Atoll Mangrove Progradation Patterns: Analysis from Jaluit in the Marshall Islands

Abstract

Low-lying islands are vulnerable to coastal erosion, and mangroves, which can mitigate erosion, have suffered enormous losses in recent decades owing to human impacts. Previous studies have little investigated mangrove shores on atolls, which may face combined multiple threats. We analysed the large Marshall Islands atoll of Jaluit, at a higher resolution than previous spatial change studies, finding that mangrove shorelines prograded seawards over the last seven decades. Biogeomorphic colonisation processes were characterised from transects along ~ 14.6 km of shorelines. Mangrove progradation occurred in patterns of arc-shapes evident of long-shore drift deposition, patch expansion of offshore mangrove colonisers, and linear shoreline advance. Significant differences in the rates of expansion were identified, with arc-shaped colonisation showing the fastest rates of expansion. However, linear shoreline advance was the most frequent expansion pattern showing greater than three-fold more classified transects than arc-shaped colonisation and patch expansion. These results have implications for low island mangrove restoration. Applying mangrove planting patterns mimicking these different natural colonisation processes may enhance restoration success in ecosystem-based adaptation projects to mitigate sea level rise vulnerability. Results from this study show that atoll mangrove shorelines demonstrate resilience during past sea level rise rates, and that rates of expansion vary according to patterns of biogeomorphic colonisation.

IUCN Red List of Ecosystems, Mangroves of the Central Pacific

“…However, owing to the small scales of many Central Pacific mangroves, higher resolution spatial analyses are needed (Crameri and Ellison 2022). For example, atoll islands commonly have mangrove coasts of <30 m width and the whole island is <50 m width (Crameri and Ellison 2024), so many critical habitat details are not picked up by large scale determinations. Thus, the Central Pacific mangrove province is assessed as Data Deficient (DD) under subcriterion D2b.…”