The GBA (Hong Kong-Guangdong-Macau Greater Bay Area) ocean consists of Pearl River Estuary and the adjacent continental shelf, which forms a coupled estuary-shelf system (ESM). It links with Pearl River upstream and broad South China Sea downstream as a continuum of land, river and ocean. In the last 20-year, the Hong Kong University of Science and Technology and Xiamen University have jointly led >35 cruises in the ESM under supports of National 973 projects and projects of Theme-based Research Scheme (TRS) and Area of Excellence (AoE) of Hong Kong. We have conducted interdisciplinary mappings and buoy-based long-term time series monitoring in the ocean. These observational studies have provided the most comprehensive, time-dependent, 3D physics and biogeochemistry data in the GBA ocean. The endeavors have provided many new findings and scientific understandings in the ESM and set new insights for the studies of land-river-ocean continuum globally. The highlighted achievements include, for example, the findings of  1: seasonal eutrophication drove bottom hypoxia along the coastal transition zone (CTZ) between the Pearl River Estuary and the adjacent continental shelf; 2: hypoxia in the CTZ exhibited remarkable spatiotemporal variability due to changes of the path of the nutrient-rich river plume, strength of mixing, and length of water residence time regulated by variable coastal current; 3: combined biogeochemical processes and the unique hydrodynamic condition provide the sufficient condition for hypoxia formation in the CTZ; 4: eutrophication-derived organic matter (OM) accounted for ~65% in the bottom hypoxic waters, while terrestrial OM directly contributed ~30%; 5: bottom hypoxia reduced the acid-base buffering capacity of seawater and seriously aggravated the coastal acidification; 6: oyster farming is a promising approach to mitigate eutrophication and hypoxia. Oyster farming area of 10 to 200 km² yields a hypoxic volume reduction of 6% to 67%, which equals the impact of reducing 10% to 60% of river nutrient load; 7: The shoreward transport of OM from the subsurface chlorophyll maximum (SCM) offshore can trig hypoxia when river discharge is low. Our research provides science-based knowledge for mitigation and cost-effective strategy to ensure sustainable water quality in GBA ocean.