Resilient ecosystems, resilient communities-situational analysis of the Moata'a community and mangrove enviroment

Abstract

Mangrove ecosystems are under pressure due to anthropogenic stressors and sea level rise. The resilience of mangroves will depend on the rate of accretion of sediments compared to the rate of sea-level rise and their capability to colonise higher elevation areas or buffer zones. This will also be affected by the measures to protect the existing mangroves against anthropogenic pressures. The Secretariat of the Pacific Regional Environment Programme (SPREP) and the University of Newcastle signed a partnership agreement to analyse the ecosystem sustainably and determine possible adaptation interventions against climate change of the Moata’a mangroves, Samoa. This report presents the Output 4, “Final Moata’a Mangrove Ecosystem Analysis Report” from the project: “Mangrove, catchment and climate change impacts assessment and mangrove ecosystem analysis”. The report comprises an analysis of background studies relevant for the project and the available information, including data quality assessment and gap filling, a description of the modelling tools and assumptions, results from hydro sedimentological and ecosystem assessments, and resilience analysis of the Moata’a mangrove ecosystem considering future changes in climate, land use and infrastructure construction. The data and background information were used to develop hydrosedimentological models of the Moata’a catchment and also of the Vaisigano catchment using the freely available software SWAT. The Vaisigano catchment was included in the analysis because it contributes important amounts of sediment and water to the Moata’a mangrove ecosystem during large floods, The results from the hydrosedimentological analysis were used in conjunction with tidal flows and sediments to analyse the response of the mangrove ecosystem to sea level rise using an ecogeomorphological model. The ecosystem response assessment indicated that approximately 40% of the mangrove area can be lost over the next 100 years due to the sea level rise that corresponds to a high emission pathway (RCP8.5 of AR6 IPCC report). Mangroves can respond to sea level rise by increasing their elevation when capturing sediments and organic material in the soil or by migrating to higher ground, but the high values of 2 sea level rise limit the effectiveness of these mechanisms. Analysis of uncertainties in the model topographic data indicated that the loss of mangroves can be between approximately 30% and 60% over the next 100 years. The resilience of the Moata’a mangrove was studied by analysing the response of the ecosystem under different scenarios of climatic and man-made alterations at the catchment scale, in conjunction with sea level rise. A scenario that considered the construction of levees and a dam for flood protection resulted in a reduced resilience of the ecosystem due to a reduced delivery of sediment to the wetland. Conversely, a scenario that considered an increase in rainfall intensity due to a 4°C temperature rise greatly improved the resilience of the ecosystem because of a higher production of sediment due to erosion. A scenario that considered land use changes due to replacement of forest by agriculture in areas of the Vaisigano catchment had little effect on the mangroves resilience as the sediment inputs increased only marginally and water quality effects were not accounted for. The results indicate that the mangrove ecosystem resilience is very sensitive to reductions in sediment availability. Maintaining the connectivity of flow and sediment within the mangrove wetland and with the Vaisigano river and the coastal areas is vital for the resilience of the mangrove ecosystem. In addition, implementation of buffer zones at elevations that can promote migration are recommended to accommodate future mangrove colonization. Regular monitoring is required to assess the effectiveness of any preventive measures and to be able to implement adaptive management strategies.