A Conceptual Model for the Responses of Mangrove Forests to Sea Level Rise

Analyzing the ecological and physiological properties and processes of mangrove forests, as well as the physical environment where they occur, and including knowledge of their responses to sea-level changes in the Quaternary, a conceptual model for the response of mangroves to sea-level changes was developed. The model considered the rate of sea-level rise, the balance of sedimentation, regional oceanographic properties, and coastal geomorphology. The local topography determines the availability of areas for mangrove migration landward. The rate of sediment input determines the capacity of mangrove forests to resist a sea-level rise. This compensation process will depend not only on sediment input but also on the deposition rate, which will be influenced by marine forces that are responsible for sediment transport. The efficiency of sediment accumulation in keeping pace with rising sea level will also depend on the area and geology of the drainage basin, in addition to the geomorphology and coastal dynamics, all of which will determine the rate of sediment retention in the intertidal zone. If the rate of sediment deposition is smaller than the rate of sea-level rise, an erosion process can take place, which may lead to the local extinction of the ecosystem, or to the persistence of part of the ecosystem as a refuge mode. Another possible response, if there is no compensation process, is the migration of mangrove forests landward. This migration is only possible if the topography is suitable for mangrove colonization, and if the rate of relative sea-level rise is compatible with the mangrove life cycle. Following this retraction process, a maintenance mode can be attained, when a new equilibrium is reached and the ecosystem persists. Two scenarios might lead to the loss of the ecosystem: (a) if the rate of sea-level rise is greater than the rate at which the mangroves are able to migrate landward; (b) if after an initial retraction, the mangroves encounter an obstacle that prevents their further migration. Despite the current knowledge about mangrove responses to environmental changes, the fully validation of the proposed model will only be possible with the incorporation of new data, through the maintenance of monitoring networks.