# Mapping National Mangrove Cover for Belize Using Google Earth Engine and Sentinel-2 Imagery
## 1 Introduction  
Mangroves are a diverse group of halophytic plants that are found along tropical and subtropical coastlines [1]. These highly productive ecosystems perform critical ecological functions (e.g., fish nurseries [2, 3] and bird rookeries [4]), and they provide important ecosystem services to coastal communities (e.g., flood protection [5] and erosion prevention [6]). Mangroves are particularly important for their roles in climate change adaptation (e.g., coastal accretion in response to sea level rise [7]) and mitigation (e.g., carbon sequestration [8]). Within the Mesoamerican Reef (MAR) ecoregion, a transboundary resource shared by Mexico, Belize, Guatem, and Honduras, mangrove protection averts significant flooding of land, and Belize is ranked the number one country globally for its benefits of mangroves (i.e., averted flood damages to property) relative to GDP [5]. Mangroves within the MAR are under considerable stress from both anthropogenic (e.g., coastal development [9, 10, 11, 12]) and natural (e.g., localized eutrophication in combination with sea level rise [13]) threats, which resulted in mangrove cover declining by approximately 30% across the region between 1990 and 2010 [14]. Within the MAR, there is increasing interest to manage and conserve mangrove ecosystems, with countries looking to include mangroves in their Nationally Determined Contributions (NDCs) for the Paris Agreement [15, 16, 17]. Additionally, a regional strategy for the management, conservation, restoration, and monitoring of mangroves was launched in 2020 [18]. One of the main priorities identified by the management strategy and by [14]'s review of mangrove managementin the region was the need for improved mangrove mapping at high spatial resolutions and with a consistent methodology implemented across the region. Previous national-scale (e.g., [19; 20] for Belize, [21] for Mexico) and global-scale (e.g., [10; 22]) projects have used remote sensing to map mangrove cover for individual countries within the MAR and/or for the MAR as a whole. However, each of these studies used relatively coarse-resolution data (in most cases, 30 m Landsat data). Throughout the MAR, many mangrove caves are too small to be accurately mapped using 30 m resolution data. These small mangrove caves and small patches along the mainland coastal zone are among the most immediately threatened by human activity and sea level rise. Therefore, there is an urgent need to monitor changes to the MAR's mangrove ecosystems at finer spatial resolutions. This high-resolution monitoring must be conducted using a consistent methodology throughout the region to ensure comparability among countries and locations. In this paper, we present the first stage of our ongoing initiative to address these research needs. We used Google Earth Engine-based processing of Sentinel-2 satellite imagery to map mangrove ecosystem area in Belize as of 2020. This new map is both (a) the highest-resolution map of Belizean mangrove ecosystems currently available, and (b) the first stage of our ongoing project to map mangrove ecosystems across the entire MAR using high-resolution satellite imagery and a consistent classification methodology. We chose Belize as the pilot country for this broader project for three reasons. First, we have a combined 40-plus years of field work experience among the mangrove ecosystems of Belize, including several recent remote-sensing based studies of local-scale mangrove changes that provided us with a \"library\" of georeferenced field observations with which to train our image classification algorithm. Second, the Belize Barrier Reef (BBR) hosts hundreds of the small caves discussed above, and therefore presents an important case study for evaluating the effectiveness of Sentinel-2 imagery in the mapping of these caves. Third, the Belizean government has included mangroves within their NDCs [15; 17], and a high-resolution map of mangrove cover will assist in providing the most accurate estimate of mangrove carbon within Belizean mangroves.