Biogeosciences, 17, 1805–1819, 2020 https://doi.org/10.5194/bg-17-1805-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Distribution and flux of dissolved iron in the peatland-draining rivers and estuaries of Sarawak, Malaysian Borneo Xiaohui Zhang1, Moritz Müller2, Shan Jiang1, Ying Wu1, Xunchi Zhu3, Aazani Mujahid4, Zhuoyi Zhu1, Mohd Fakharuddin Muhamad4, Edwin Sien Aun Sia2, Faddrine Holt Ajon Jang2, and Jing Zhang1 1State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 200241 Shanghai, China 2Faculty Engineering, Computing and Science, Swinburne University of Technology, Sarawak Campus, 93350, Kuching, Sarawak, Malaysia 3School of Ecological and Environmental Sciences, East China Normal University, 200241 Shanghai, China 4Faculty of Resource Science & Technology, University Malaysia Sarawak, 94300, Sarawak, Malaysia Correspondence: Xiaohui Zhang ([email protected]) Received: 24 May 2019 – Discussion started: 18 June 2019 Revised: 12 February 2020 – Accepted: 24 February 2020 – Published: 3 April 2020 Abstract. Dissolved iron (dFe) is essential for multiple bio- in the dFe yield, such as in the Serendeng tributary of the geochemical reactions in oceans, such as photosynthesis, Rajang River and Simunjan River, where intensive oil palm respiration and nitrogen fixation. Currently, large uncertain- plantations were observed. ties remain regarding the input of riverine dFe into coastal oceans, especially in tropical rivers in southeastern Asia. In the present study, the concentrations of dFe and distribu- tion patterns of dFe were determined along the salinity gra- 1 Introduction dient in the Rajang River and three blackwater rivers that drain from peatlands, including the Maludam River, the Se- Iron (Fe) is an essential element for enzymes and is deemed buyau River and the Simunjan River. In the Rajang River, the to be responsible for photosynthesis, respiration and nitro- dFe concentration in freshwater samples (salinity < 1 PSU gen fixation (Moore et al., 2009; Raven, 1988; Williams, – practical salinity units) in the wet season (March 2017) 1981). Over the past 4 decades, Fe has been identified as a was higher than that in the dry season (August 2016), which micronutrient that significantly supports primary productiv- might be related to the resuspension of sediment particles and ity in oceans (Brand and Sunda, 1983; Moore et al., 2009; soil erosion from cropland. In the Rajang estuary, an intense Tagliabue et al., 2017). In particular, after a series of in situ removal of dFe in low-salinity waters (salinity < 15 PSU) fertilization experiments, researchers have verified the occur- was observed, which was likely due to salt-induced floccu- rence of Fe limitation on the growth of phytoplankton and its lation and absorption of dFe onto suspended particulate mat- critical effect on CO2 fixation (Boyd et al., 2007; Martin, ter (SPM). However, increases in the dFe concentration in 1990). the wet season were also found, which may be related to dFe At the global scale, the amount of riverine dissolved iron desorption from SPM and the influences of agricultural activ- (dFe) transported to coastal oceans is estimated to be 1:5 × 9 −1 ities. In the blackwater rivers, the dFe concentration reached 10 mol yr (Boyd and Ellwood, 2010; de Baar and de Jong, 44.2 µmol L−1, indicating a strong contribution to the dFe 2001; Jickells et al., 2005; Milliman and Farnsworth, 2011; budget from peatland leaching. The dFe flux derived from Saitoh et al., 2008). Tropical rivers might contribute a signif- the Rajang estuary to the South China Sea was estimated to icant quantity of dFe based on studies of the Amazon River be 6:4±2:3×105 kg yr−1. For blackwater rivers, the dFe flux (Bergquist and Boyle, 2006; Gaillardet et al., 1997) and the was approximately 1:1 ± 0:5 × 105 kg yr−1 in the Maludam Congo River (Coynel et al., 2005; Dupré et al., 1996). How- River. Anthropogenic activities may play an important role ever, few studies have assessed the dFe concentrations and transport patterns of tropical rivers in southeastern Asia, even Published by Copernicus Publications on behalf of the European Geosciences Union. 1806 X. Zhang: Distribution and flux though those rivers can account for approximately 30 % of 2 Materials and methods fluvial discharge to oceans (Milliman and Farnsworth, 2011). Estuaries, which are the interaction zone between surface 2.1 Study area loading and coastal oceans, effectively modulate dFe concen- trations during mixing and hence change the magnitude of Malaysia has the second largest peatland area (approximately the riverine dFe flux. A wide range of studies on the behav- 2:6 × 104 km2) in southeastern Asia (Mutalib et al., 1992). iors of dFe in estuaries have been conducted, and several dis- The state of Sarawak accounts for the largest peatland area tribution patterns have been documented (Boyle et al., 1977; of Malaysia and has widespread blackwater rivers (Joosten, Herzog et al., 2017; Oldham et al., 2017; Zhu et al., 2018). 2012; Wetlands International, 2010). Approximately 23 % of Generally, estuaries act as a sink for dFe due to flocculation the peatland in Malaysia is defined as relatively undisturbed, occurring between cations and high-molecular-weight col- of which 17 % is in Sarawak (Wetlands International, 2010). loids (Bergquist and Boyle, 2006; Boyle et al., 1977; Stolpe Since the mid-1980s, the rubber, textile, metal, food process- and Hassellov, 2007). In some rivers with high concentra- ing, petroleum and electronic industries have been developed tions of dissolved organic matter (DOM), dFe has been found in the area, and they are the major economic supporters in to be conservative because of the chemical connection of Fe Malaysia (Trade Chakra, 2009). As a response, the deforesta- to DOM (Oldham et al., 2017; Sanders et al., 2015; Stolpe tion rate in Sarawak increased to 2 % per year from 1990 to et al., 2010). The magnitude of dFe removal from estuaries 2010 (Miettinen et al., 2012), and this rate is attributed to oil can be quantified by removal factors (RFs). Anthropogenic palm and rubber tree plantations (Joosten, 2012). activities related to coal mining, the ore industry and agricul- The Rajang River, i.e., the largest river in Malaysia, which ture activities could significantly impact the concentrations has a length of 530 km, flows from Iran Mountain to the and distributions of dFe in estuaries (Braungardt et al., 2003; South China Sea (Fig. 1a, b). The drainage basin is 51 × Morillo et al., 2005; Xue et al., 2016). 103 km2 (Milliman and Farnsworth, 2011; Staub and Esterle, Currently, only limited records of dFe concentrations have 1993). The drainage area of the Rajang estuary is 6500 km2, been provided for peatland-draining rivers (Batchelli et al., and 50 % of it is covered with extensive peat at depths greater 2010; Krachler et al., 2010; Oldham et al., 2017). The dFe than 3 m (Staub and Gastaldo, 2003). The Rajang River is distribution in peatland-draining estuaries is also largely un- approximately 5–10 and 8–20 m deep during the dry season known. Coastal belts in southeastern Asia are covered by a and the wet season, respectively. The mainstream flow veloc- large area of peatlands, reaching approximately 9 % of the ity ranges from 0.2 to 0.6 and 0.8 to 1.2 m s−1 during the dry global peatland coverage (Dommain et al., 2011; Joosten, season and the wet season, respectively (Tawan et al., 2019). 2012). To the best of the authors’ knowledge, the dFe con- The discharge rate for the Rajang River reaches 6000 m3 s−1 centrations in Malaysia have only been determined (1) in in the wet season (December to March), with an average Pelagus, where high concentrations of dFe were observed discharge of approximately 3600 m3 s−1 (Staub et al., 2000; in freshwater due to the sediments there (Siong, 2015), and Staub and Gastaldo, 2003). The climate in the Rajang water- (2) in Bebar, a blackwater river in Pahang, Malaysia, where shed is classified as the tropical ever-wet type (Morley and the concentration of dFe was up to 30 µmol L−1. However, Flenley, 1987), while the precipitation rate varies between information about the distribution and biogeochemistry of the dry and wet seasons. The city of Sibu is assumed to be dFe is lacking (Shuhaimiothman et al., 2009). Such limita- the boundary line of the Rajang drainage basin and Rajang tions in dFe data may markedly influence the regional esti- estuary according to physiographic conditions (Staub et al., mations of the dFe budget. 2000; Staub and Esterle, 1993), and saltwater intrusions can To fill this gap in knowledge, two cruises were conducted reach downstream of the city (Jiang et al., 2019). Apart from in the state of Sarawak, Borneo, Malaysia, which included mineral soils transported from the upper stream, the Rajang the largest river in the state of Sarawak (the Rajang River) estuary also receives materials from the adjacent hill regions and three blackwater rivers. This study aims to determine and the Retus River (Staub and Gastaldo, 2003). There are (1) the concentration and distribution of dFe in the studied several tributaries for the Rajang River in the estuary, includ- rivers and their estuaries, (2) the seasonal variation in the ing Igan, Hulu Serendeng (further separated into two tribu- concentration and distribution of dFe in the Rajang River and taries: Paloh and Lassa), Belawai and Rajang. The Igan trib- its estuary, and (3) the dFe yield and the magnitude of river- utary is the major outlet for freshwater (Jiang et al., 2019). ine dFe flux to the coastal areas. Mangroves are distributed in the brackish-water area in the southwestern estuary near the Rajang and Serendeng trib- utaries, and some freshwater trees, such as Casuarina, are observed in the northeastern and coastal areas (Scott, 1985).