Baodong Wang

Recognition on the forming-vanishing process and underlying mechanisms of the hypoxia off the Yangtze River estuary

On the basis of compiled multidisciplinary historical data in 2006–2007 and incorporation of relevant simulation results and remote sensing data, we performed an in-depth study of the generation and dissipation process of the hypoxic zone and its distribution morphology and structure off the Yangtze River estuary. Based on the hydrological circulation dynamics, reproduction of phytoplankton (leading to the decomposition of organic matter), and other factors, we comprehensively and systematically investigated the generation and dissipation of the hypoxic zone and underlying mechanisms for the seasonal variation in its position, explored the multi-factorial synergistic reactions during the generation and dissipation process of the hypoxic zone, and revealed the controlled mechanism for the morphology and structure of the hypoxic zone’s distribution. Our studies indicate that in the winter and spring seasons, the hydrological environment off the Yangtze River estuary provides a water body with relatively low contents of dissolved oxygen (DO), which is the background for the formation of a hypoxic zone. After entering into the summer season, the hypoxic zone gradually develops towards the north and becomes mature. Because of the impact of the terrain, local decomposition of organic matter, and upwelling of the Kuroshio subsurface water in July–August, the hypoxic zone off the Yangtze River estuary exhibits the characteristics of discontinuous distribution in space and has a south and north “dual-core” structure in the inner continental shelf. In addition, there is a hypoxic core in the eastern outer continental shelf. The degrees of hypoxia vary for different areas; they are strongest overall in the north, next strongest in the south; they are weakest on the outer continental shelf. In summer, the hypoxic zone in the north is related to the northward differentiation of the southern hypoxic zone and results from local development and intensification. In August, the hypoxic zone in the north reaches its peak, and after September, it rapidly retreats southward and disappears because of weakening stratification. In the fall, there is hypoxic zone along the coast of Zhejiang in the south, and there is also a low-DO area to the southwest of Jeju Island, with both zones disappearing rapidly. In addition, the change of dynamic environment also causes the low-DO area of the outer continental shelf to move outward in the fall. The variation in the intensity of the stratification and its cumulative effects as a barrier of vertical DO transportation over long periods of time have a significant impact on the degree of hypoxia in the hypoxic zone. In addition, the seasonal variations in the size of the stratified region, intensity of each current system/water mass, upwelling, front, and high-value area of phytoplankton biomass jointly restrict the extension of the hypoxic zone in the inner continental shelf and latitudinal (south-north direction) movement of its location off the Yangtze River estuary. The combined effect of dynamic factors, such as that of the Kuroshio subsurface water, causes a low-DO core in the outer continental shelf. The bottom cold water to the north of the East China Sea is the dynamic basis for the formation of the low-DO area to the southwest of Jeju Island during the fall season. The special seabed topography and mud area distribution off the Yangtze River estuary have a certain degree of influence on the development of the hypoxic zone. The generation and dissipation of the hypoxic zone and its distribution morphology off the Yangtze River estuary, and seasonal variation of its structure and position are a result of the synergistic effects of various factors.

Water Quality in Marginal Seas off China in the Last Two Decades

Based on historical data, variations in seawater quality and pollutant origins in marginal seas off China in the recent two decades are overviewed. Analysis shows that the total area of nonclean water was increasing continuously in the 1990s, but it has been decreasing since 2000; however, the total polluted area (sum of areas of slightly, medium, and heavily polluted waters) in Chinas standard fluctuated without a clear trend of decline or increase, indicating that although the increasing trend of marine environmental pollution has been kept effectively within certain limits, seawater quality has not been improved essentially in marginal seas off China. Furthermore, the occurrence frequency and affected area of HABs increased rapidly in marginal seas off China in the last 20 years. The amount of industrial and municipal discharges from the mainland China increased from the end of the 1980s to the end of the 1990s but decreased in the 2010s whereas riverine input of nutrients and pollutants from marine sources increased continuously.

Effects of Tamarisk shrub on physicochemical properties of soil in coastal wetland of the Bohai Sea

There are many different and even controversial results concerning the effects of Tamarisk on the physicochemical properties of soil. A year-round monitoring of soil salinity, pH and moisture is conducted beneath the Tamarisk shrub in a coastal wetland in the Bohai Sea in China, to ascertain the effects of Tamarisk on the physicochemical properties of soil in coastal wetland. Compared with the control area, the soil moisture content is lower around the area of the taproot when there is less precipitation in the growing season because of water consumption by Tamarisk shrub. However, the soil moisture content is higher around the taproot when there is more precipitation in the growing season or in the non-growing period because of water conservation by the rhizosphere. The absorption of salt by the Tamarisk shrub reduces the soil salinity temporarily, but eventually salt returns to the soil by the leaching of salt on leaves by rainfall or by fallen leaves. The annual average soil moisture content beneath the Tamarisk shrub is lower than the control area by only 6.4%, indicating that the Tamarisk shrub has little effect on drought or water conservation in soils in the temperate coastal wetland with moderate annual precipitation. The annual average salinity beneath the Tamarisk shrub is 18% greater than that of the control area, indicating that Tamarisk does have an effect of rising soil salinity around Tamarisk shrubs. The soil pH value is as low as 7.3 in summer and as high as 10.2 in winter. The pH of soil near the taproot of the Tamarisk shrubs is one pH unit lower than that in the control area during the growing season. The difference in pH is less different from the control area in the non-growing season, indicating that the Tamarisk shrub does have the effect of reducing the alkalinity of soil in coastal wetland.

A historical overview of coastal eutrophication in the China Seas

Chinas rapid economic and social development has led to an acceleration in nutrient inputs to coastal waters, which, in turn, has resulted in severe coastal eutrophication. On the occasion of the 40th anniversary of Chinas reform and opening up, the evolution of the causative factors and the state as well as future prospects for coastal eutrophication in the China Seas are analyzed and summarized. Results showed that the coastal eutrophication situation was not so serious at the beginning of reform and opening up, but it worsened rapidly from the end of the 1980s to the mid-2000s. In the last decade, the worsening trend has been curbed but the status of coastal eutrophication has not been substantially improved. Much work is still needed to be able control the total amount of nutrients entering coastal waters and enable comprehensive treatment of coastal eutrophication in the China Seas.

The biogenic silica composition, behavior and budget in the Changjiang Estuary

Three comprehensive surveys were performed in the Changjiang (Yangtze River) Estuary (CJE) to understand the biogenic silica (BSi) composition, behavior and budget. It is indicated that the BSi is composed of phytoliths, phytoplankton and sponges; phytolith BSi has 16 forms and account for 23% to 83% of the bulk BSi in the maximum turbidity zone. The budget shows that the major exogenous BSi source in the water column of the CJE is the riverine input, accounting for 95% of the total BSi input. Dominant processes that maintain BSi levels in the water column are the primary production (55 Gmol/a) and the subsequent BSi sedimentation (46 Gmol/a); and the BSi pool produced by the primary production represents two point three times the BSi loading of the Changjiang River and 63% of the BSi output, respectively. The net export (26 Gmol/a) of BSi from the CJE to the East China Sea and Yellow Sea roughly equals the riverine BSi loading. The observed total accumulation of BSi is one point seven times larger than the loading of total BSi output, with 53% to 88% of phytolith BSi and their assemblage, indicating that there has already been a “filter” of terrestrial BSi. The reverse weathering in sediments is an important process for the reactive silica removal in the CJE due to authigenic alterations. It is indicated that the phytolith fluxes in the suspended load represent a significant BSi source in the estuary, and the CJE would act as a net BSi sink.

Does reduced sediment load contribute to increased outbreaks of harmful algal blooms off the Changjiang Estuary

Harmful algal blooms (HABs) have been increasingly frequent in coastal waters around the world over the last several decades. Accelerated coastal eutrophication, resulting from the increased anthropogenic loadings of nutrients, is commonly assumed to be the primary cause of this increase. However, although important, accelerated coastal eutrophication may not be the only explanation for the increasing blooms or toxic outbreaks in estuarine waters. Changes in riverine material fluxes other than nutrients, such as sediment load, may significantly affect biological activities and HAB incidence in estuarine and coastal waters. A case study off the Changjiang (Yangtze River) Estuary indicated that with the increasing riverine loadings of nutrients, the sediment load from the Changjiang River has been reduced by 70% over the past four decades. A comparison of long-term data revealed that the phytoplankton biomass maximum has expanded to a region of much lower salinity due to the drastic reduction in riverine sediment load and the subsequent improvement in light penetration in the Changjiang River plume. Furthermore, there was an apparent mirror-image relationship between the sediment load from the Changjiang River and the HAB incidence off the Changjiang Estuary over the past four decades, and the number of HAB incidents was significantly negatively correlated with the sediment load. Therefore, it is argued that the drastic decline in sediment load from the Changjiang River reduced turbidity in the Changjiang Estuary and thus contributed to the increased frequency of HABs in the buoyant discharge plumes.