Abstract
Abstract. Human-induced excess nitrogen outflowing from land through rivers to oceans has resulted in serious impacts on terrestrial and coastal ecosystems. Oceania, which occupies < 2.5% of the global land surface, delivers 12% of the freshwater and dissolved materials to the ocean on a global scale. However, there are few empirical data sets on riverine dissolved inorganic nitrogen (DIN) fluxes in the region, and their dynamics are poorly understood. In this study, a river monitoring network covering different types of land uses and population densities was implemented to investigate the mechanism of DIN export. The results show that DIN concentration/yield varied from ∼20 μM/∼300 kg-N km−2 yr−1 to ∼378 μM/∼10 000 kg-N km−2 yr−1 from the relatively pristine headwaters to the populous estuary. Agriculture and population density control DIN export in less densely populated regions and urban areas, respectively, and runoff controls DIN at the watershed scale. Compared to documented estimates from global models, the observed DIN export from the Danshui River is 2.3 times larger, which results from the region-specific response of DIN yield to dense population and abundant runoff. The dominating DIN species change gradually from NO3− in the headwaters (∼97%) to NH4+ in the estuary (∼60%) following the urbanization gradient. The prominent existence of NH4+ is probably the result of the anaerobic water body and short residence time, unlike in large river basins. Given the analogous watershed characteristics of the Danshui River to the rivers in Oceania, our study could serve as a first example to examine riverine DIN fluxes in Oceania.
Highlights
Nitrogen, a vital element in living organisms, plays a critical role in controlling primary production in the biosphere
Oceania has been identified as a hotspot of global dissolved inorganic nitrogen (DIN) export, much less attention has been paid to the rivers of Oceania, where development is rapid
The positive correlation between runoff and DIN yield/flux reveals that hydrology exerts strong control over DIN export, i.e. DIN in our study area is transportdominated
Summary
A vital element in living organisms, plays a critical role in controlling primary production in the biosphere. Excess nitrogen discharged from land to oceans has resulted in seasonal coastal hypoxia, harmful algal blooms, and decreased fishery production in ecosystems (Howarth et al, 1996; Rabalais, 1996; Jickells, 1998; Boesch, 2002; Turner et al, 2003; Galloway et al, 2004; Duan et al, 2007; Conley et al, 2009; Billen and Garnier, 2007; Diaz and Rosenberg, 2008). Most current knowledge about dissolved inorganic nitrogen (DIN = NH+4 +NO−3 +NO−2 ) export has derived from larger river systems in developed countries of Europe and North America; much less attention has been paid to developing countries in Oceania in the tropical western Pacific Ocean. Oceania is surrounded by stratified oligotrophic water with limited bio-available nutrients, nitrogenous nutrients (Jiao et al, 2007; Martha and Kristen, 2012)
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