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

which are critically important habitats, could become unavailable by the end of the century even under the more conservative estimate of 1.5 m SLR if accretion levels drop to below 8 mm/yr. This will greatly reduce the capacity of the Ramsar site to support the numbers of waterbirds that currently use it. A lower accretion rate will inundate the mangroves, and estua- rine water intrusion could extend into the ponds inside the nature reserve. At high tide there will be more extensive inundation of the nature reserve and some of the marshes in the north- ern extents of the Wetland Conservation Area (Fig 3). These marshes—categorized as such for the SLAMM—are represented by fishponds. An analysis that applied SLAMM to coastal wet- lands in Korea also showed similar results, where the tidal flats were lost to rising sea levels, with inundation of adjacent rice fields [28]. In China’s Yangtze River estuary, models predict that accelerating sea level rise, land subsidence and low sediment accretion could result in con- siderable or complete loss of coastal wetland habitats [16]. Thus, the climate change scenario in Mai Po is consistent with the dynamics in other coastal wetlands along the flyway. The gei wai and ponds inside the Mai Po Nature Reserve, which forms the core zone of the Ramsar site, are intensively managed by WWF Hong Kong for about 40 years, under contract from the Agriculture, Fisheries and Conservation Department (AFCD) of the HKSAR Gov- ernment, according to 5-year prescriptive management plans. However, the tidal flats and the

coastal mangroves remain largely unmanaged, with the exception of a 45 ha area in front of four floating bird watching hides to remove invasive sedges, grasses and mangrove seedlings that encroach into the mudflat. Management of the ponds in the nature reserve include main- taining islands, draining the ponds to re-contour the bottom to maintain the diversity of deep and shallow water habitats to support the assemblage of birds that require different habitats, and adapting the open areas to accommodate different species based on analysis of pond use by birds. While deeper ponds are maintained for the ducks and grebes, shallow water ponds and islands are maintained as high tide roosting sites for the shorebirds and waders when the tidal flats are inundated. Although these habitats are relatively well protected from direct anthropogenic land conversion due to the protected areas status of the nature reserve, the SLAMM outputs suggests that these habitats are vulnerable to the manifestations of global cli- mate change. If SLR and declining sedimentation results in inundation of the tidal flats, coastal mangroves, and ponds in the nature reserve, the birds will lose habitat. Because of sediment trapping by two large dams upstream in the Pearl River, there is expected to be a >70% decrease in fluvial sediment supply, and the coastal zone will be starved sediment deposition causing the Pearl River Delta to erode and shrink [23–25]. Thus, there is a high likelihood that the tidal flat will also erode, and happen even faster, and large areas of

important habitat for the wading and shorebirds will be lost sooner than expected. But the climate model suggests that the fishponds along the inland boundary and the south- ern areas of the Ramsar site, especially in Lut Chau and Nam Sang Wai, will be relatively less impacted by SLR (Figs 2 and 3). A few of the southern fishponds are outside the Ramsar site boundary but are still within the Wetland Conservation Area. Securing these fishponds for conservation should be a climate change adaptation strategy for Mai Po, since they could become wetland habitats for birds under future climate change scenarios, especially serving as high tide roosting sites when the ponds in the nature reserve are inundated. Artificial wetlands such as fishponds already serve as supratidal habitats for waterbirds during high tide when tidal flats are inundated even in other stopover sites and wintering areas across the flyway [56, 57]. But, while some of the wetlands are owned by the HKSAR Government and under the management jurisdiction of the AFCD, most wetlands in the Wetland Conservation Area are privately owned, including by property developers who have leased them to aquaculturists to manage and operate as commercial fishponds. Under HKSAR Government regulations, the wetlands and fishponds in the Wetland Conservation Area are protected but can be converted 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 11 / 16