S. M. Liu

Human impacts on the large world rivers : Would the Changjiang (Yangtze River) be an illustration?

The “Three Gorges Project” (TGP) in the upstream of Changjiang (Yangtze River) has resulted in great concern of scientific society and public conversations on the economic, environmental, and human health issues. Data of nutrients from main stream and 15 large tributaries indicate that Changjiang receives a large part of its nutrient burden from the drainage area upstream the “Three Gorges Dam” (TGD). A model calculation shows that the construction of TGD may cause further eutrophication in the upstream region with phosphate as a limiting factor relative to nitrogen for photosynthesis. The estimated carbon fixation within the reservoir may equal to 10–20% of the actual particulate organic carbon (POC) budget for the Changjiang. Taking into account the fact that dissolved inorganic nitrogen (DlN) in the Changjiang has increased by a factor of 2 in the last 10–20 years, the expected N:P ratio of the river would reach 300–400 after the year 2010. Such a change in nutrient and organic carbon budgets of the Changjiang will significantly influence the environment and health of ecosystems of the adjacent shelf region.

Distributions and fluxes of methane in the East China Sea and the Yellow Sea in spring

[1]xa0Distributions and fluxes of methane were determined during two surveys in March–May 2001 in the Yellow Sea and the East China Sea. Methane concentrations in the surface and bottom waters range from 2.52 to 5.48 and 2.81 to 8.17 nM, respectively. The distributions of methane are influenced obviously by the Yangtze River effluent and Kuroshio water. CH4 input via the Yangtze River is estimated to be 3.17 mol/s, of which a considerable part may be lost by air-sea exchange during estuarine mixing. Net CH4 flux exported from the shelf to the Kuroshio is about 1.84 mol/s. Methane enrichments in bottom waters occur widely, which reveals sediment sources of CH4. However, the CH4 input from the sediments of the studied region in spring is lower than other shelf regions due to low organic carbon in the sediments and high O2 contents in the water column. The sea-to-air methane fluxes are estimated to be 1.36 ± 1.45 and 2.30 ± 2.36 μmol m−2 d−1 using Liss and Merlivat [1986] and Wanninkhof [1992] relationships, respectively, and the estimated spring emission rate of methane ranges from 9.32 × 10−3 to 15.7 × 10−3 Tg CH4 yr−1. However, these estimations suffer from the neglect of seasonal variability and should be taken as a low limit. Therefore more measurement campaigns should be carried out to enhance our understanding of this particular oceanic region.

Nitrogen species in rainwater and aerosols of the Yellow and East China seas: Effects of the East Asian monsoon and anthropogenic emissions and relevance for the NW Pacific Ocean

[1]xa0Rainwater and aerosol samples were collected from a coastal urban area (Qingdao) and remote islands (Qianliyan and Shengsi) and along cruise tracks in the Yellow Sea and East China Sea from 1997 to 2005. The samples were analyzed for nitrogen species (NO3−, NO2−, NH4+, and organic nitrogen) and other important elements. The nitrogen species concentrations showed considerable temporal and spatial variations for wet as well as dry atmospheric depositions. In addition, there was a dramatic reduction in the influence of anthropogenic emissions on nitrogen species with increasing distance from coastal urban stations to remote areas across the Yellow Sea and East China Sea. The monsoon climate of East Asia also had prominent effects on the atmospheric composition of nitrogen, with higher loadings in northerly (i.e., winter) than southerly (i.e., summer) monsoon periods, owing to strong emissions from the East Asian landmass. Dust storms in spring dramatically reduced the periodically high concentrations of atmospheric pollutants (e.g., nitrogen species) across the NW Pacific Ocean, but this was accompanied by a twofold-to-fourfold increase in the temporal deposition flux, which showed broad spatial dimensions. Finally, our study identified a strong gradient of wet as well as dry nitrogen deposition fluxes from East Asia to the interior of the North Pacific Ocean. The gradient reflected changes in emission sources and chemical reactions (e.g., forming secondary aerosols), rainfall and scavenging, and change in air mass trajectory.

Dissolved silicate in coastal marine rainwaters: Comparison between the Yellow Sea and the East China Sea on the impact and potential link with primary production

[1]xa0Rainwater samples are collected from islands and seagoing cruises in the Yellow Sea and the East China Sea to determine concentration and to estimate deposition flux for dissolved silicate (DSi), together with other nutrients (e.g., nitrogen) by spectrophotometry. Concentrations of dissolved silicate in rainwater show considerable variation in time and space, with 0.5–15 μM at the Yellow Sea and the East China Sea. The (NO3− + NH4+)/DSi ratio in rainwater changes over up to 2 orders of magnitude, with high levels in winter and low in autumn. Levels of DSi falls with higher rainfall, and a positive relationship can be established with amount of total particles in rainwater samples. The deposition of DSi via rainfall is 0.97 × 109 mol yr−1 for the Yellow Sea and 2.0 × 109 mol yr−1 for the East China Sea, respectively, 20–40% higher than the atmospheric dry deposition estimated by previous work but shows critical importance to the marine ecosystems at low trophic level in terms of comparison of chemical budget and new production. The extrapolation to the global ocean indicates that aeolian inputs (i.e., wet and dry depositions) of dissolved silicate can be on the order of 0.9 × 1012 mol yr−1, with ∼20% of this amount is distributed in the continental margin.

Dissolved aluminum and silica in the Changjiang (Yangtze River): Impact of weathering in subcontinental scale

[1]xa0Water samples were collected from the river mouth upstream over a distance of 3500–4000 km in the Changjiang (Yangtze River) and its 15 tributaries in April–May 1997, and at Nantong close to the river mouth in 1996–1998. Dissolved Al and Si were determined and compared with data of water flow, total suspended matter, and alkalinity. Data obtained show that dissolved Al in the Changjiang illustrates a rather uniform distribution from upstream toward river mouth, and a broad increasing of Al with higher Si to alkalinity ratio in tributaries has been found. The plots of Al:Si ratio against specific runoff and sediment yields reveal different weathering mechanisms involved in tributaries. At Nantong, concentration of Al is considerably stable relative to important seasonal variations of water discharge and sediment load. Acidification of bulk riverine samples releases an amount of Al that can be one order of magnitude higher than the natural concentrations, indicating potential remobilization by acid deposition and/or waste drainage. Compared with other world rivers, concentration of Al in the Changjiang is low and similar to those from primitive systems.

Nutrient dynamics in the winter thermohaline frontal zone of the northern shelf region of the South China Sea

[1]xa0As the first attempt to estimate the nutrient transport across the winter thermohaline frontal zone on the northern shelf of the South China Sea, the nutrient dynamics around the front and the effects of cross-frontal water exchange on nutrient transport were investigated using wintertime field observations. Both water temperature and salinity increased from coastal to oceanic waters, showing the presence of a thermohaline front. The concentrations of dissolved inorganic nutrients decreased oceanward, especially across the thermohaline front, while those of dissolved organic nutrients (i.e., dissolved organic nitrogen (DON) and dissolved organic phosphorus) showed patchy distributions. Ammonium was the major constituent of dissolved inorganic nitrogen, and DON was the main component of total dissolved nitrogen. Molar ratios of PO43−/total dissolved phosphorus decreased from coastal to oceanic waters, indicating that PO43− was rapidly removed and/or consumed from the water column and that organic matter degradation increased offshore, replenishing PO43−. Molar ratios of NO3−/(NH4+ + DON) were 0.01–0.6, indicating dominance of regenerated nitrogen. Surface water convergence and bottom water divergence were identified in the across-shore velocity field, and the calculated across-shore nutrient fluxes suggest that the presence of the winter thermohaline front promotes the offshore transport of nutrients from coastal waters. The transport path begins with convergence of surface coastal waters toward the front, followed by the sinking in the frontal region and the oceanward movement through the bottom layer of the front offshore side. With an assumption of 500 km as the length of thermohaline front on the northern shelf of the South China Sea, the calculated offshore fluxes of nutrients across the entire front are larger than those from the Zhujiang (Pearl River) and the Changjiang (Yangtze River).

Tracing Nitrogen Biogeochemistry During the Beginning of a Spring Phytoplankton Bloom in the Yellow Sea Using Coupled Nitrate Nitrogen and Oxygen Isotope Ratios

Nitrogen biogeochemistry during the beginning of a spring phytoplankton bloom in the Yellow Sea was investigated based on nutrient concentrations, benthic fluxes of nutrients, the nitrogen and oxygen isotope composition of NO3−, and the hydrological conditions. The δ15N and δ18O values for NO3− in the Yellow Sea were more variable in surface waters than in near-bottom waters and were generally 6.3–8.2‰ for δ15N and 6.2–9.7‰ for δ18O, except in the Yellow Sea Coastal Current Water (YSCCW), where the maximum δ15N and δ18O values found were 13.2‰ and 18.8‰, respectively. Both the δ15N and δ18O values varied among the different water masses with higher values in the YSCCW than in the Yellow Sea Mixed Water and the Yellow Sea Warm Current Water. Higher δ18O relative to δ15N in NO3− in the Yellow Sea reflects contributions from multiple NO3− sources, including nitrification and atmospheric deposition with the later accounting for 71% of total external NO3− input. A simple nitrogen budget indicated the importance of nutrient regeneration from sediments as a source for water column NO3− representing 74% of total NO3− input in the YS.