Along the coast of the northern Gulf of Mexico lies the estuary of Grand Bay, trimmed with an expanse of marsh spanning across the Mississippi-Alabama border. Here, the Grand Bay National Estuarine Research Reserve overlaps with the Grand Bay National Wildlife Refuge, providing well-known benefits such as habitat for commercially and ecologically important species and serving as popular destinations for recreational activities like hunting, fishing, and hiking. In addition, these vast coastal marshes offer protection for the surrounding upland habitats, communities, and infrastructure by buffering waves and flooding, and sequestering carbon from the atmosphere.

Despite the apparent vastness of these public lands, the marsh has been shrinking for over a century. A team from the USGS St. Petersburg Coastal and Marine Science Center has been visiting Grand Bay multiple times a year a year since 2013 to figure out why. Their research aims to discover what drives change to the marsh landscape through time, track how the shoreline position is moving as a result, and predict the future of this vulnerable ecosystem. Kathryn Smith, one of the Principal Investigators on the Estuarine and Marsh Geology (EMrG) project conducting this research, stated,

We study sediments—the foundation of coastal marshes—to see how they move between the marsh platform and the estuary. Where is the sediment going? Where is it coming from? The answers to these questions can help us better understand how the ecosystem changes and what its future holds.

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Learn more about how the team studies Grand Bay in this interactive geonarrative: A Century of Change in Grand Bay, Mississippi and Alabama.

The EMrG team uses a variety of methods to study sediments, such as net sedimentation tiles (NSTs) – handy little tools you could make at home. They include a tile (like what you might have in your bathroom or kitchen) on top of a piece of PVC piping, stuck into the mud and even with the top of the sediment platform. After a few months, they revisit and remove the tiles to measure how much sediment has accumulated on the NST – a sign of how much marsh accretion is occurring over time.

In some areas, the marshes in Grand Bay are eroding well over 2 meters (~6 feet) per year. That’s a major concern for our federal and state partners who manage these areas, so we’re working here to try to better understand why.

The team also uses a combination of historical maps, aerial imagery, and satellite data to track how the position of the marsh shoreline has changed through time. They also use a GPS to mark shoreline positions during their field work. While the team has seen high rates of sediment delivery into the marsh near the shoreline, the gradual retreat of the marsh shoreline appears to be the source of much of this sediment—essentially, the sediment is taken away from the edges and deposited on top.

A new geonarrative allows users to explore shoreline change at Grand Bay.

As sea levels rise, water from the estuary can add sediment to the top of the marsh platform and allow the marsh to grow vertically—and therefore keep pace with increasing sea-level. Even if the shoreline is retreating, the marsh can adapt to the impacts of climate change by moving inland and converting the upland dry habitats to marsh. However, it can only retreat so far before it runs out of space or erosion out-paces upland transgression.

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Models developed by Karim Alizad, a member of the EMrG team, show that over the next few decades, the productivity of the marshes in Grand Bay will decline substantially, and the shoreline will continue to retreat until the marsh runs out of space due to coastal infrastructure, agricultural lands, and urbanized areas. Ultimately, once sea level rises high enough, the marshes could disappear and put coastal communities at risk. However, the team is working to refine and improve these model predictions with additional sediment parameters, a critical component of marsh development.

Understanding marsh shoreline change is increasingly important due the diverse estuarine habitats at risk from habitat loss and expanding coastal communities under increasing threat from sea level rise and storms. This information can also be used for making decisions regarding living shoreline projects, habitat management for protected species, land-use planning, and coastal restoration.