Nutrient budgets for large Chinese estuaries

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 (PO 3− 4 ) with very high DIN: PO 3− 4 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 NH + 4 . The flux of NH + 4 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 massbalance 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 areCorrespondence to: S. M. Liu (sumeiliu@ouc.edu.cn) 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.

Distribution, fluxes and decadal changes of nutrients in the Jiulong River Estuary, Southwest Taiwan Strait

“…This indicates that most DIN in the JRE may result from chemical fertilizer application [33] and soil erosion in the watershed. This phenomenon is similar to the Yangtze [41], but it is different from the upper reach of the Pearl River Estuary of China. The latter is significantly impacted by municipal sewage from Guangzhou, where NH 4 -N is the dominant chemical species of DIN [35].…”

“…This is consistent with the silicate-rich Jiulong geography. The neighboring Minjiang watershed (another medium-large river system in Fujian Province of China) is also characterized by a similar areal DSi yield rate of 5.0×10 3 kg Si km 2 a 1 [41], owing to a similar regional geology and weathering background.…”

“…Although it was lower than the Ganges and Danshui Rivers, which are in tropical and/or subtropical Asia, their human NO 3 -N productivities per person are at similarly high levels ( Figure 7). We make comparisons to several large rivers in China [41], as follows. Although water discharge of the Jiulong River is only 1.1% that of the Yangtze River, 2.5% that of the Pearl River, and 21.8% that of the Minjiang River (Fujian Province, China), areal DIN and SRP yield rates from the Jiulong River watershed are respectively 4.1 and 7.1 times those from the Yangtze River watershed, 2.4 and 2.4 times those from the Pearl River watershed, and 3.1 and 1.7 times those from the Minjiang River watershed.…”

Inter-shelf nutrient transport from the East China Sea as a major nutrient source supporting winter primary production on the northeast South China Sea shelf

“…Because we did not find noticeable inter-winter variations in DIN concentrations in the TWS segment of the CCC from our own observations or from the literature, we adopted the average DIN concentration of 11.0 ± 2. Albeit the large uncertainty of the estimated DIN flux resulting from variations in both the DIN concentration and volume transport fluxes, this DIN flux was substantial as compared to the concurrent DIN discharge from the Changjiang in wintertime, which is estimated to be ∼ 760 mol s −1 (Liu et al, 2009). Because the DIN loading of the Pearl River is small in winter and PRP flows southwestward driven by the northeast monsoon in winter (Liu et al, 2009;Ou et al, 2009) with little contribution to the NESCS, and the winter monsoon favors coastal downwelling (S. M. Gan et al, 2013b), the CCC-supplied nutrients were critically important to the NESCS shelf, which would otherwise be low in productivity despite its favorable light and temperature conditions.…”

“…In detail, the DIN : PO 4 ratio in the CCC was ∼ 33.8 (Fig. 9a), substantially lower than that in the Changjiang (100-160 : 1, Liu et al, 2009). The rapid reduction of the DIN : PO 4 ratio from the Changjiang to the CCC might be induced by mixing with the ambient seawater with low DIN : PO 4 ratios as shown in Figs.…”