Abstract. Dissolved inorganic nitrogen (DIN), including nitrate, nitrite and ammonium, frequently acts as the limitation for primary productivity. Our study focused on the transport and transformation of DIN in a tropical estuary, i.e., the Rajang River estuary, in Borneo, Malaysia. Three cruises were conducted in August 2016 and February–March and September 2017, covering both dry and wet seasons. Before entering the coastal delta, decomposition of the terrestrial organic matter and the subsequent soil leaching was assumed to be the main source of DIN in the river water. In the estuary, decomposition of dissolved organic nitrogen was an additional DIN source, which markedly increased DIN concentrations in August 2016 (dry season). In the wet season (February 2017), ammonium concentrations showed a relatively conservative distribution during the mixing, and the nitrate addition was weak. La Niña events induced high precipitations and discharge rates, decreased reaction intensities of ammonification and nitrification. Hence similar distribution patterns of DIN species in the estuary were found in September 2017 (end of the dry season). The magnitude of riverine DIN flux varied between 77.2 and 101.5 t N d−1, which might be an important support for the coastal primary productivity.

<p><strong>Abstract.</strong> Dissolved inorganic nitrogen (DIN), including nitrate, nitrite and ammonium, frequently acts as the limitation for primary productivity. Our study focused on the transport and transformation of dissolved inorganic nitrogen in a tropical estuary, i.e. Rajang river estuary, in Borneo, Malaysia. Three cruises were conducted in August 2016, February–March and September 2017, covering both dry and wet seasons. Before entering the coastal delta, decomposition of the terrestrial organic matter and the subsequent soil leaching was assumed to be the main source of DIN in the river water. In the estuary, decomposition of dissolved organic nitrogen was an additional DIN source, which markedly increased DIN concentrations in August 2016 (dry season). In the wet season (February 2017), ammonium concentration showed a relatively conservative distribution during the mixing and nitrate addition was weak. In September 2017 (dry season), La Niña induced high precipitation and discharge rates, decreased reaction intensities of ammonification and nitrification and hence the distribution of DIN species in the estuary water was similar with the trend found in the wet season. The magnitude of riverine DIN flux varied between 77.2 and 101.5 ton N d<sup>-1</sup>, which might be an important support for the coastal primary productivity.</p>

<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>

Abstract. Tropical peatlands are one of the largest pools of terrestrial organic carbon (OCterr); however, our understanding of the dynamics of OCterr in peat-draining rivers remains limited, especially in Southeast Asia. This study used bulk parameters and lignin phenol concentrations to investigate the characteristics of OCterr in a tropical peat-draining river system (the main channel of the Rajang and three smaller rivers: the Maludam, Simunjan, and Sebuyau) in the western part of Sarawak, Malaysian Borneo. The depleted δ13C levels and lignin composition of the organic matter indicates that the most important plant source of the organic matter in these rivers is woody angiosperm C3 plants, especially in the three small rivers sampled. The diagenetic indicator ratio, i.e., the ratio of acid to aldehyde of vanillyl phenols ((Ad∕Al)V), increased with decreasing mean grain size of sediment from the small rivers. The selective sorption of acid relative to aldehyde phenols might explain the variations in the (Ad∕Al)V ratio. Elevated (Ad∕Al)V values observed from the Maludam's sediments may also be attributed to source plant variations. The (Ad∕Al)V ratio appears to be related to the C∕N ratio (the ratio of total organic carbon to total nitrogen) in the Rajang and small rivers. In small rivers, a quick decline of C∕N ratios is a response to the slower modification of (Ad∕Al)V ratios due to better preservation of lignin phenols. An accumulation of lignin phenols with higher total nitrogen percentages (TN%) in the studied systems was observed. Most of the OCterr discharged from the Rajang and small river systems was material derived from woody angiosperm plants with limited diagenetic alteration before deposition and thus could potentially provide significant carbon to the atmosphere after degradation.

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