Mangrove forests are home to a wide variety of fish, birds, crustaceans, and other animals, making them some of the most productive and biologically diverse ecosystems on the planet. Mangroves create such rich communities in part because of their unique adaptations to living in brackish water, which allow them to grow along coastlines and in coastal rivers that are too salty for other plants. These same adaptations mean that mangrove forests’ structure and composition are highly dependent on local hydrological conditions, as constantly shifting mixtures of fresh- and saltwater from pulsing tides and seasonal river flows create coastal gradients of species adapted to different levels of salinity. Sea level rise and extreme storms are expected to drive seawater increasingly inland under climate change, making it critical to understand how changes in salinity affect mangrove forests. Yet because mangrove forests can be hard to get to and even harder to study, little is known about if or how water salinity levels in these ecosystems vary across space and time.

In a recent study supported by the South Central CASC, researchers developed a sophisticated hydrological model to simulate water flow and salinity in mangrove forests along the Shark River estuary in Everglades National Park, Florida. Their Riverine MANgrove HYdrological model (RHYMAN) accounted for freshwater inflows from the Shark River, saltwater inundation from coastal tides, and, uniquely, seepage into underground groundwater stores characteristic of the region’s karstic geology to simulate local hydrological conditions. In particular, they estimated soil porewater salinity, the quantity of salt found in water samples permeating the soil beneath mangrove ecosystems. They simulated water conditions at three study sites along the Shark River (downstream, midstream, and upstream), each of which had distinct mangrove structural and functional properties and different dominant mangrove species. The researchers then compared their simulated values to a long-term data set collected at the sites from 2004-2016.

Across all three study sites, RHYMAN simulations were functionally comparable to field-collected values, indicating that the model is useful in estimating mangrove hydrological conditions. The researchers also found that salinity in mangrove forests varied seasonally (wet versus dry season) and as you move farther upstream. In particular, the upstream site was the least salty and all three sites experienced more inundation events in the dry season. They further found that simulated changes in sea level rise and river flow management altered mangrove soil porewater salinity, with higher sea levels leading to more salty conditions. This could contribute to future mangrove range expansions similar to those being observed across the southeast. The RHYMAN model may be useful to researchers and resource managers evaluating hydrological changes, in areas such as the Everglades, to forecast mangrove water salinity under future climate and sea-level scenarios.