A climate adaptation strategy for Mai Po Inner Deep Bay Ramsar site: Steppingstone to climate proofing the East Asian-Australasian Flyway

maintained at current levels [23]. Under this scenario, the coastal zones will be starved of approximately 6,000 Mt of fluvial sediment until the end of this century, which may also result in the erosion and shrinking of the delta, including the inter-tidal mudflat [23]. In fact, recent studies of the sub-aqueous bathymetry of the Pearl River Delta has shown that sections of the delta are already eroding, or accreting at lower rates [24, 25]. The Mai Po Inner Deep Bay Ramsar site in Hong Kong is an important staging and winter- ing site along the EAAF and supports large numbers of migratory birds, including 24 globally threatened bird species associated with wetlands. Thus, rising sea levels, stronger storm surges and shrinking mudflats will combine to decrease the availability of habitats for the waterbirds in Mai Po in the future, and adaptation plans to counter the threats from climate change and offshore subsidence are needed now to ensure persistence of migrations and flyway functional- ity. Here, we modelled the impacts of climate change and decreasing sedimentation rates on the important bird habitats in the Mai Po Inner Deep Bay Ramsar site to support a climate adaptation strategy that will continue to host migratory waterbirds. To do this, we applied the SLAMM (Sea Level Affecting Marshes Model) version 6.7 [26] to simulate habitat conversion under selected sea-level rise scenarios for coastal areas in the Mai Po Inner Deep Bay Ramsar site and adjacent areas surrounding it. This model has been widely used for coastal resource management across the world, includ-

ing for selected wetlands in the Yellow Sea estuary of China [16, 27] and Korea [28] in the EAAF region. But we note that the lack of data on historic wetland conversion, especially out- side North America [29] hinders retrospective analysis, which could be used to calibrate and validate the model. Analyses have shown that elevation data is the most important factor for output accuracy since conversion among habitat classes is mainly governed by elevation [29– 31]. Therefore, a high-resolution DEM is important for generating consistent results. Other site-specific parameters such as tidal range and accretion also affect the model outputs and should therefore be as accurate as possible [30]. Despite these shortcomings, SLAMM is the only marsh migration model that handles uncertainty in spatial inputs (DEM, VDATUM, etc.) and parameter choices, and this capability has contributed to its wide use [31]. Mai Po Inner Deep Bay Ramsar site The Mai Po Inner Deep Bay Ramsar site (hereafter Mai Po) is located in the North West New Territories of Hong Kong Special Administrative Region (HKSAR), along the southern edges of the Deep Bay of the Pearl River estuary (Fig 1A). The Ramsar site is comprised of tidal mud- flats, coastal mangroves, marshes, and fishponds (Fig 1B), and now represents Hong Kong’s largest remaining wetland. It is about 1,500 ha in extent, with a 380 ha nature reserve zoned within it (Fig 1B). Mai Po supports about 80,000 wintering waterbirds and another 20,000–30,000 use it as a

staging area during their annual migration in spring and autumn. Regular visitors to Mai Po include the Endangered Black-faced Spoonbill (Platalea minor) and Nordmann’s Greenshank (Tringa guttifer), and, occasionally, the Critically Endangered Spoon-billed sandpiper (Calidris pygmaea). Mai Po played a significant role in the population recovery of the Black-faced spoonbill, and now regularly supports about 7–10% of the global population [32]. In the 1950s, most of the mangroves in Mai Po and surrounding coastal areas were con- verted to a system of traditional aquaculture pond management known as gei wai, where the ponds were connected to the bay via a sluice gate that allows water exchange with the bay. Some mangroves were left to remain in the ponds, and the biomass from the mangroves nour- ished the shrimp, oysters and fishes that were stocked in the ponds through tidal water exchanges, which also aerated and regularly flushed the ponds. Shrimp were harvested in the PLOS ONE Climate adaptation strategy for Mai Po Inner Deep Bay Ramsar site PLOS ONE | https://doi.org/10.1371/journal.pone.0239945 October 21, 2020 3 / 16