Distribution and Flux of Dissolved Iron of the Rajang and Blackwater Rivers at Sarawak, Borneo

Abstract

<p><strong>Abstract.</strong> Dissolved iron (dFe) is essential for biogeochemical reactions in oceans, such as photosynthesis, respiration and nitrogen fixation. Currently, large uncertainties remain on riverine dFe inputs, especially for tropical rivers in Southeast Asia. In the present study, dFe concentrations and distribution along the salinity gradient in the Rajang River in Malaysia, and three blackwater rivers draining from peatlands, including the Maludam River, the Sebuyau River, and the Simunjan River, were determined. In the Rajang River, the concentration of dFe in fresh water (salinity < 1) in the wet season (March 2017) was higher than that in the dry season (Auguest 2016), which might be related to the resuspension of sediment particles and soil erosions from cropland in the watershed. In the Rajang Estuary, an intensive removal of dFe in low salinity waters (salinity < 15) was observed, likely due to the salt-induced flocculation and the absorption onto suspended particulate matters (SPM). However, dFe concentration enhancements in the wet season occured in some sampling sites, which may be related to the desorption from SPM and agriculture activities. On the other hand, dFe was conservatively distributed in high salinity waters (salinity > 15), which may result from the association between dFe and pelagic organic matters. In the blackwater rivers, concentrations of dFe reached 44.2 μmol L<sup>−1</sup>, indicating a great contribution from peatland. The dFe flux derived from the Rajang Estuary to the South China Sea was (6.4 ± 2.3) × 10<sup>5</sup> kg yr<sup>−1</sup>. For the blackwater river, the dFe flux was approximately (1.1 ± 0.5) × 10<sup>5</sup> kg yr<sup>−1</sup>, in the Maludam River. The anthropogenic activities may play an important role in the dFe yield, such as the Serendeng tributary of the Rajang River, and Simunjan River, where intensive oil palm plantations were observed.</p>

From canals to the coast: dissolved organic matter and trace metal composition in rivers draining degraded tropical peatlands in Indonesia

“…The two fluorophores are typical of terrestrial input of DOM (Yamashita et al, 2008) and similar to observed fluorophores in other blackwater rivers in Borneo (Harun et al, 2015;Zhou et al, 2019). An increase in this C1/C2 ratio reflects a shift toward lower wavelengths and therefore toward lower aromaticity and lower molecular weight (Austnes et al, 2010;Zhou et al, 2019). Moreover, a decreasing trend in SUVA values is observed along Table 2.…”

“…The fluores- Table 1. cence index has relatively high values for tropical peatlands, where most of the DOM is of terrestrial origin (Gandois et al, 2014;Zhou et al, 2019). The FI values vary widely in the drainage canals (1.60 ± 0.17) and blackwater river (1.55 ± 0.06) but are more uniform in the whitewater river, both upstream (1.55 ± 0.03) and downstream (1.55 ± 0.05) of the confluence with the blackwater river.…”

The distribution and behaviour of Fe, Al, Si, Mn, Cu and Ni in ombrotrophic tropical peat draining blackwater estuaries on Borneo Island

“…Malaysia is rich in peat swamp forests distributed across the various states with 23 percent retaining near-pristine nature; 17 percent are located in the state of Sarawak (Wetlands International, 2010). The state of Sarawak on the island of Borneo has the largest tropical peat swamp, thereby making Malaysia the second largest home to tropical peat swamp forests with an area of approximately 2.6 x 10 4 km 2 (Zhang et al, 2020). The establishment of major industries such as metal, food processing, electronics, petroleum, rubber, and textile in Malaysia (the mid-1980s) contributed to an increase in the deforestation rate in the country.…”

“…The establishment of major industries such as metal, food processing, electronics, petroleum, rubber, and textile in Malaysia (the mid-1980s) contributed to an increase in the deforestation rate in the country. The conversion of tropical peat swamp forests for agricultural purposes including rubber, oil palm, and sago plantations between 1990 and 2010 accounted for a 2 percent increase in the deforestation rate yearly in Sarawak (Zhang et al, 2020). Indonesia accounts for about 80 percent of the tropical peat swamp forests globally, although about 94,000 km 2 (above 36,000 square miles) have been lost due to anthropogenic activities (Dargie et al, 2017).…”

“…River water certified reference material (CRM, SLRS 6 Riverine Water, Canada) for trace elements from National Research Council, Canada was prepared using the same procedure as the water samples and analyzed using HR-ICP-MS. Table 2 shows the results for the CRM and trace element analyses, including the percentage recovery for the CRM. We prepared standards for both fresh and seawater samples using trace element concentrations in Southeast Asia (Ishak et al, 2016;Tashakor et al, 2018;Zhang et al, 2020). 150 µl of seawater sample was added to the standards to create similar conditions and used for calculating trace element concentration in seawater samples.…”