This is a Theme-based Research Scheme project on Ocean Circulation, Ecosystem and HypoxiA around Hong Kong waters (OCEAN-HK) with the ultimate goal of this project being to identify the factors driving the increasing eutrophication and hypoxia and to provide analytical tools and a scientifically-based strategy for stabilizing or even reversing eutrophication and hypoxia and for ensuring the overall sustainability of the marine environment in Hong Kong. The research topics are covered by four interlinked tasks to determine: (1) sources and sinks of nutrients and their biogeochemical controls, (2) ecosystem dynamics and biological controls, (3) pollutant and ecosystem impacts, and (4) physical controls, synthesis and future trends in the river-estuary-shelf (RES) waters.

Coastal eutrophication is caused by excessive nutrient loading which stimulates phytoplankton blooms when physical, chemical and biological conditions are favorable. It may lead to harmful algal blooms (HABs) and hypoxia (or “dead zones”, where dissolved oxygen is generally below 2 mg/L), both of which threaten the ecosystem. Coastal eutrophication has been a global environmental issue for decades, yet its persistence reflects the scientific and socio-economic complexities involved in alleviating the problem.

The coastal waters around Hong Kong are also affected by persistent and increasing eutrophication. This deteriorating situation may increase the frequency of HABs, expand the area of hypoxic zones and lead to other ecosystem disruptions and worse of all, offset the environmental improvements achieved through the costly Harbour Area Treatment Scheme over the last decade.

Eutrophication/hypoxia in Hong Kong waters is primarily caused by the ecosystem’s responses to the increasing nutrient discharge from the Pearl River and local sewage effluent. Meanwhile, increasing discharge of organic pollutants also modulates the biogeochemical pathways and ecological consequences and it further increases the severity of eutrophication/hypoxia. Highly variable oceanic currents transport the nutrients in the RES waters around Hong Kong, which undergo complex coupled physical-biogeochemical processes and modulate eutrophication/hypoxia. To date, these key processes have not been investigated in a comprehensive manner in our RES waters and they remain largely unresolved in similar ecosystems elsewhere in the world. Understanding the full spectrum of intrinsic coupled physical, biogeochemical and pollution processes in eutrophication is crucial to predicting and mitigating the impacts of eutrophication and it remains a huge scientific challenge regionally and globally.

By adopting a global and local perspective and by conducting an interdisciplinary study with world-class methodology, the project team will investigate holistically the coupled physical-biological-chemical processes in this interactive RES system and thereby develop tools for diagnosing and forecasting eutrophication/hypoxia. The project team will develop a novel, state-of-the-art marine monitoring system by conducting interdisciplinary mapping and time-series measurements, from which the team will further develop a novel coupled physical-biogeochemical-pollutant modeling system.

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Field survey conducted by OCEAN-HK

Figure 1. Conceptual model of the coupled physical and biogeochemical dynamics that control the summer hypoxia in the coastal transition zone off the Pearl River Estuary.

Figure 2. Overview of the characteristic forcing, hydrodynamics, sinking flux of riverine and oceanic organic matter and the bottom water hypoxia distribution off Pearl River Estuary. The 3D view shows model-simulated surface salinity on the top and bottom dissolved oxygen (DO) concentration superimposed on topography. The model domain extends to 80 m depth, but only the top 30 m is shown. In the surface colormap, the red curvy solid lines represent the simulated bottom salinity contour of 10, which together with the red straight solid lines denote the western and eastern coastal transition zone (CTZ) used in our analysis.