Protected mangrove extent changes and their relation with wave power exposure on Aldabra Atoll

Annabelle Constance and Paul J. Haverkamp, Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland, Annabelle Constance and Gabriela Schaepman-Strub, University Research Priorty Programme Global Change and Biodiversity (URPP GCB), University of Zurich, Zurich, Switzerland, Nancy Bunbury, Seychelles Islands Foundation, Victoria, Seychelles

Background/Question/Methods
Mangrove forests occur along tropical coastlines at the boundary between the land and sea. Because of their location, mangroves are threatened by sea level rise, storm surge and drought, especially on low-lying islands where forests have limited land area to adapt. Mangrove forests are vital for the conservation of biodiversity, protection of coastlines and carbon capture. Here, we examine mangrove extent changes between 1997 and 2018 on the protected Aldabra Atoll (the largest area of mangroves in Seychelles) using Landsat satellite imagery and change detection methods. Further, we assessed the relationship of wave power with mangrove stability along the lagoon coastline.
Results/Conclusions
Over the 21-year study period, mean mangrove extent was 1283 ha with an overall net increase of 60 ha (0.23% year-1). The majority of extent changes were small (< 2 ha) and contiguous to existing mangroves. Mean wave power was three times higher in areas where mangrove cover changed between 1997 and 2018, compared to areas with stable mangrove cover (p<0.001). We identified a wave power threshold of 7.1 Watts m-1 for mangrove occurrence, above which mangroves do no occur on Aldabra, and a threshold of 2.3 Watts m-1 for areas with stable mangrove extent. These thresholds are useful for assessing sites with the greatest potential for mangrove restoration across similar areas worldwide. Our results highlight the importance of quantifying mangrove extent changes at a local scale to assist with planning for the protection and restoration of this valued habitat, and broader coastal areas, related to adaptation and mitigation of climate change impacts, especially sea level rise.