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
Nitrogen (N) is an essential element for life. The concentration of N may
significantly influence species composition and diversity in terrestrial,
freshwater and ocean ecosystems (Vitousek et al., 1997). Apart from nitrogen
gas (
Estuaries are the linkage between terrestrial surface water and coastal sea (Seitzinger et al., 2002) and have received much attention from researchers and coastal managers with regard to the quantification of terrestrial DIN transport (e.g., Falco et al., 2010; Holmes et al., 2012; Li et al., 2013; Kuo et al., 2017). The mixing between freshwater and saline water develops physiochemical gradients in various parameters such as dissolved oxygen (DO), salinity and pH (Spiteri et al., 2008), which influence the growth of distinct bacterial communities (e.g., Goñi-Urriza et al., 2007; Spietz et al., 2015). These bacteria actively participate in DIN transformation processes, such as denitrification, DNRA (dissimilatory nitrate reduction to ammonium) and anammox (anaerobic ammonium oxidation; Burgin and Hamilton, 2007). These processes strongly influence DIN concentrations (Canfield et al., 2010), adding uncertainties to the precise estimation of DIN fluxes. Moreover, environmental parameters related to these gradients vary significantly between seasons, leading to a highly dynamic DIN export. Additionally, riparian regions are the focus of anthropogenic processes, including agriculture, manufacturing and wastewater treatment. (Richardson et al., 2007). These activities strongly influence DIN cycling in rivers and estuaries.
Researchers increased the sampling frequency and introduced regional
modeling work to improve the understanding of DIN transport and
transformation processes. More importantly, stable isotope fractions, e.g.,
Despite the significant advances made, knowledge gaps in DIN transport via estuaries still exist, with geographic coverage as one of the major shortcomings. As aforementioned, previous research work was intensively conducted in temperate and sub-tropical zones. The tropical zone, characterized by the high annual temperature and intense precipitations, hosts substantial rivers and streams, while studies on DIN transport are scarce, especially in Southeast Asia. The lack of information potentially hampers the holistic understanding of DIN transport from rivers to oceans and increases uncertainties in the global DIN budget estimation (Voss et al., 2013). In addition, since the Second World War, a rapid development in tropical countries has been witnessed. For instance, from 1960 to 2008, the gross national income (GNI) per capita in Malaysia (3.3–5.3) was much higher than the global average level (0.7–1.8; Tran, 2013). Coupled with urbanization, land-use change and population increase, the tropical zone is becoming a hotspot for DIN production, utilization and transport (Seitzinger et al., 2002). The ecological and environmental response in tropical estuaries to the DIN-related anthropogenic pressure is less documented and the magnitudes of river-borne DIN fluxes to coastal lines in the tropical zone are less evaluated. Moreover, the coastal ecosystem in the tropical zone is often complex in structure and harbors substantial seagrass meadow, fishes and coral reefs (Sale et al., 2014). The enhanced intrusion of allochthonous DIN from estuaries to coastal regions might be an ecological risk for local systems (Barbier et al., 2011). Adding these together, it is of urgency and importance to apply robust DIN studies in the tropical zone, and cooperation between researchers from multiple disciplines is highly necessary.
In the present study, three cruises in a typically tropical estuary, i.e.,
the Rajang River estuary (hereafter Rajang estuary), in Malaysia were
conducted, from August 2016 to September 2017, covering both dry and wet
seasons. Each cruise started from the upper stream and extended to the
coastal ocean. Concentrations of PN, dissolved organic nitrogen (DON) and
DIN were determined, and isotope fractions of
The Rajang River (hereafter Rajang) is located in Sarawak state, Borneo
(Fig. 1a and b). Sarawak is one of the largest states in Malaysia, with an
intensive tropical forest coverage. By 2000, the population in Sarawak was
2.5 million with an urbanization level of 47.9 %
(
Maps of the sampling area:
The Rajang is the largest river in Malaysia, with a length of ca. 530 km. It
originates from the Iran Mountains, flows through several cities, such as
Kanowit, Song and Sibu, enters the Rajang delta and discharges into the
South China Sea (Staub et al., 2000). The watershed coverage is
approximately
Three cruises were conducted during 2016–2017, covering two dry seasons
(August 2016, September 2017) and a wet season (February to March 2017). The
sampling in each cruise included the river water sites and brackish water
sites in the Igan, Lassa–Paloh and Rajang tributaries. High-salinity water
samples (
The river water and coastal seawater were collected into 1 L acid-prewashed high-density polyethylene (HDPE) sampling bottles via a pole sampler that decreases the contamination from boat surface and engine cooling water (Zhang et al., 2015). Apart from four stations in September 2017, salinity, temperature, DO and pH were measured in situ by an Aquaread® multiple-parameter probe (AP-2000). For porewater samples, a sampling hole with a depth of approximately 20 cm was dug at low tide. The seeped pore water that accumulated in the bottom of the sampling hole was discarded three times before collection. Subsequently, pore water was sucked into a 50 mL syringe and then transferred to 250 mL acid-prewashed sampling bottles. The rainwater was collected under the roof at a local primary school in a strong precipitation event. The rainwater from the first 10 min was discarded. Salinity of the pore water and rainwater was determined by a refractometer.
The filtration was conducted immediately after sampling. The harvest water
samples were shaken and then divided into two portions (excluding pore water
and rainwater). The first portion was filtered via polycarbonate membrane
filters (0.4
In September 2017, a mixing experiment was conducted to explore the
influence of river-borne suspended particles on DIN transformation along the
salinity gradient. In particular, river water samples (salinity: 0) from
Sibu (10 km downstream from the city dock) and the coastal ocean (salinity:
32 ‰) were collected. All the seawater and half of the
river water were filtered through polycarbonate membrane filters to remove
particle matters. The first treatment group was assigned to be the mixture
between the filtered seawater and the particle-free river water. In
practice, they were mixed and placed in 1 L acid prewashed HDPE bottles with
a volume of 500 mL. The percentage of river water in the system was 0 %
(purely filtered seawater), 25 %, 50 %, 75 % and 100 % (pure river
water). The second treatment group contained filtered seawater and
unfiltered river water, while the total volume and percentage of river water
were identical to the first treatment. The HDPE bottles were placed in the
dark at 25–26
After thawing and thorough remixing, concentrations of
To understand the addition or removal of solutes during the mixing, a
two-endmember mixing model (Liss, 1976) used for the conservative
distribution of solute concentration and related isotope fractionations was
invoked:
Total DIN fluxes (
All the statistical analyses, such as a Student's
The spatial distribution of each parameter, e.g., salinity, temperature and solute concentration, was plotted in Surfer 14.0 (Golden Software Inc., USA), and the dot plots were done in Sigmaplot 12.5 (Systat Software Inc., USA). Given the limited space, a portion of the plots were displayed in the Supplement (Figs. S3 to S11).
Distribution of salinity, suspended particle matter (SPM) and
In August 2016, the salinity in the sampled water ranged from 0.02 ‰ to
31.2 ‰ (Fig. 2). A similar salinity range was
observed in the remaining cruises. The water temperature ranged from 27.7 to
31.8
In August 2016, SPM concentrations ranged from 24 to 120 mg L
Concentrations of
Compared with the PN concentration, the content of dissolved fractions was
relatively minor. In August 2016, DON concentrations varied from 2.6 to 14.8
Distribution of
Along the salinity gradient, positive offsets (a positive deviation from
conservative mixing) for
Distribution of the
In the particle-free (filtered) group, the mean DON concentration was 7.1
Concentrations of DON,
The salinity of the rainwater was 0 (Table 1). The concentration of
Chemical properties in the rainwater and pore water from mangrove swamp, sandy beach and peatland.
The Rajang is the largest river in Sarawak and receives substantial
materials from its watershed. In the present study, we collected samples in
the drainage basin (river water, salinity 0) and the estuarine. In the river
water, the proportion of DIN in the N inventory was minor, accounting for
20 % to 30 % (Fig. 7a). In comparison to rivers located in dense
population areas, such as the Pearl River in China, the Mississippi River in
the USA, the Danshui River in Taiwan, China, and the Mekong River in
Vietnam, concentrations of
A global view of the
In Fig. 7b,
Notably, levels of
In August 2016, concentrations of
Sketch of the N input pathways in the Rajang estuary
N transformations, including ammonification, nitrification, and DNRA (Burgin and
Hamilton, 2007), may also markedly contribute to the enhancement of
The elevation in the concentrations of
The distribution of DIN species and related isotopes in the mixing zone
between cruises was also observed, indicating the seasonal variability. In
February 2017, the wet season in the Sarawak (Müller et al., 2015),
productions of
In the Rajang estuary, the PN content in SPM frequently ranged from 0.1 %
to 0.3 %, mainly terrestrial-derived solids because of the low
concentration of chlorophyll (Martin et al., 2018), smaller than other
tropical rivers located in adjacent regions, e.g., the Wonokromo River
(0.5 %) and the Rorong River (0.85 %) in Indonesia (Jennerjahn et al.,
2004) and the Godavari River in India (0.36 %; Gupta et al., 1997). In the
mixing experiment, small differences in DON concentration between groups
were found, indicating that the decomposition of PN was weak. The oxic
consumption of these particles in the upper stream might be the reason for
the low reactivity for particles in the degradation potential. Therefore,
the presence of high concentrations of PN cannot enhance
In September 2017, the end of the dry season according to historical
records, the DIN distribution trend was markedly different from the pattern
from August 2016. Specifically, the generation of
After the consumption/addition in estuaries, DIN injects into coastal
oceans. As outlined in Table S1, the magnitude of
The DIN concentration in the river water varied between seasons in the
Rajang, mainly resulting from the decomposition of terrestrial organic
matter. Strong precipitations, induced by La Niña events, might inhibit
soil ammonification in the watershed and hence decreased
The datasets used and/or analyzed in the present study are available from the corresponding author on reasonable request.
The supplement related to this article is available online at:
JZ, MM, YW and SJ designed the study. JZ, EA, FJ and MM performed the sample collection and in situ measurements for the first cruise. SJ, KZ, AM, EA, FJ and MM performed samplings and in situ measurements for the second and third cruises. SJ, JJ, GZ, YW, KZ, and TR completed laboratory analyses. All the co-authors equally participated in the interpretation and discussion of the results. SJ prepared the manuscript with suggestions from all the 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 would like to thank the Sarawak Forestry Department and Sarawak Biodiversity Centre for permission to conduct collaborative research in Sarawak waters under permit numbers NPW.907.4.4(Jld.14)-161, Park Permit No WL83/2017, and SBC-RA-0097-MM. Lukas Chin and the “SeaWonder” crew are acknowledged for their support during the cruises. Technical support by Patrick Martin and Gonzalo Carrasco at Nanyang Technological University during the cruises and Lijun Qu, Wanwan Cao, Xiaohui Zhang, and Xunchi Zhu at East China Normal University in the laboratory analyses is gratefully acknowledged. We also appreciate the great assistance from Zhiming Yu at the Institute of Oceanology, Chinese Academy of Sciences, for the stable isotope analyses.
This research has been supported by the Newton-Ungku Omar Fund (grant no. NE/P020283/1), the China Postdoctoral Science Foundation (grant no. 2018M630416), the MOHE FRGS 15 Grant (grant no. FRGS/1/2015/WAB08/SWIN/02/1), the Ministry of Education of the People's Republic of China (grant no. B08022), and the State Key Laboratory of Estuarine and Coastal Research (grant nos. SKLEC-KF201610 and 2017RCDW04).
This paper was edited by Tim Jennerjahn and reviewed by Zhiming Yu and one anonymous referee.