As a biogeochemist, Ashley Helton, assistant professor in the Department of Natural Resources and the Environment (NRE), is interested in how climate change and land development affect pollution in aquatic ecosystems.
She says, “What really fascinates me about biogeochemistry is that it has the potential to explain the underlying mechanisms of many big environmental issues we’re trying to address today.”
Helton is involved in several research projects that examine some of these issues. In a two-year project funded through Connecticut Sea Grant, Helton and master’s student April Doroski are investigating how sea level rise and wetland restoration efforts may alter the capacity of fresh water wetlands to process urban contamination. The study involves approximately thirty-five wetlands along several Connecticut coastline river systems.
Excess nitrogen and metals such as copper, lead and zinc are common pollutants that enter waterways from urban communities. Wetlands serve an important ecosystem service by retaining metals and removing nitrogen runoff through denitrification, a process in which microbes remove nitrogen from the water and release it to the atmosphere as nitrogen gas.
Helton says, “As sea levels rise, many fresh water and brackish wetlands experience elevated salinity, and we think it may affect the way wetlands process these urban contaminants.”
Salinity levels in freshwater and brackish coastal wetlands are increasing with sea level rise. The microbes involved in denitrification may be sensitive to this increase in salinity, thereby reducing their capacity to remove nitrogen. Additionally, an increase in salinity reduces the capacity of wetland soils to bind and retain nitrogen and metal ions.
“Denitrification is the only process that permanently removes nitrogen so it’s not transported to sensitive coastal areas such as Long Island Sound, where excess nitrogen contributes to dead zones,” Helton explains. Dead zones are areas in the sound unable to support marine life.
Wetland restoration projects in Connecticut have been very successful. The Connecticut Department of Energy and Environmental Protection’s coast habitat restoration program has restored more than 1700 acres of tidal wetlands at approximately forty sites in the past twenty-five years, and the US EPA Long Island Sound Study’s Habitat Restoration Initiative has worked with a variety of federal, state, municipal, nonprofit and private sources to fund more than fifty-six wetland restorations. However, their primary focus has been on habitat improvements, restoring tidal hydrology and improving fish passage, as well as combating invasive plant species. Less is known about microbial processing of nitrogen and contaminant retention and how sea level rise will affect these important ecosystem functions. This research could be important when developing strategies for future wetland restoration projects.
In a project funded through the Connecticut Institute for Water Resources (IWR), Helton and collaborator Tracy Rittenhouse, assistant professor in NRE, are studying ephemeral wetlands, wetlands that have standing water during only part of the year. Ephemeral wetlands provide habitat to many sensitive amphibian species and play important roles in the cycling of carbon and nitrogen on the forest floor. The project is unique because four undergraduates manage the research.
The students are examining the effects of road salt runoff on ephemeral wetland ecosystems. The use of road salt in the United States to melt snow and ice during the winter roughly tripled from 1975 to 2005. The team is measuring water quality, greenhouse gas emissions and denitrification and using amphibian egg mass counts to determine how road salt affects the functioning of the ephemeral wetlands. “Our hope is to directly quantify linkages between road salt use and ephemeral wetland ecosystem function, which has implications for managing how road salts are applied near these protected ecosystems,” Helton points out.
These headwater streams play an important role in transporting nutrients and other contaminants from the landscape to larger river and coastal ecosystems. “Nitrogen export from streams and rivers to coastal areas, including Long Island Sound, is the primary cause of seasonal hypoxia, or dead zones,” Helton says. “Nonpoint sources of nitrogen, particularly during storm events, can contribute substantially to excessive nitrogen loads, but predicting where and when this occurs from landscapes is difficult.”
Climate change will increase intense rainfall events and associated runoff in New England. This project focuses on measuring nutrient fluxes in streams during storms of various sizes to understand how future changes in storm timing and intensity may affect nutrient flux in headwater streams.
In addition to climate change, changes in land use related to urban development can also affect the patterns of surface runoff to stream ecosystems. Helton is collecting storm datasets from five streams that vary in the amount of urban land use in their watersheds.
The goal of the project is to more fully understand the flux of nutrient pollution to streams and rivers as both land development and storm intensity increase. “These results will be relevant for future planning of storm water management under a changing climate that includes more intense storm events,” Helton says.
Helton is part of a collaborative project recently funded through the National Science Foundation. The five-year, $4.48 million grant is designed to fund research focused on quantifying and predicting the rates and timing of whole stream ecosystem metabolism. The research will provide a more accurate assessment of the role of fresh waters in global carbon cycling and increased understanding of how carbon resources for aquatic food webs are affected by both climate and land use change.
Helton will be partnering with researchers from Duke University, University of Florida, Arizona State University, University of Wyoming, University of New Hampshire, University of Wisconsin and the United States Geological Survey.
To determine whole stream metabolism, sensors are placed within streams to continuously record oxygen and carbon dioxide concentrations. Plant and microbial life in a stream both consume and produce oxygen and carbon dioxide, and the difference between oxygen consumption and carbon dioxide production is whole stream metabolism. Thus, whole stream metabolism is analogous to metabolism within an individual organism. It represents how energy is created and used within an aquatic ecosystem.
Stream metabolic rates are tremendously variable in time due to frequent droughts or storms that may remove organic matter from the stream bed or due to changes in the supply of light, nutrients or other resources. By measuring and modeling patterns of daily, seasonal and disturbance recovery trajectories across many streams, Helton and her colleagues will be able to detect and predict the response of stream ecosystems to disturbances.
For her part in the study, Helton will develop predictive models for six study regions to extrapolate the team’s findings from individual streams to whole stream networks.
“We are developing methods and models to estimate stream recovery trajectories under different disturbances such as storms, drought and land use changes,” Helton says. “We can often measure how a particular stream responds to a specific disturbance, but our goal is to develop a framework to predict how entire stream networks will respond to changes over time. Then we can make informed decisions on a broader scale.”