Learning to Understand and Protect our Marshes: the Importance of Long-term Studies
It’s low tide, and as you look out over the expanse of grasses and tidal creeks of The Plum Island marshes and estuaries, you are likely to see wader-clad shapes slogging through the mud and bent over their rakes, collecting soft-shelled clams for dinner or market. These hardy souls are just the latest generation of fishermen engaged in one of many traditional uses of this natural resource. Or you might see another set of muddy individuals, out and about the marsh in all kinds of weather and carrying unusual tools and paraphernalia. These are members of a team of scientists, engaged in studying the marsh and estuary as part of the Plum Island Ecosystems Long Term Ecological Research program, or PIE-LTER.
A couple of those strange scientists out on the marsh
(Dr. Anne Giblin and author heading out to collect sediment cores)
The marshes and estuaries studied by PIE-LTER are part of The Great Marsh, the largest expanse of intertidal marsh in the Northeast. Estuaries are often described as areas where rivers meet the sea, and in the PIE-LTER case, those rivers are the Parker, Rowley, and Ipswich, all three of which flow into Plum Island Sound. The Sound then meets the ocean at the southern end of Plum Island, a barrier island that shields the shallow waters and marshes behind it from the full force of the ocean. In fact, the formation of the marshes and estuaries ~ 4000 years ago began only after the formation of the barrier island. Thus scientists classify the Plum Island estuaries as coastal plain, bar-built estuaries. They are further classified as macrotidal, meaning their circulation is dominated by tidal flow. PIE has two high and two low tides each day, spanning a tidal range of 3 meters, and resulting in well-mixed waters in the estuaries and tidal creeks.
StartFragmentA. PIE Marsh-Estuary
B. The three watersheds that feed into the marsh estuary
C. Location of PIE-LTER site in Massachusetts
The Great Marsh includes extensive areas of productive, tidal marshes. On the salty to brackish end, smooth cordgrass, (Spartina alterniflora) lines the creek banks and saltmarsh hay (S. patens), with its characteristic “cow-lick” appearance, is found on higher ground of the marsh platform. On the freshwater end, the salt marsh grasses give way to cattails (Typha) and sedges (Scirpus and Carex). The estuaries include numerous creeks and wide areas of mudflats, home to a diversity of small fish, shellfish, crabs, and other invertebrates that are key components of the marsh-estuarine food web. Seasonal inhabitants of the estuary like striped bass and American eel are also supported by this food web, and link the system to the coastal ocean. Similarly, the marsh-estuary is a critical habitat and food resource for migratory birds, as well as year-round inhabitants.
Rowley river, with Plum Island Sound and GOM in the distance; low tide shows mud flats;
evidence of people: house, boat, ditches on marsh
Rowley River at low tide, exposing mud flats, and with Plum Island Sound and the Gulf of Maine in the distance. Straight-line ditches, as well as houses and boats, are evidence of human use of the marsh-estuary.
Humans have also long used the resources of the marshes. Before European settlement, Native Americans hunted and fished the area for subsistence, and also engaged in some planting and fur trading. European settlers arrived at the PIE area in the early 1600s, and in addition to fishing and hunting, began a more systematic and widespread transformation of the landscape for agriculture. The high marsh provided a “ready-made” pasture for livestock, and its grasses were harvested as salt marsh hay, a lucrative crop. Although salt marsh haying has declined in recent years, the estuaries continue to provide harvests of shellfish and finfish, fueling local economies through both commercial and recreational fishing, most famously for the softshell clam (Mya arenaria). The Great Marsh is also a popular destination for birders, duck hunters, boaters, artists and photographers, and those who simply enjoy the natural beauty of the area.
Marsh-estuary ecosystems also do a number of things for humans that are not so obvious, but are increasingly important in the face of global climate change. They buffer uplands from the impact of severe storms and help prevent coastal flooding. They are extremely productive systems, taking up large quantities of carbon dioxide (CO2) and ultimately storing it in the ground as dead roots and rhizomes. In this way, they represent a disproportionate carbon sink for the land area they cover and are therefore important players in the global carbon cycle.
Setting up the tall eddy flux tower
Sam Kelsey, Inke Forbrich, and Anne Giblin setting up an instrument tower used to measure the balance between CO2 release and uptake by the marsh.
At the PIE-LTER site, scientists are working to understand how climate change and other human impacts act -- and interact-- to alter these marshes and estuaries. To do so requires a multi-year and multi-disciplinary approach, so a large team of scientists is involved, conducting research on topics ranging from nutrient cycling to fish migrations, and combining research approaches of long-term monitoring and experimentation.
The primary effect of climate change on the Plum Island marshes is expected to be sea-level rise and specifically the future rate of sea-level rise. Marshes maintain their surface elevation relative to sea level by accumulating sediment and organic matter at least as fast as sea level is rising. At the rate of sea-level rise the marshes were facing over the last 100 years, about 2.8 mm/yr, PIE marshes have been able to maintain themselves. However, that rate has increased to ~4.8 mm/yr, and some areas of the marsh are not gaining elevation equal to the higher rate. One reason is that there is very little sediment delivery to the marshes from the rivers because dams trap the sediment upstream. With limited sediment supply, will PIE marshes be able to keep up with sea level rise? To address this question, PIE-LTER scientists measure inputs of sediment to the marshes from the rivers and the coastal ocean. We also measure sources of sediment and organic matter that come from the marsh itself. In a process coined “marsh cannibalization,” we have found that some of the sediment eroded from the exposed edges of the marsh is re-deposited back onto the surface of the marsh. In addition, we are assessing how much the marsh grasses themselves contribute to building elevation by quantifying how much of the plant material (organic matter) they produce each growing season gets buried and stored in marsh soils.
Another aspect of climate change affecting the PIE system is rising ocean temperatures. The Gulf of Maine is experiencing more rapid temperature increases than 99% of the world’s oceans (Pershing et al. 2015), a trend implicated in the decline of cod and lobster fisheries. Warmer coastal waters may push cold-water species north, and they also open the door to new habitat for more southern species. Already we have documented a northerly range expansion of the fiddler crab into Plum Island marshes and further. Blue crabs also seem to be more common. We are watching for other species shifts, both into and out of the PIE system.
Climate change is also predicted to cause more extreme weather events in the GOM region. In fact, several storms with very high precipitation have occurred in the last 20 years, causing flooding in the watersheds of Plum Island Sound. Events such as these lead to large amounts of water running off paved surfaces and lawns in the ever-expanding development in the watersheds, carrying with it debris, fertilizers, and other pollutants that end up in streams, rivers, and ultimately the estuaries. On the other end of the spectrum, the area has recently experienced severe drought, with streams and ponds drying up and causing fish and other aquatic organisms to be isolated from their full habitat, or killed. Among these fish are species like river herring and alewives that normally return to the estuary after spawning upriver; changes in these populations could alter estuarine food web. PIE scientists study the effects of watershed events on the estuary in projects that monitor the transport and fate of water and other materials from the watershed to the estuaries (including sediments), and by studying food web dynamics in multiple habitats within the estuary and marshes.
With changes in sea level and freshwater inputs, estuaries may experience shifts in typical salinity distributions, particularly in the upper reaches of the estuary where fresh and salt water mix. These changes have implications for the organisms that live there, including the microbes. Though unseen, these microbes are fundamental to the functioning of the marshes and estuary. As a community, they possess an extraordinary range of metabolic capabilities, performing functions such as decomposition and denitrification, which also help buffer the coastal zone from inputs from land. Research at PIE has found that some of these functions are sensitive to salinity.
The large team of scientists conducting research in association with the PIE-LTER includes students at all levels. Students from regional middle and high schools are involved in ongoing field projects that conduct surveys of marsh plants and help monitor invasive species like purple loosestrife and Phragmites. Students also take part in field studies to assess the local effects of sea level rise, using a spotting level on area beaches to locate and flag predicted high tide lines. Hands-on activities like these impart real-life awareness about the function and vulnerabilities of our coastal systems, while also contributing to the overall knowledge base about them. (For more see Lauren Healey’s article in STUDENT FORUM).
The PIE marshes and estuaries are complex and dynamic, and that makes them exciting, challenging, and rewarding for scientists to study. The numerous benefits they provide to humans (so-called ecosystems services) make them important for society. In the face of climate change and other anthropogenic impacts, it is critical to understand how these systems function and protect them. PIE-LTER scientists seek to share what they learn and to learn from others, by partnering with local stakeholders and by involving students in our research. By doing so, we hope to contribute not only facts and figures about the marsh, but to impart an appreciation for the intrinsic value of these systems and the threats they are facing and to encourage and support public stewardship.
Ms. Tucker works at the Marine Biological Laboratory’s Ecosystem Center in Woods Hole, MA, the home institution of the PIE-LTER. She studies nutrient cycling in marine and estuarine sediments at PIE and other coastal systems. Her primary role is to analyze samples and process and synthesize data to determine rates and pathways of nitrogen cycling. Research conducted at the PIE-LTER is funded by the National Science Foundation.
Jane was always interested and curious about the natural world. She claims to have been a “nerdy kid” who asked for a microscope for Christmas but also loved to be outdoors. She had an aquarium as well as a variety of rescued pets, loved to identify seashells and tree leaves, and of course watched Jacques Cousteau and the Wild Kingdom. She grew up on the coast of NC, and loved the oceans and marshes and always wanted to know what and why. So for her, a career in science was the obvious path to take.