Widespread Expansion of Salt Marsh Pools Observed on Maine Marshes Since 2009

Salt marshes provide critical habitats, protect coastal infrastructure, and are increasingly exposed to sea level rise, with many having a history of agricultural use and ditching over the centuries. Pool area coverage can be considered an indicator of marsh health but is rarely quantified. In this study, we digitized marsh pools using aerial imagery to quantify changes in pool area and density on 12 salt marshes in Maine from 2009 to 2021 as a case study of marsh response to environmental changes. We categorized pools into three types: mega-pools, individual pools, and perimeter pools, based on morphology and examined whether pools remained singular, split, or combined. We found a 15.7% increase in pool area from 2009 to 2021 on all marshes, primarily driven by mega-pool expansion, whereas individual and perimeter pools remained relatively constant. The rate of pool expansion across all marshes was 49,000 m² a⁻¹ with mean mega-pool size 6,400 ± 400 m². There was an increase in pool count per km² on all marshes except for the York River marsh, which still experienced area expansion. Pools primarily increase in cover through merging or being engulfed by mega-pools. Area cover change was not substantial when pools remained singular, split into many pools, or were only present in 2009 or 2021. Mega-pools were larger on lower marsh elevations and expanded at a greater rate when overlapping ditches, suggesting influence by sea level rise and historic agriculture. Marsh restoration projects that promote the drainage and re-vegetation of mega-pools may reverse this trend.