“Are you sure the GPS is right? You set it for the right point?” I asked as we floated towards our destination. Not because I didn’t believe the GPS, or thought our captain steered us wrong, but because I couldn’t believe what we were seeing. Instead of the usual view of West Raccoon, a barrier island and popular shorebird nesting area 15 miles off the coast of Louisiana, we saw nothing. No grasses. No birds. Not even a sandbar. Only endless water and a distant horizon. The boat rocked back and forth with the waves over what used to be land, and we all paused, mourning the small island and stark reminder of the wetland loss facing Louisiana.
Louisiana’s wetland loss and restoration efforts have been all over the news in the last few years. Relocation of the United States’ first climate refugees, Louisiana’s potential buyout plan for coastal residents, and a NY Times series on coastal land loss are just a few of the issues to make national headlines. All this talk about coastal wetland loss is for good reason: the state is home to 40% of the continental United States’ wetlands, but experiences 80% of the country’s wetland loss. Due to a combination of erosion, subsidence, sea level rise, and human alterations to land and waterways, Louisiana lost over 1,000,000 acres of land in the last 90 years (Figure 1). In the next 40 years, another 1,000,000 acres are expected to disappear. It’s not surprising then that there is a growing conversation about restoring and creating wetlands in coastal Louisiana.
WHY SHOULD YOU CARE ABOUT WETLANDS?
Coastal wetlands like marshes provide ecosystem services (or benefits) that directly and indirectly impact humans. Marshes provide habitat for fish and wildlife, protect inland areas from storm surge and flooding, filter nutrients and toxins out of water, and capture carbon dioxide from the air and store it in the ground through plant growth (carbon sequestration). Carbon sequestration, in particular, is important because of the increasing carbon dioxide concentrations in our atmosphere. A single acre of wetland can capture and store up to 5,000 tons of carbon each year, half of the carbon emissions a typical car produces in a year. When wetlands turn into open water or are drained for land development, these ecosystem services are lost. The disappearance of a small piece of marsh may not seem that important, but the loss of millions of acres of wetlands – like what is now happening in Louisiana – has dramatic consequences for local residents, wildlife populations, water quality in the Gulf of Mexico, and the level of greenhouse gases in the atmosphere.
A wetland’s ability to provide these important ecosystem services is controlled by soil properties, hydrology (timing and movement of water through the system), and plant productivity. Soil properties like sediment type, organic matter content, and nutrient content all influence the processes that occur in wetland soils. These biogeochemical processes (or the interaction of microbes, soil, and nutrients) impact what nutrients are available to plants. Nutrient levels influence plant productivity, which in turn influences how much carbon can be stored in the soil. The hydrology of the wetland is impacted by soil properties and in turn impacts biogeochemical processes that influence plant productivity and wetland services (Figure 2).
HOW IS LOUISIANA RESTORING AND CREATING WETLANDS?
To maintain wetland ecosystem services and combat the dramatic wetland loss in Louisiana, the state and federal government are working to restore and create new coastal wetlands. Restoration projects for existing marshes include marsh enhancement through plantings or sediment addition, restoring natural hydrology, and diverting sediment and water flow from the Mississippi River into nearby marshes. Another option is to create new marshes. This means dredging or removing sediment from local waterways like the Mississippi River and pumping that sediment into shallow open water or next to existing marsh (Figure 3). In a region where wetland loss is so prevalent, creation of new marshes is crucial for preserving current levels of wetland ecosystem services.
IS THERE A DIFFERENCE BETWEEN NATURAL AND CREATED WETLANDS?
For most wetland creation projects, “success” of the project is normally measured based on the kind of plants that grow and how much of the land those plants cover. It is often assumed that if wetland plants grow on a created wetland, that area is providing the same ecosystem services as a natural wetland. However, a few recent studies show that may not be the case for all created wetlands, particularly for soil properties that influence wetland function.
Two recent studies evaluated differences between natural and created wetlands. During construction, many newly created marshes are built higher than the normal tides because they are expected to gradually compact and become closer to sea level. Feher, Willis, and Hester (2018) showed that the higher surface elevations of created salt marshes led to different plant communities and lower plant productivity than natural saltmarsh. In a similar study, Wood, White, and Armbruster (2017) studied the soil and microbial properties of a created and natural marsh in Barataria Bay, Louisiana. They found that created marshes were drier and had less organic matter, nutrients, 85% lower microbe activity, and fewer microbes overall. In both studies, the biogeochemical processes of created marsh did not function like natural marsh due to differences in wetland construction, soil properties, and hydrology (Figure 2). These differences made it harder for plants to colonize the created marsh. Even though the created marsh areas had plants and the projects may have been termed a restoration “success”, Feher (2018) and Wood (2017) demonstrated that important ecosystem services were not fully restored in created marshes due to changes in hydrology or microbe activity.
HOW CAN WE IMPROVE WETLAND CREATION?
The goal of wetland creation and restoration projects is to return ecosystem services to the environment in the most cost and time effective way. Even though it is clear that previous wetland creation projects may not be perfect, dredging sediment and creating new wetlands is one of Louisiana’s best tools for maintain marsh land cover and protecting the coast. So how can we make created wetlands more effective at performing the ecosystem services we need?
Consider what sediment is used to create new wetlands, as soil type influences wetland function (Figure 2).
Build new wetlands at elevations within the tidal range. If wetlands are built too high, the hydrology is changed and they no longer function as wetlands.
Add organic matter to the soil of newly created wetlands during construction. This makes a better habitat for soil microbes and encourages plant growth.
Create stream channels or inlets to allow for “natural” flooding and nutrient exchange.
Soil development is slow in any environment, and expecting created wetland soils to function like natural wetlands soon after creation may be unrealistic. However, it is important to understand what influences plant and soil processes to maximize the ecosystem services provided by created wetlands. As coastal areas around the world face global sea level rise and growing rates of wetland loss, perhaps the lessons learned in Louisiana’s wetland restoration and creation projects will be used to improve future efforts.
L.C. Feher, J.M. Willis, and M.W. Hester. 2018. Importance of site history and environmental setting on soil properties in restored Louisiana back-barrier island salt marshes. Journal of Coastal Research. 34:58-66. http://www.jcronline.org/doi/abs/10.2112/JCOASTRES-D-16-00149.1
S.E. Wood, J.R. White, and C.K. Armbruster. 2017. Microbial processes linked to soil organic matter in a restored and natural coastal wetland in Barataria Bay, Louisiana. Ecological Engineering. 106:507-514. http://dx.doi.org/10.1016/j.ecoleng.2017.06.028