Guosen Zhang

Biogeochemistry of bulk organic matter and biogenic elements in surface sediments of the Yangtze River Estuary and adjacent sea

This study investigated the distribution and roles of total organic carbon (TOC), biogenic silicon (BSi), various forms of nitrogen (N) and phosphorus (P), and the stable carbon isotope (δ(13)C) in surface sediments of the Yangtze River Estuary (YRE) and adjacent sea. Terrestrial input accounted for 12-63% of total organic matter in the study area. The distribution of biogenic elements was affected by the Changjiang Diluted Water, the Jiangsu Coastal Current, human activities, marine biological processes, and the sediment grain size. Potentially bioavailable N and P accounted for an average 79.6% of the total N (TN) and 31.8% of the total P (TP), respectively. The burial fluxes for TOC, BSi, TN and TP were 39.74-2194.32, 17.34-517.48, 5.02-188.85 and 3.10-62.72 μmol cm(-2) yr(-1), respectively. The molar ratios of total N/P (1.2-5.0), Si/P (5.0-14.8) and Fe/P (21-61) indicated that much of the P was sequestered in sediments.

Silicon isotopic chemistry in the Changjiang Estuary and coastal regions: Impacts of physical and biogeochemical processes on the transport of riverine dissolved silica

The dissolved silica (DSi) concentration and silicon isotopic composition (δ30Si) of surface water samples from the Changjiang Estuary was measured in summer and winter to study the behavior of DSi fluvial inputs into the estuary. The DSi concentration decreased away from the estuary and had a linear relationship with salinity, suggesting that mixing between river water and seawater is the dominant effect on DSi levels in the study area. Measured δ30Si in the Changjiang Estuary ranged from +1.48‰ to +2.35‰ in summer, and from +1.54‰ to +1.95‰ in winter. As a result of low light levels and abundant DSi riverine inputs, DSi remains relatively unaffected by biological utilization and fractionation in the near-shore region, and the isotopic imprint of water from the Changjiang can still be detected up to a salinity level of 20 in summer. An obvious increase in δ30Si was observed beyond this salinity level, indicating a significant increase in biological utilization and fractionation of DSi in high salinity waters. Lower water temperatures and light levels that prevail over the winter lead to the reduced fractionation of DSi compared with that in summer. The fractionation factor (30ɛ) was estimated using a steady state model to the high salinity waters, yielding a value of −0.95‰, which is in agreement with previous results obtained for Skeletonema costatum in cultivation experiments. The results of this study suggest that silicon isotopes can be used to identify the impact of biological utilization on the behavior of DSi in highly dynamic estuarine environments.

Nutrients’ distribution and their impact on Pangani River Basin’s ecosystem – Tanzania

ABSTRACT Surface and groundwater from Pangani River Basin (PRB) were sampled in dry and wet seasons, analysed for dissolved organic and inorganic nutrients (N, P, Si and Urea). There was spatial and seasonal nutrients’ variability, with enrichment of dissolved inorganic fractions accumulated from natural and anthropogenic sources. Silicates increased in dry season, whereas nitrate, ammonium, phosphate and urea increased in wet season; except for phosphate, other nutrients increased from upstream to the river mouth. High rate of chemical weathering possibly due to tropical climate and volcanic rocks has caused PRB to have higher concentration of silicates than average freshwater African Rivers. Contribution of PRB to the coast of Indian Ocean was 2.6, 39.0, 45.2, 67.4 and 5444.8 (mol/km2/yr) for nitrite, phosphate, ammonium, nitrate and silicates, respectively, which were lower than most of the tropical rivers in the world. Levels of nitrate and phosphate for most of the stations were higher than recommended levels for aquatic ecosystem health. Furthermore, observed hypoxia condition in some stations threatens aquatic life. This study recommends the efficient use of fertilizers to reduce nutrients’ uptake into the lakes and rivers so as to meet the recommended level for aquatic and human health.

Hypoxia off the Changjiang (Yangtze River) estuary and in the adjacent East China Sea: Quantitative approaches to estimating the tidal impact and nutrient regeneration

Large areas of hypoxia have been reported off The Changjiang Estuary and in the East China Sea. Five cruises, covering winter, spring, and summer, were carried out from 2007 to 2013 in this region, and in August 2013 (summer), an extensive hypoxic event (11,150km2) was observed, which was characterized by an estimated bulk oxygen depletion of 5.1 million tons. A strong tidal impact was observed associated with the bottom oxygen depletion, with the periodicity of diel variations in dissolved oxygen being 12h (i.e., similar to the tidal cycle). A conservative estimate of nutrient regeneration suggested that during the hypoxic event of August 2013, the amount of regenerated nitrogen (as nitrate) and phosphorus (as dissolved inorganic phosphorus) was 27,000-30,000 tons and 1300-41,000tons, respectively. Estimates of the absolute (bulk) regenerated nutrient fluxes were much greater than the conservative estimates.

Hypoxia and nutrient dynamics affected by marine aquaculture in a monsoon-regulated tropical coastal lagoon

The Laoyehai (lagoon) is located at the east coast of Hainan Island in the South China Sea and has been subject to perturbations from human activities, notably marine aquaculture, and has eutrophic surface and hypoxic near-bottom waters. A lack of knowledge of hydrodynamic and biogeochemical processes is a challenge to the sustainable management of lagoon at the ecosystem level in science. Five field campaigns, including three during the southwest monsoon and two in the northeast monsoon periods, were carried out at the Laoyehai in 2008–2011. The aim of this study is to investigate the impacts of dynamic processes of hydrography and human activities on nutrient geochemistry and their relationships to the system eutrophication and hypoxia in the lagoon. In this coastal system, high levels of ammonium relative to nitrate are found, elevated phosphate skews the DIN/DIP relative to the Redfield ratio, and the dissolved silicate concentration is high because of submarine groundwater discharge. The organic fraction in the Laoyehai accounts for a large proportion of the total nutrients associated with the release of wastes from marine aquaculture. The hypoxia of near-bottom waters in the Laoyehai is created and maintained by heterotrophic processes that are fueled by organic matter, which are exacerbated by poor water exchange as a consequence of the geomorphology and weak tidal circulation.

Distribution of organic carbon: possible causes and impacts in the Pangani River Basin ecosystem, Tanzania

Environmental context Understanding the sources of organic carbon and its spatial and seasonal variation is essential for implementing measures to control water pollution. There is, however, only limited information about organic carbon in east African rivers. This study reports the distribution of dissolved and particulate organic carbon in the Pangani River Basin, using isotopes to trace sources of carbon to the basin and its flux to the Indian Ocean. Abstract There is limited information on organic carbon in African rivers, especially from the eastern side. Here, we report distribution and impacts of total suspended matter (TSM), and dissolved and particulate organic carbon (DOC & POC) in the Pangani River Basin (PRB) ecosystem together with their fluxes to the Indian Ocean. δ13C was also used to trace sources of carbon in the basin. Results showed that the basin is supplied with carbon from allochthonous sources dominated by C3 plants, with higher levels of TSM and DOC in the wet season than in the dry season. Several factors, including altitude, temperature, rainfall, lithology and anthropogenic activities, have a significant influence on the seasonal and spatial distribution of organic carbon in the basin. High discharge in the wet season mobilised terrestrial organic carbon to elevate concentrations of DOC, POC and TSM. Mean concentrations of DOC, dissolved inorganic carbon (DIC), POC and TSM in PRB were in ranges comparable to that in other tropical rivers but their fluxes were lower than in most tropical rivers around the world. Diverting water from the river for irrigation and hydroelectric power production was one of the factors that reduced the flux of carbon. Observed hypoxic conditions in the reservoir indicates that the quality of water for human and aquatic ecosystem health is possibly threatened by a high level of organic carbon; furthermore, the trends of increasing population, deforestation, temperature and rainfall will likely increase the concentration of organic carbon in the future. Better management of waste, afforestation and reforestation are recommended to restore degraded natural forest, so as to reduce uptake of organic carbon from the terrestrial environment.