Abstract
Abstract. Chinese rivers deliver about 5–10% of global freshwater input and 15–20% of the global continental sediment to the world ocean. We report the riverine fluxes and concentrations of major nutrients (nitrogen, phosphorus, and silicon) in the rivers of the contiguous landmass of China and Korea in the northeast Asia. The rivers are generally enriched with dissolved inorganic nitrogen (DIN) and depleted in dissolved inorganic phosphate (PO43−) with very high DIN: PO43− concentration ratios. DIN, phosphorus, and silicon levels and loads in rivers are mainly affected by agriculture activities and urbanization, anthropogenic activities and adsorption on particulates, and rock types, climate and physical denudation intensity, respectively. Nutrient transports by rivers in the summer are 3–4 times higher than those in the winter with the exception of NH4+. The flux of NH4+ is rather constant throughout the year due to the anthropogenic sources such as the sewer discharge. As nutrient composition has changed in the rivers, ecosystems in estuaries and coastal sea have also changed in recent decades. Among the changes, a shift of limiting nutrients from phosphorus to nitrogen for phytoplankton production with urbanization is noticeable and in some areas silicon becomes the limiting nutrient for diatom productivity. A simple steady-state mass-balance box model was employed to assess nutrient budgets in the estuaries. The major Chinese estuaries export <15% of nitrogen, <6% of phosphorus required for phytoplankton production and ~4% of silicon required for diatom growth in the Chinese Seas (Bohai, Yellow Sea, East China Sea, South China Sea). This suggests that land-derived nutrients are largely confined to the immediate estuaries, and ecosystem in the coastal sea beyond the estuaries is mainly supported by other nutrient sources such as regeneration, open ocean and atmospheric deposition.
Highlights
The coastal ocean represents an area of only 10% of the global ocean surface, but accounts for ∼25% of global ocean primary production and 80% of global organic carbon burial (Berner, 1982; Smith and Hollibaugh, 1993)
Drastic increases in delivery of river-borne nutrients owing to landuse changes and anthropogenic emissions are known to result in eutrophication, modifying aquatic food webs and provoking more severe hypoxic events in coastal marine environments (Humborg et al, 1997; Ragueneau et al, 2002; Pahlow and Riebesell, 2000; Turner and Rabalais, 1994; Turner et al, 2003)
Eighteen rivers including Huanghe and Changjiang, four coastal seas of Bohai, Yellow, East China and South China were subjected to our analysis (Fig. 1, Tables 1, 2, and 5)
Summary
The coastal ocean represents an area of only 10% of the global ocean surface, but accounts for ∼25% of global ocean primary production and 80% of global organic carbon burial (Berner, 1982; Smith and Hollibaugh, 1993). Drastic increases in delivery of river-borne nutrients owing to landuse changes and anthropogenic emissions are known to result in eutrophication, modifying aquatic food webs and provoking more severe hypoxic events in coastal marine environments (Humborg et al, 1997; Ragueneau et al, 2002; Pahlow and Riebesell, 2000; Turner and Rabalais, 1994; Turner et al, 2003). Seaward continuum of rivers, modify riverine nutrient fluxes to the sea significantly through biogeochemical processes. About 5–10% of global freshwater input and 15–20% of the global continental sediment (Table 1) are delivered to the ocean by the Chinese rivers (Zhang, 2002). Their drainage area covers a region between 5 to 55◦ N latitude, ranging from tropical to cold temperate climate. Most of the major Chinese rivers originate in the western part of the country, the Himalayan glacial plateaus above 4000 m altitude, and flow eastward zonally, notably the Huanghe (Yellow River) and Changjiang (Yangtze River), Published by Copernicus Publications on behalf of the European Geosciences Union
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