guan and Martin, 1991; Hung and Shy, 1995; Abdel-Moati, 1998; Yao et al., 2006; Chang et al., 2016). The
marine endmember of the DISe concentrations in the sampled estuaries (salinity
In contrast to DISe, DOSe concentrations were high in the rivers and decreased in a seaward direction as salinity increased (Figs. 3, S5 in the
Supplement). DOSe has been shown to behave non-conservatively in other estuaries, with concentrations decreasing along salinity gradients or with
mid-estuarine input (Cutter, 1989a; Guan and Martin, 1991; Hung and Shy, 1995; Abdel-Moati, 1998). DOSe concentrations in the estuaries studied in
Sarawak were higher than those reported in other estuaries (0.1 to 2.5
Coal deposits in Kanawha County in the USA have been interpreted as a dome-shaped peat swamp, analogous to those in Malaysia. Coal Se contents reached
10.7
Conceptual diagram of the behaviour of Se species in the Maludam estuary. HMW, LMW and DMSe represent high molecular weight, low molecular weight and dimethylselenide, respectively.
During the estuarine mixing, the negative correlation between DOSe
Information about the biogeochemistry of peat-draining rivers is scarce, thus their possible quantitative significance for the oceanic TDSe budget is
unexplored as of yet. The TDSe flux was estimated to be
To the best of our knowledge, this is the first study of seasonal variations in Se speciation in peat-draining rivers and estuaries in Southeast Asia. Contrary to the results from studies elsewhere, DOSe, not DISe, was the major species in the peat-draining rivers and estuaries of Sarawak, Malaysia. Contrary to our expectations, reversed DISe concentration–salinity relationships were observed in those estuaries, indicating a marine origin, while DOSe concentrations decreased with salinity, indicating terrestrial sources. The DOSe fractions may be associated with high molecular weight peatland-derived aromatic and black carbon compounds. The DOSe yields in the peatland-draining rivers were 1 or even 2 orders of magnitude higher than other reported rivers. The TDSe flux delivered by the peat-draining rivers exceeded other small rivers, and it is quantitatively more significant than previously thought. The impact of the sizable Se from increasing anthropogenic disturbing of peat to the ecosystem should be evaluated in the future.
The datasets used and/or analysed in this study are available from the corresponding author on reasonable request.
The supplement related to this article is available online at:
JZ, MM, YW, SJ and YC conceptualized the research project and planned the field expeditions. SJ, AM, ESAS, FHAJ and MM performed sample collection and in-situ measurement for the cruises. YC, WWC, JGQ, JLR, EMR and XLW completed laboratory analyses. YC, XLW, YW, SJ, XNW, JZ and MM processed and analysed the data. All co-authors participated in the interpretation and discussion of the results. YC prepared the manuscript with suggestions from all co-authors.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Biogeochemical processes in highly dynamic peat-draining rivers and estuaries in Borneo”. It is not associated with a conference.
The authors thank the Sarawak Forestry Department and the Sarawak Biodiversity Centre for giving them permission to conduct collaborative research in Sarawak waters under permit nos. NPW.907.4.4(Jld.14)-161, WL83/2017 and SBC-RA-0097-MM. Lukas Chin and the SeaWonder crew are acknowledged for their support during the cruises. Technical support from Patrick Martin and Gonzalo Carrasco at Nanyang Technological University during the cruises is gratefully acknowledged.
This research has been supported by the Newton-Ungku Omar Fund 3/1 (grant no. NE/P020283/1), the National Natural Science Foundation of China (grant no. 41806096), the China Postdoctoral Science Foundation (grant nos. 2018M630416 and 2018M632062), the MOHE FRGS 15 grant (grant no. FRGS/1/2015/WAB08/SWIN/02/1), the Overseas Expertise Introduction Project for Discipline Innovation (grant no. B08022), the SKLEC Open Research Fund (grant no. SKLEC-KF201610), the Scientific Research Foundation of SKLEC (grant no. 2017RCDW07), and the Fundamental Research Funds for the Central Universities.
This paper was edited by Steven Bouillon and reviewed by Oleg S. Pokrovsky and one anonymous referee.