After sea levels retreated about 4,000 years ago the exposed wetlands provided a new habitat for biodiversity. Recently, much effort has been made to protect the remaining coastal marshes in North America.
If the goal of that protection is for example, maintaining barriers to storm surges and providing wildlife refuges, then understanding how the marshes were formed and maintained is critical. If the goal is to return the land to it’s “natural” state, then the history of the marshes, at least for some, may come as a surprise.
Logging of forests in the 1700s and 1800s may have been responsible for creating some of the coastal wetlands in New England, notes Matthew Kirwan of the U.S. Geological Survey.
Writing in the journal Geology, Kirwan and a team of environmental scientists from Duke, Woods Hole Oceanographic Institution, and East Carolina University, describe radiocarbon-dated fossil plant rhizomes, or rootstocks, from Plum Island Estuary in Massachusetts. The oldest rootstocks from around the marsh’s fringes, dated back 4,000 years, fitting the theory of coastal marsh development following sea level retreat. But the most recent, in the middle of the marsh, were just 200 years old.
The researchers note that the area was settled in the 1630s and that by 1700 grist mills and sawmills were commonplace. They further note that, as recently as 1830, a local map identified a large area now covered by marsh as open water named Shad Creek Bay.
Kirwan and co-authors postulate that massive logging by settlers and subsequent generations led to a substantial increase in sedimentation in the bay area, creating fertile ground for marshes to become established. Agricultural development, mining, and logging, are well known examples of upstream land use that influence sediment deposit rates along the coast. Chesapeake Bay, San Francisco Bay, Newport River in North Carolina, and even the Firth of Thames in New Zealand are just a few examples of coastal cities that have changed their waterfront views as a result of upstream activity over the course of the 19th century.
With this better understanding of the history of the Plum Island Estuary marsh lands, what should be done next?
The authors stress that this specific site’s history may be more common among other North American wetlands as well and that “widespread efforts to restore valuable coastal wetlands actually prevent some systems from returning to a natural state.” Their conclusion creates something of a conundrum, given the tremendous ecosystem services such wetlands provide.
But regardless of how they got there, coastal wetlands are still surviving in much of the United States despite the fact that sediment loads have more recently decreased as a result of changes in agricultural practices, reforestation, and the construction of reservoirs. Kirwan and his colleagues argue that this is because, as sediment deposits decline, water inundation increases, prompting growth of vegetation and enabling the marshes to live in a “metastable equilibrium.”
That equilibrium, however, can be – and, write Kirwan and colleagues, is being – disrupted. Without sediment input, coastal wetlands cannot rebuild once they are lost, and the combined effect of changing land-use practices and sea level rise means that some wetlands (for example in coastal Louisiana) are submerging rapidly. Which means evaluation of what the best land use practice should be for these areas needs stronger consideration.
Photograph: Wetlands in Cape May, New Jersey, by Anthony Bley, U.S. Army Corps of Engineers