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

summer, and the gei wai were drained during the Autumn-Winter for harvesting bigger fishes and maintenance. During this time the birds are able to use them as feeding areas. Thus, these gei wai are a good example of ‘wise use’ management of wetlands that supported both migra- tory waterbirds and local fisher livelihoods. Since the 1960’s, however, most gei wai have been converted to non-traditional fishponds with no connections to the bay and used for commer- cial grow out of fishes, mostly exotic species. However, the gei wai in the Mai Po Nature Reserve are operated in the traditional way to maintain habitat for waterbirds. The nature reserve is also part of the Site of Special Scientific Interest (SSSI), zoned because of its biological importance, and is comprised of seven Biodiversity Management Zones man- aged under controlled hydrological regimes to provide shallow and deep open water areas, reedbed dominated ponds, rain-fed ponds, and gei wai to provide the range of habitats and food for waterbirds and other birds that use the nature reserve. Several active, commercial fish- ponds surround the nature reserve, inside the landward areas of Mai Po and beyond, in an area designated as a Wetland Conservation Area (Fig 1B). Methods We projected impacts for 3 time intervals, 2050, 2075, and 2100 in relation to 2019 for two dif- ferent SLR scenarios, 1.5 m and 2.0 m until 2100, and for four accretion rates; i.e., 2mm/yr, 4 mm/yr, 8 mm/yr, 15 mm/yr. We used these accretion rates because historical records (between

1898 and 1949) estimate accretion at 8 mm/year, but the rate had increased to about 15 mm/yr in the 1980’s and 1990’s, attributed to land clearing for agriculture and development in Shen- zhen [23, 33]. From the 1990’s onwards, there has been a decrease in fluvial sediment supply to the coast, and models suggest that sediment loads are expected to decrease further, by about 70% in the future with sediment trapping behind the Yangtan and Longtan mega dams [23]. Therefore, we applied 2 and 4 mm/yr accretion rates, which are approximately 30% of the 8 and 15 mm/yr rates, assuming a proportionate deposition based on declining sediment loads. Models of SLR [21] projected an average rise of about 0.67 m and 0.84 m for Representative Concentration Pathways (RCP) 4.5 and 8.5 respectively, in the seas off Hong Kong, which is 0.2m higher than the global mean values projected by the Fifth Assessment Report of Intergov- ernmental Panel on Climate Change [34]. Global temperature increases between 1.9 and 3.1˚C can bring about fast Antarctic disintegration that can contribute an additional 0.21 to 0.74 m to SLR this century within 5 and 95% quantiles, respectively, of the RCP 8.5, and contributions of up to 0.45 m for the 95% quantile under RCP 4.5 [35]. The temperature range includes the 2˚C target set by the Paris CoP. Thus, in this analysis we used 1.5 and 2 m SLR scenarios to approximate the 1.5 m (median) and 2 m (95% quantile) for RCP 8.5 [35], because experts believe that even the upper bound of the ‘business as usual’ RCP 8.5 trajectory could likely be

exceeded as few countries are adhering to their Nationally Determined Contributions to miti- gate global climate change [36]. This justifies the use of SLR scenarios that even exceed 2 m SLR for future casting and planning [37]. To execute the SLAMM, we first reclassified the habitat and land use-land cover in the Ramsar site and the Wetland Conservation Area (Fig 1) into corresponding SLAMM catego- ries (Table 1). We used expert opinion based on our experience in the Mai Po area and ecologi- cal definitions of the SLAMM categories in Clough et al. [26] to do the cross-walk. We used a Fig 1. The location of the Mai Po Inner Deep Bay Ramsar site in relation to the Deep Bay (A, above) and major habitat types (B, below) that were cross-walked to the SLAMM. categories, as indicated in Table 1. The Shenzhen River borders the northern boundary of the Wetland Conservation Area. The Kam Tin River is south of the Mai Po SSSI. Basemap Sources: Esri, DeLorme, HERE. Landuse maps and boundary layers for the wetland conservation area, Ramsar site, and Nature Reserve are from WWF Hong Kong. https://globil-panda.opendata.arcgis. com/datasets/mai-po-wetland-habitats. https://doi.org/10.1371/journal.pone.0239945.g001 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 5 / 16