Impacts of sea-level changes on mangroves from southeastern Brazil during the Holocene and Anthropocene using a multi-proxy approach

Abstract

Globally, mangroves are expected to move inland as sea-level rises. However, local characteristics mainly related to mangrove structure, geomorphology and coastal hydrodynamics may change the mangrove response to sea-level rise. The most useful evidence to support projections for the future likely comes from mangrove history reconstruction corresponding to past sea-level changes. This study characterized modern (1985–2018 CE) and past (<6300 cal yr BP) mangrove dynamics according to sea-level changes along two estuarine valleys on the southeastern Brazilian coast to predict the mangrove response to sea-level rise by 2100. Relative Sea-Level (RSL) rise triggered changes from a tidal flat occupied by herbs, palms, tree/shrubs to a lagoon surrounded by mangroves between ~6300 and ~4230 cal yr BP. More recently, the RSL fall converted that lagoon into flats occupied by herbs, trees/shrubs, and palms on higher surfaces, and mangroves on lower flats during the mid-late Holocene. The last thousand years were characterized by a mangrove contraction between 390 and 77 cal yr BP, caused by a RSL fall. By contrast, mangrove expansion began at 77 cal yr BP (1873 CE) and continued after 1950 CE, migrating onto higher tidal flats previously occupied by herbs, palms, and trees/shrubs. Spatial-temporal analysis also indicated a mangrove invasion onto higher flats since 1985 CE. These trends are likely related to RSL rise since the end of the Little Ice Age and they intensified during recent decades, when the lower mangrove boundaries remained stable, and mangroves expanded to higher surfaces. Mangroves, confined between steep surfaces, will expand ~22ha under the influence of a sea-level rise of 98cm by 2100. However, an upstream mangrove migration by 2100 on low and extensive fluvial plains (~4030 ha) will depend mainly on the interaction between fluvial discharge/sea-level rise.