Deep Bay is home to the largest remnants of mangrove forests and tidal mudflats in the Pearl River estuary and serves as a vital refueling ground along the EAAF, which is part of the Greater Bay Area (GBA) with a population of 86 million, making it the world's largest megalopolis. Principal component analysis revealed changes in water quality over four decades, particularly due to nutrient loading and seasonal flushing effects. Large shifts in water quality were accompanied by changes in wintering waterbird numbers. Prior to 2003, water quality was primarily affected by organic nutrients from animal husbandry. However, post-2003, massive reclamation and increased impervious surface coverage driven by the development of Shenzhen (with a population of 17.7 million) altered the hydrodynamics of Deep Bay. These changes led to sedimentation, pollution input, and alterations in macrobenthos. These changes had negative impacts on migratory birds using Deep Bay as a refueling station. This study emphasizes the importance of controlling these factors and limiting urbanization along sensitive coastlines by policymakers.
Anthropogenic habitat loss poses a significant threat to wildlife populations worldwide. With over 50% of the global human population residing in urban areas and 10% living along coastlines, coastal ecosystems, particularly wetlands, are especially vulnerable to the effects of urbanization. This pressure is most pronounced in tropical estuarine wetlands, which are often the locations of megacities worldwide. These areas have been facing unsustainable aquaculture and agriculture, leading to the drainage and conversion of these biodiverse and productive habitats. Additionally, these wetlands provide essential ecosystem services, including climate regulation through blue carbon sequestration and acting as critical breeding, staging, and wintering sites for migratory waterbirds. The network of wetlands along the East Asian-Australasian Flyway (EAAF) supports over 50 million migratory waterbirds from more than 250 species, many of which are of conservation concern.
However, these estuarine wetlands are under the threat of urbanization due to their historical desirability for human settlement. Coastal urbanization has contributed to the rapid population decline of migratory birds due to habitat loss and degradation from both direct and indirect factors. The EAAF, spanning coastal regions in Japan, South Korea, China, and Indonesia, serves as a vital pathway for migratory birds. These areas have historically acted as staging sites and overwintering locations for these birds. In recent decades, many of these coastal settlements have developed into megacities, leading to habitat destruction, changes in hydrology, and geomorphological modifications. Furthermore, the majority of global population growth occurs in coastal areas, increasing the pressure on coastal wetland ecosystems. Ignoring this issue could lead to irreversible damage to ecosystems supporting a wide range of plant and animal life and providing crucial ecological services.
Notably, the Greater Bay Area (GBA), consisting of Hong Kong, Shenzhen, and nine other cities within the Pearl River Delta in South China, has merged to form the world's largest megalopolis with 86 million inhabitants. The high rate of urbanization and infrastructure development around the delta area poses severe threats to the remaining habitats. Deep Bay, situated in the Pearl River Estuary (PRE), is a shallow embayment with high urbanization levels on both sides. Despite its urban setting, Deep Bay is an important stopover site for migratory birds along the EAAF. Annually, tens of thousands of migratory birds refuel in the marshes and mudflats of the internationally acclaimed Mai Po Nature Reserve in Hong Kong during their migration between Siberia and New Zealand.
This study is one of the first to combine long-term environmental and ecological datasets to investigate the interaction between environmental changes and the capacity to provide key ecosystem services, such as wildlife conservation, in a peri-urban estuarine wetland. It reveals that water quality significantly changed in Deep Bay around 2003-2005, coinciding with rapid urbanization and reclamation. The changes in land use due to reclamation have also affected water circulation.
Remote sensing surveys of Deep Bay from 1988 to 2020 show that the coverage of mangroves and impervious surfaces increased at a similar rate, with a 55% increase. Meanwhile, water bodies, including mudflats, fishponds, and gei wais, decreased by 30%. This transformation was primarily due to the reclamation in Deep Bay, resulting in the conversion of 15.6 km2 of open water and mudflats into impervious surfaces for urban development. The reclamation led to a 12.6% loss of water surface area.
Coastal human populations have been rapidly expanding, particularly in East Asia, and wetlands have been under pressure from unsustainable aquaculture and agriculture before the recent wave of urbanization. The development of many tropical estuarine coastal wetlands into megacities has led to significant declines in waterbird species along the EAAF.
The impact of urbanization is mainly attributed to changes in hydrology regimes and geomorphological modifications. Impervious surfaces from urban areas increase surface runoff and sediment input, influencing water quality and increasing the frequency and magnitude of floods. These changes result in decreased habitat sinuosity and heterogeneity in wetland streams. As the majority of human population growth occurs in coastal areas, it is highly probable that coastal wetland ecosystems will continue to face significant repercussions unless decisive action is taken to address these factors.