If you are heading to the beach this summer, you are likely to pass by coastal wetlands on your way to the shore. These wetlands vary from bottomland hardwoods to marshes to seagrass beds but all occur at the intersection of land and sea, where fresh water from land meets saline tidal waters.
Coastal wetlands provide an array of ecosystem services. They protect shores from flooding, erosion and storm surge; provide habitat for wildlife; filter pollutants from water and sequester carbon. A group of researchers led by Assistant Professor Beth Lawrence of the Department of Natural Resources and the Environment (NRE) and UConn’s Center for Environmental Science and Engineering (CESE) is studying and quantifying the ecosystem services of carbon and nitrogen cycling to determine how these areas are responding to rising oceans.
The coast of the eastern United States is expected to experience elevated levels of sea level rise compared to the global average. Several factors, including water temperature, salinity, currents, the melting of glaciers and ice sheets and various geological and geographical elements, affect the rate of sea level rise. Scientists forecast global sea level rise in the range of 8 inches to 6.6 feet by 2100. The pace at which and amount the oceans rise will depend largely upon carbon and methane emissions that accelerate the melting of the planet’s ice and increase ocean temperatures. Heat causes water to expand, further escalating sea level rise.
Ashley Helton, an assistant professor in NRE, and Associate professor Chris Elphick of the Department of Ecology and Evolutionary Biology (EEB) are collaborating with Lawrence. Aidan Barry, a graduate student in NRE, is also helping conduct the research. The funding for the project comes from Connecticut Sea Grant, the Long Island Sound Study and the Connecticut Institute for Resilience and Climate Adaptation.
While the predicted rate of sea level rise varies, there is no doubt it is occurring and coastal wetlands are strongly affected by advancing waters.
The interdepartmental study launched in March and its initial focus is on movement in vegetation zonation and carbon and nitrogen cycling. The research will also examine how wetland restoration techniques affect these ecosystem services. Their investigation builds upon coastal wetlands research conducted by recent NRE graduate April Doroski.
“Evidence suggests that coastal wetlands are either drowning or shifting further inland,” says Lawrence. “We want to see how this is influencing ecosystem services like carbon and nitrogen cycling. The fact that coastal wetlands have strong zonation patterns helps us. Because plant species have different salt tolerances and oxygen needs, based on their proximity to the ocean, we can study how the environment in these individual bands is being affected. We can then forecast how projected rates of sea level rise might alter vegetation zones and their associated rates of carbon sequestration and nitrogen removal.”
Any changes to the vegetation of coastal wetlands affects their ability to carry out important functions like nitrogen removal and carbon storage.
“There is significant runoff of nitrogen from urban and agricultural watersheds. Wetlands work to remove nitrogen. In Connecticut, some of this runoff reaches coastal wetlands before it makes it into Long Island Sound. This microbial process of denitrification limits eutrophication,” says Lawrence.
Eutrophication occurs when nutrients, such as nitrogen or phosphorus, are not filtered out before reaching a body of water. The excessive nutrients promote algae growth; when the algae die, decomposers use all the available oxygen, killing fish and other vegetation. Nitrogen runoff is typically from fertilizers and sewage.
Coastal wetlands also store carbon in their organic rich soils. Their soils are often anoxic, meaning they lack oxygen, which slows the rate of decomposition and the release of carbon. Plants grow quickly in wetlands and so they sequester considerable amounts of carbon.
“Coastal wetlands also don’t produce as much methane as fresh water wetlands,” says Lawrence. Methane is a more potent greenhouse gas than carbon dioxide, meaning that it is more efficient at trapping head in the atmosphere, but carbon dioxide is the most plentiful greenhouse gas contributing to climate change.
“Since we know that coastal wetlands support these important functions, we need to quantify how much they’re doing, if that amount is changing with sea level rise and different management strategies, and eventually what we can do to ensure they keep benefiting us,” Lawrence says.
The team is currently surveying potential locations for future collection of plant materials and soil from thirty coastal wetland sites along Long Island Sound to select locations for the study.
“We are in the process of identifying wetlands that have areas dominated by three different species: common reed (Phragmites australis), marsh hay (Spartina patens) and salt grass (Spartina alterniflora). We want to study those plants in areas that have been subjected to different management techniques: tidal restoration, Phragmites control and reference areas, where no restoration has occurred. This will help us understand how plant composition and management techniques alter the processes of carbon and nitrogen cycling,” says Lawrence.
Non-native Phragmites are reeds that can be damaging to wetlands. They grow rapidly, over 6 feet in a single year, and create wide networks of horizontal roots, known as rhizomes, that can extend as far as 60 feet, resulting in growth all along the expanse. Invasive Phragmites can outperform other plant species, including native Phragmites, reduce wildlife habitat and is a fire hazard in dry weather. A combination of herbicide application and mowing is the most effective control for Phragmites and is commonly used by coastal managers in Connecticut. While a large management concern, due to its prolific growth Phragmites may sequester more carbon and remove more nitrogen than native coastal marsh species.
Collected soil samples will be quantified for carbon mineralization, an index of how stable the carbon is, and denitrification potential, a measure of how much nitrogen is removed to the atmosphere. Biomass both above and below ground will be measured as those results can indicate important information about the health of the ecosystem and help assess management practices.
Next year, they will conduct a series of experiments further exploring the interconnectedness of elements in coastal wetlands to learn the effects on the efficacy of carbon and nitrogen ecosystem services.
“This summer, we’re gathering root and plant tissue samples, assessing sediment chemistry and completing microbial assays from wetlands. We plan to implement an experiment next summer manipulating hydrology, salinity and plant species to determine how these individual factors affect carbon and nitrogen cycling,” says Lawrence.
“The next step will be to integrate our data with sea level rise projections and expected shifts in vegetation to forecast how these shifts will alter ecosystem service provision in Long Island Sound coastal wetlands. We also aim to promote understanding of this system with diverse audiences. We will be working with high school teachers to develop a regionally relevant, inquiry-based outreach module that will highlight key concepts linking climate change and coastal ecosystems.”