Elsevier

Journal of Hydrology

Volume 530, November 2015, Pages 447-461
Journal of Hydrology

Rising flux of nutrients (C, N, P and Si) in the lower Mekong River

https://doi.org/10.1016/j.jhydrol.2015.10.005Get rights and content

Highlights

  • Multiple-scale concentrations, stoichiometric ratios and fluxes of nutrients are studied.

  • High-flow months show unprecedented contribution to annual nutrient loads.

  • Anthropogenic activities significantly increase concentrations and fluxes of nutrients.

  • Human activities alter nutrient limitation from N to P for phytoplankton growth.

  • Nutrient fluxes are (×109 mol/y): DIC (414.5), DSi (62.8), DIN (4.1) and DIP (0.3).

Summary

Changing human land use is accelerating global element cycling and appreciably altering ecosystems in the riverine–estuarine systems. This is evident by dramatic shifts in the supply of nutrients (C, N, P, Si). However, very little is known about the magnitude and rates of these changes. We examine the Mekong, one of the world largest rivers, to assess on a whole-of-system scale, the spatial, monthly and inter-annual flux of nutrients and stoichiometric ratios using a huge data-set (1985–2011). Seasonal and spatial patterns are apparent and linked to hydrology. The estimated mean flux (×109 mol/y) of elements from the Mekong at Pakse are 414.5 for DIC, 4.1 for DIN (3.3 for NO3-N and 0.8 for NH4+-N), 10.2 for TN, 0.32 for DIP, 0.46 for TP and 62.8 for DSi, respectively, which are intermediate relative to other large rivers. However, compared to the river reach at Pakse, the total river fluxes are two-fold greater for C, N, P and DSi. Annual flux increases significantly for DIC, NO3-N, DIN, TN and DSi, and is especially pronounced for all N and P species during the recent decade (1998 onward). Distinct shifts of nutrient stoichiometry, with far-reaching changes for phytoplankton productivity in the Mekong estuary are also evident, as the system shifts from potentially N limited to P limited. These notable changes to the exports and ratios of nutrient variables and anthropogenically-driven nutrient concerns are becoming a defining feature of the Mekong Plume to the South China Sea.

Introduction

Agricultural fertilizer application and discharge of domestic and industrial sewage have increased at an alarming rate, more than doubling global riverine nutrient transports to the oceans in the past few decades. For example, riverine nitrate-N flux is estimated to have increased from 0.86 × 1012 mol/y in 1970 to 1.5 × 1012 mol/y in 1995, while dissolved inorganic phosphorus (DIP) is estimated to have increased from 25.8 × 109 mol/y to 83.9 × 109 mol/y (cf. Turner et al., 2003, Seitzinger et al., 2005, Seitzinger et al., 2010). Such increases in biologically available forms of nitrogen (N) and phosphorus (P) are linked to eutrophication in both freshwater and coastal marine ecosystems and consequent declines of ecological function, reduction of vital water supplies, and increasing frequency of toxic algal blooms, hypoxia and incidents of massive fish kills (Turner et al., 2003, Conley et al., 2009). Projections of ever increasing N and P loads in the surface waters under the weight of human population expansion and industrialisation (cf. Seitzinger et al., 2010, Le et al., 2005, Le et al., 2010, Luu et al., 2012), is a sensitive issue amongst downstream communities and nations, who share common water resources.

In addition to the changes in amounts and forms (dissolved inorganic, organic, particulate) of riverine nutrients, the ratios have received much attention as a predictor of unnatural phytoplankton community composition and production rate, both globally and regionally (Redfield et al., 1963, Turner et al., 1998, Turner et al., 2003, Chai et al., 2009, Elser et al., 2009). The riverine exports of nutrients are a key influence on the receiving coastal marine systems (Justic et al., 1995, Seitzinger et al., 2002). Redfield et al. (1963) proposed a fixed element ratio (C:N:P:Si = 106:16:1:16) for sustained growth of aquatic organisms. Although universally known and widely applied to the study of aquatic environments, at the global-scale these nutrient ratios are known to deviate from the optimal stoichiometry of C:N:P:Si, for instance, DIN:DIP:DSi = 14:1:83 (Turner et al., 2003), DIN:DIP = 8–45 (Klausmeier et al., 2004), C:N:P = 88:14:1 (Seitzinger et al., 2005) and DIN:DIP:DSi = 30:1:114 (Seitzinger et al., 2010). These significant deviations from Redfield ratio indicate the growth-limiting deficiency of nutrient element for phytoplankton (Justic et al., 1995).

A number of detailed studies have linked riverine nutrient exports with anthropogenically driven changes on freshwater, coastal and marine ecosystems at global scales (cf. Meybeck, 1982, Turner et al., 2003, Seitzinger et al., 2005, Seitzinger et al., 2010), as well as at the river basin scale such as Yangtze (Liu et al., 2003, Duan et al., 2007, Duan et al., 2008, Muller et al., 2012), Mississippi (Turner and Rabalais, 1994, Turner et al., 2003, Lane et al., 2004, Raymond et al., 2008) and European rivers (Ludwig et al., 2009, Ludwig et al., 2010). Anthropogenic sources contribute widely to riverine fluxes of nutrients, such as nitrate and phosphate, which have demonstrated sharp increases in some systems. For example, N and P have increased by a factor of >5 in the Alboran of the Mediterranean (142.9 × 106 mol N/y in 1963 to 714.3 × 106 mol N/y in 1998, while 3.2 × 106 mol P/y in 1963 to 16.1 × 106 mol P/y in 1998) (Ludwig et al., 2009), and also in the Yangtze (the largest Asian monsoon river), up to five-fold vs 13-fold increase rates of the annual fluxes for DIN and DIP were reported (DIN flux increased 22.5 × 109 mol/y in 1963–1984 to 113.6 × 109 mol/y in 2009–2010, while the DIP budget varied between 96.8 × 106 mol/y and 1225.8 × 106 mol/y) (Muller et al., 2012 and references therein).

The Mekong River is one of the largest Asian rivers and like others in this region, is undergoing many stresses from rapid population growth, dam construction, and intensive agricultural expansion in particular, especially in the most recent two decades (MRC, 2003, MRC, 2010, Campbell, 2009, Grumbine and Xu, 2011). We hypothesize that such rapid development will be evident as a rising shift in nutrient flux notably for nitrate, phosphate and ammonium (anthropogenic pollution signature), and a decreasing shift in stoichiometric ratios of DSi:DIN and DSi:DIP. However, very little is known about the historical flux of nutrients from the Mekong to the oceans and/or, the stoichiometry of C:N:P:Si in the Anthropocene. Nevertheless, even from preliminary reports from just one hydrological monitoring station (Vientiane), exceptional increases in N and P concentrations are indicated (i.e., nitrate increased by a factor of 1.4 from 1996–1998 to 2003–2005, while TP and PO4-P increased by a factor of 2 in the similar period) (Lida et al., 2011, and reference therein). Eutrophication and algae blooms could become far more severe due to constantly elevated concentrations of N and P, and reservoir impoundments combined (Campbell, 2007, MRC, 2003, MRC, 2010). The implications for element cycles on local and global scales and river water quality for domestic and industrial use are potentially immense.

In order to address the aforementioned gaps in our understanding of the Mekong River system, a huge data-set was used to examine the multiple-scale concentrations, stoichiometric ratios and fluxes of nutrients in the Mekong River under anthropogenic effects. Here, we hypothesize the significant shifts of seasonal, spatial and inter-annual nutrient loads, and changing nutrient limitation of phytoplankton growth in the Mekong estuarine and adjacent sea areas. These results help to assess human intervention influences on water quality in the Mekong River, and inform new and much needed development of global riverine nutrient budget models.

Section snippets

Study area

This study focuses on the Mekong River (8°52′–22°53′N, latitude; 98°91′–108°99′E, longitude), the 8th largest in terms of water discharge (15,000 m3/s or 470 km3/y). It carries substantial loads of dissolved and particulate materials to the South China Sea. For example, annually it transports 145 Mt/y of suspended sediment (Wang et al., 2011) and 123 Mt/y of solute to the ocean (Gaillardet et al., 1999). The headwaters arise from the high elevation (5200 m) Qinghai–Tibet Plateau (QHTP), converging

Results

A step-change increase in nutrient flux over the past couple of decades is clearly evident. This result and related hydrological processes are described in detail in the following relevant sub-sections.

Discussion

N and P are mostly from non-point sources (i.e., precipitation, agricultural fertilizer and soil erosion) and point sources (i.e., industrial waste and residential sewage) (Li et al., 2009a, Seitzinger et al., 2005). Prior reports attributed anthropogenic non-point sources as 62% of the DIN load at global scale (Seitzinger et al., 2005) and in the adjacent river-Yangtze (Shen, 2003), and understandably precipitation and agricultural sources dominate the nitrate and DIN in the Mekong basin, a

Conclusion

We report unprecedented shifts of seasonal, spatial and inter-annual nutrient loads, and critical nutrient limitations to phytoplankton growth in the Mekong River. Concentrations and fluxes of C, N, P and Si species in the River displayed remarkable spatial differences. The DIC concentration significantly decreased downstream with a range of 1360–1780 μM, while N, P and Si had no universal spatial trends. DIC flux from the upper River comprised 35.8% of the flux at Pakse, the contribution was

Author contributions

SYL analysed the data, generated model outputs and data interpretation, and drafted the manuscript. Both the authors contributed to manuscript writing and discussion.

Acknowledgements

This study was jointly supported by the “Hundred-talent Project” of the Chinese Academy of Sciences, China and SCUs postdoctoral research fellowship, Australia. We are grateful to MRC for data supplement (GIAI number: 9506000003818_E0100eil). Special thanks are given to three anonymous reviewers and Prof. Laurent Charlet (editor in chief) and Prof. Nicolas Gratiot (Associate editor) for their constructive comments and suggestions to improve the paper.

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