Rise and fall of an avian oasis: Tracking the impacts of land use change in a key coastal wetland in the world's largest megalopolis.

Tropical estuaries support wetlands with high biodiversity value and provide essential ecosystem services. Many of these systems, however, are global hotspots for urbanization, particularly in Asia, where this process has resulted in rapid conversion, fragmentation, and degradation of 80 % of the wetlands along the East Asian-Australasian Flyway (EAAF) for migratory birds. However, the impact of such landscape scale changes on migratory birds at a key stopover site along the EAAF has not been evaluated. Here, we used long-term data (> 40 years) from Deep Bay (Hong Kong), a shallow embayment in the Pearl River estuary (PRE) in south China, to investigate the impact of urbanization on (1) catchment land use and water quality, and (2) its impact on the capacity of the wetland to support populations of migratory waterbirds. Deep Bay supports the largest remnants of mangrove forests and tidal mudflats in the PRE and is an important refueling ground along the EAAF. It is also part of the Greater Bay Area (GBA, population 86 million), the world's largest megalopolis. The principal component analysis highlighted the nutrient loading and cleansing effect from seasonal flushing as characterizing variation in water quality in Deep Bay over four decades. Major shifts in water quality during the study period were accompanied by contemporaneous changes in wintering waterbirds numbers. Prior to 2003, the main drivers of water quality were organic nutrients from animal husbandry. Following large-scale reclamation and increases in impervious surface cover post-2003, primarily due to the development of the megacity of Shenzhen (population 17.7 million), the hydrodynamics of Deep Bay have changed, with knock-on effects of sedimentation, input of pollutants, and changes in the macrobenthos. The wintering waterbirds community responded to these changes both in total numbers and the relative importance of feeding guilds. Where total bird counts are positively influenced by benthic biomass, and the benthic biomass is positively correlated to the water quality that is driven by the cleansing effect of tidal flushing. These anthropogenic drivers have negatively impacted migratory birds that use Deep Bay as a refueling station. This study highlights the need for policymakers to control these drivers and limit the level to which sensitive coastlines are urbanized.