o-Yi Zhu

Bacterial diversity in the surface sediments of the hypoxic zone near the Changjiang Estuary and in the East China Sea

Changjiang (Yangtze River) Estuary has experienced severe hypoxia since the 1950s. In order to investigate potential ecological functions of key microorganisms in relation to hypoxia, we performed 16S rRNA‐based Illumina Miseq sequencing to explore the bacterial diversity in the surface sediments of the hypoxic zone near the Changjiang Estuary and in the East China Sea (ECS). The results showed that numerous Proteobacteria‐affiliated sequences in the sediments of the inner continental shelf were related to both sulfate‐reducing and sulfur‐oxidizing bacteria, suggesting an active sulfur cycle in this area. Many sequences retrieved from the hypoxic zone were also related to Planctomycetes from two marine upwelling systems, which may be involved in the initial breakdown of sulfated heteropolysaccharides. Bacteroidetes, which is expected to degrade high‐molecular‐weight organic matter, was abundant in all the studied stations except for station A8, which was the deepest and possessed the largest grain size. In addition, dissolved organic carbon, water depth, percentage ratio of clay to silt, salinity, and sedimentary grain size were environmental effectors that shaped the sedimentary microbial community structure. Our results showed that putative Gammaproteobacteria‐affiliated sulfur‐oxidizing bacteria may not only detoxify hydrogen sulfide produced by sulfate‐reducing prokaryotes, but also serve as the primary producers in the marine sediments. Specific groups of aerobic Bacteroidetes and Planctomycetes participated in degrading organic matter, which might contribute to the oxygen depletion in the hypoxic zones.

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.

Organic Matter and Biomarkers of the Changjiang Estuary and East China Sea Shelf

As one of most populated river–estuary–shelf regions, the biogeochemistry of organic matter in the Changjiang Estuary and East China Sea (ECS) is critical for a quantitative understanding of global biogeochemical cycles. This chapter summarizes the spatial variation of organic matter and biomarkers from the watershed to the shelf. Monthly data collected from the downstream of the watershed help to elucidate the seasonal variation of biomarkers (e.g., pigments and amino acids) along with hydrological and biological processes. The geochemical characterization of biomarkers in the ECS is well studied, and this chapter provides an overview of the potential controls on biomarkers. Hydrological sorting, in situ primary production, and diagenetic processes contribute to the variable biomarker distribution, composition, and burial in the ECS. Anthropogenic disturbance is illustrated by an evaluation of the impact of the Three Gorges Dam (TGD) on the composition of terrigenous organic matter and pollutants in the study system. Climate change (e.g., extreme drought), impoundment by the TGD, and the effects of tributaries and lakes in the middle and lower streams of the river may play different roles in the delivery of organic carbon. Although there is high pressure from anthropogenic activities compared with other regions in the world, pollutant concentrations are relatively low and potential ecological risks are limited. With changing fluxes from rivers, and significant modification and burial of organic matter on the shelf due to global change, long-term observations are necessary to develop our knowledge of the biogeochemistry of organic matter in this highly dynamic shelf region.

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.