This post belongs to a special series of posts written by students in Dr. Simon Engelhart’s Coastal Geologic Hazards course at the University of Rhode Island. In this course students learn about coastal processes, including storm surges and sea level rise, and how these impact people and the environment.
Narragansett Bay in Rhode Island is home to 3,600 acres of salt marsh. Salt marshes are coastal wetlands that are dictated by saltwater inundation from the changing tides. Those fortunate enough to live on a coast will tell you of their beauty and serenity. However, there is so much more to these ecosystems beyond their beauty. Often overlooked is the integral role salt marshes play as a transition boundary between saltwater and freshwater ecosystems. They act as a filter, taking up pollutants before they reach coastal waters. Additionally, they can act as a coastal buffer, limiting flooding during intense storm events. Want to know more about salt marshes? Find yourself in a salt marsh, and the answers literally lie at (and below!) your feet. Here is a short guide on how to see what is going on in this unique environment.
Let’s first look above ground. What species of plants are there? Where exactly are they located? Spending just five minutes walking around can give you an incredible amount of information about that marsh and how it functions on the day to day. See taller, shrub species? Those mark the upper edges of the marsh and only thrive above mean higher high water (MHHW). If you come across salt marsh aster, salt hay, or sea lavender, you know you’ve moved to the zone between mean high water (MHW) and MHHW. What do those water levels actually tell you? In most places, there are two high tides a day, with one being higher than the other. MHHW is the average of the higher daily tide. MHW, on the other hand, is the average of all the high tides. In other words, if you come across salt marsh hay, you are in an area that on average is inundated by a high tide once each day.
In addition to tidal information, you can also learn about the condition of the marsh. Notice a lot of Salicornia virginica, commonly known as pickleweed? Its presence can be a sign of a recent disturbance, like excessive flooding or sediment deposition caused by an extreme storm event. These disturbances will kill many salt marsh plants. Pickleweed is often one of the first species to grow back after a disturbance event, and as time goes on, the other salt marsh species will grow back in and outcompete the pickleweed, indicating that the system has not experienced any significant events in recent years. A lot of pickleweed means that there has been a recent disturbance and that the marsh isn’t fully healthy. You can also tell if the marsh is eroding somewhere nearby if you see ribbed mussels in the mid to high marsh. Ribbed mussels grow at the edge of the marsh, so seeing them out of place means the section of marsh the ribbed mussels were attached to broke off and was deposited further up on the marsh at high tide as part of a flooding event or storm.
If you have the right tools, you can also get information about the marsh’s history. Any of a variety of sediment corers will give you a vertical profile of the marsh. Sediment corers are hollow tubes that are twisted into the sediment in order to obtain a column of sediment. Since salt marsh sediments are deposited vertically, having a vertical profile allows you to identify distinct layers that indicate different times and conditions of deposition. At the Fox Hill Salt Marsh in Jamestown, RI for example, in parts of the marsh near the beach berm you can see alternating layers of gray, coarse, sandy sediment and brown, fine, organic sediment. In this case, it is thought that the gray sediment is evidence of hurricanes over-washing sand over the beach berm onto the marsh.
This isn’t to say that you can learn everything about a marsh simply by looking at it. Much more of the marsh’s health and history can be determined by taking samples back to a lab. For example, small organisms known as foraminifera and diatoms can be identified and, when combined with radiocarbon dating (determining the age of something using carbon isotopes), can be used to track sea level at the marsh’s location through time. Using technology that is usually reserved for hospitals, like a CT scan or X-ray, can reveal depositional layers in sediment cores that are invisible to the naked eye. A scientist can make a career out of studying one specific salt marsh. But if you stop and look around, salt marshes hold clues that can be deciphered by anyone. So head to your nearest salt marsh and go for a walk. Where is MHHW? Was there a recent disturbance? Bring a corer with you and learn about how the marsh grew to what it is today.
Casey Dannhauser is currently working towards a Masters of Oceanography at the University of Rhode Island. When she isn’t focusing on her schoolwork she can be found working to improve the coastal water quality in Cotuit, Massachusetts for the Barnstable Clean Water Coalition.
Michaela Cashman is an ORISE fellowship participant at the US EPA Atlantic Ecology Division in Narragansett, Rhode Island. She is concurrently working on a Ph.D. in microplastic detection and isolation in marine sediments. Michaela is interested in emerging contaminants, microplastics, remediation technologies, and hydrogeology. Her free time is spent constructing stained glass windows and advocating for her graduate student union, URI GAU.
Zane Grissett is in the final year of his undergraduate degree (B.S. Geology & Geological Oceanography) at the University of Rhode Island. He is currently studying the environmental effects of a 9-million-year-old meteorite impact in Argentina, but when he has some free time, you can usually find him surfing or spearfishing somewhere along the coast of Rhode Island.
Feature image: URI graduate student J Padgett explains the history of Fox Hill marsh recorded in the sediment below his feet to graduate students Casey Dannhauser, Jeeban Panthi and Dan Russell, and undergraduate Logan Thomas. Photo by Simon Engelhart.