Accelerated migration of mangroves indicate large-scale saltwater intrusion in Amazon coastal wetlands

Saltwater intrusion can dramatically transform coastal ecosystems, changing vegetation and impacting wildlife and human communities who rely on these natural resources. This phenomenon is difficult to measure over large and remote areas but can be inferred from changes in the distribution of salt-tolerant vegetation, such as mangroves, observable from satellite imagery. The northern coast of Brazil has the largest continuous mangrove forest in the world and very low human occupation. Even so, saltwater intrusion and changes to the coastline have been reported in this region, with potential consequences for mangrove carbon storage and for local livelihoods, but this has not been quantified due to the remoteness of the area. This study measured changes in mangrove distribution along the Northern Brazil coast in the state of Amapá, covering ca. 15,000 km², over the last 38 years using Landsat satellite imagery. We found that mangrove area in this region is highly dynamic, with significant gains and losses occurring over the study period, but with an overall net gain of 157 km². Mangroves have been systematically expanding inland and this growth has accelerated close to the shoreline and at the head of tidal channels in the last two decades, indicating rapid and large-scale saltwater intrusion in this region. This phenomenon is likely driven by sea level rise, which also accelerated in this region in recent decades, but anthropogenic impacts such as buffalo grazing may also play an important role.