“One of the most iconic sights associated with salinization and sea level rise are ‘ghost forests’”
–Dr. Ryan Emanuel, North Carolina State University
The coastal plain of the southern United States is already seeing the effects of changing climate, but not in the way that may first come to mind. While warming temperatures and rising sea levels will lead to flooding and loss of land, with particularly dire outcomes for low-lying states in the US, such as Louisiana, Florida, and North Carolina, gradual sea-level rise is already altering our coast lines through the saltwater intrusion into freshwater-dominated systems.
Saltwater intrusion is when ocean water moves inland, either over land or through the soil. As sea levels rise, ocean water can push further inland, either gradually or quickly in the case of a hurricane or storm. This can affect groundwater resources, and it can also lead to soil salinization – or saltier soil. For forests and agricultural regions in low-lying areas, increased soil salinization results in soil and water quality degradation and a loss in soil nutrients. This leads to decreased productivity, habitat loss, and degradation.
Saltwater and the North Carolina Coast is an NSF SEES Collaborative project between the University of North Carolina, North Carolina State University, Duke University, and East Carolina University seeks to understand the complex interactions of hydrology, climate, land-use, and ecological processes associated with saltwater intrusion along the Albemarle-Pamlico peninsula in eastern North Carolina. To learn more about this project and about the impacts of saltwater intrusion on ecosystems, I spoke to one of the Principal Investigators, Dr. Ryan Emanuel, Associate Professor of Forestry and Environmental Resources at North Carolina State University.
Q: How does salt water intrusion impact those living on the coast?
The direct impacts range from permanent loss of their lands, to reduced crop productivity due to soil salinization. These are visible and tangible changes to coastal regions in this part of the world. One of the most iconic sights associated with salinization and sea level rise are “ghost forests”—stands of snags that often fringe estuarine coastlines around here. These were former upland forests, often pine-dominated, inundated by saltwater or exposed to chronically salty soil water.
We’re not studying saltwater intrusion into groundwater. That is a related, but an already well-known issue. The hydrogeological processes associated with groundwater salinization are already well known and linked to sea level rise and human groundwater withdrawal. Coastal North Carolina has issues with groundwater salinization, but we chose to focus on the less well-understood issues surrounding salinization of freshwater-dependent ecosystems. We also chose to focus on freshwater-dependent ecosystems rather than salt marsh accretion or loss, because this is another well-studied problem with existing models and tools for decision-makers.
Q: What are the indirect effects from saltwater intrusion?
There’s a gradation between direct and indirect impacts, but I guess you could say that one indirect impact would include loss of wildlife habitat. Coastal North Carolina lies along the Atlantic Flyway for migratory birds, and this is a concern for those associated with hunting, birdwatching, and related tourism. Another impact might be the release of nutrients to the estuary as seasalts spur the release of nutrients from formerly-agricultural soils. Marcelo Ardon and Emily Bernhardt, who are co-PI’s on the project, have a nice 2013 paper in Global Change Biology on that topic.
Q: Which areas are most vulnerable to saltwater intrusion?
The lowest elevations are obviously highly vulnerable, but we have a more subtle hypothesis that supposes freshwater, falling as precipitation in the interior, acts as a buffer to saltwater intrusion. Any concentrated flows of freshwater on the landscape such as natural drainages or artificial outfall canals, have the ability, during wet periods, to keep saltwater at bay. No pun intended! On the other hand, during dry periods, these same drainages and canals can serve as short circuits to bring saltwater deep into the interior of the peninsula. In this respect, we think terrain, even very subtle terrain on the peninsula, has the potential to influence vulnerability in more ways than simply “lower is more vulnerable.”
However, the role of terrain is also mediated by climate, since drainage patterns can serve opposite functions during wet or dry periods. Working independently, two groups of us, Bernhardt and Ardon, along with myself, have observed this anecdotally over the years, and discussions about this evolved into our current project. The Albemarle-Pamlico is a good place to study this concept of vulnerability, because it contains numerous artificial drainages of various sizes and configurations, those with free flowing versus one-way valves, plus a few areas of relatively natural drainage.
Q: How widespread is saltwater intrusion?
In North Carolina alone, it affects hundreds of kilometers of estuarine coastline, and part of our work aims to determine just how vulnerable the entire 6,000 square kilometer Albemarle-Pamlico Peninsula is. Depending on trajectories of sea level rise, a significant fraction of the peninsula will be permanently inundated by the end of this century; but in the meantime, we want to understand the processes associated with this slow transition from freshwater-dependent to salty, and we want to understand human dimensions as well. How do humans influence this process by managing water on their working land and will they respond to gradually saltier water moving into their working lands. This is an issue common to coastlines, especially on passive margins, with broad, flat coastal plains, like the southeastern US.
Q: How did you become interested in this project?
As I mentioned before, some of us were working on the peninsula already, independently, on focused topics like wetland restoration and forest die-off. We saw the effects of salinization firsthand, and we were curious about the regional picture, especially about how these processes scale and manifest over thousands of square kilometers. And in reality, this is only part of a larger narrative of sea level rise in the southeastern US and throughout the world. This is a big deal, and it’s not just about whose feet are wet in 2100. Salinization is a climate change impact, and it’s already here.
Moreover, as a native North Carolinian, I’m enamored by the radical diversity of natural environments across the state. The outer coastal plain, and the Albemarle-Pamlico in particular, house some truly unique ecosystems, including freshwater-dominated wetlands like pocosins and Carolina Bays and our expansive but almost absurdly shallow sounds. The peninsula is a very special place, but it’s also at the forefront of climate change.
Finally, I couldn’t pass up a chance to work with such a great team. Bernhardt and Wright are well established biologists at Duke, Todd BenDor is a talented social scientist from UNC, and Ardon is a rising star who already has a great record of research in this region. I’ve also engaged additional faculty here at NCSU including social scientists, Erin Seekamp, Sudipta Dasmohapatra and Jordan Smith and a wildlife scientist, Chris Moorman. We received an internal grant to look at adaptation strategies related to tourism, bird habitat, and forest product markets. Abi Bhattachan serves as the postdoc who helps me to bridge these two teams—the Coastal SEES and NCSU-SALT.
Q: What do you hope the most significant contribution of this work will be and where would you like to see it go?
This work can change the way we think about and prepare for the consequences of sea level rise. Tons of work exists on this topic for developed shorelines for obvious reasons. More people, more money. But much less attention has been paid to rural areas, particularly in broad, flat coastal plains like the southeastern US.
I would like to see our work inform decision-making on the Albemarle-Pamlico Peninsula and in similar regions. There are difficult decisions on the horizon about infrastructure and land-use, and I think conversations need to happen now—locally, state-wide and nationally—that consider rural lands. I hope that our work brings this issue into sharper focus by revealing ecological process insight and a refined view of human interactions. Both at scales relevant to decision-making.
I also hope our work can help shift thinking about sea level rise, generally, from a discrete increment at some future date to a gradual and continual process. People who experience sea level rise understand the gradual nature of it, I think, but I’m not sure that people in other places think about the continuity of this process. We wanted to advance discussions of sea level rise beyond a story about “X-cm of inundation by the year Y” and think about salinization as a process that occurs gradually, during a timescale corresponding to a human life. The end result may be inexorable, but there are many pathways by which we arrive there, and the ecological outcomes associated with those pathways could be very different.
Q: How can people get involved?
We are already engaging stakeholders including farmers, land managers, government agencies located on the peninsula. For now, others can stay tuned to the Coastal SEES website. As the project moves along, we’ll post updates there and, hopefully, include information for potential collaborators, education partners, and the general public.