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Duck Harbor

    MARSHES

    Scientists Measure an Ecological Transition at Duck Harbor

    How the ‘overwashes’ preview the restoration of the salt marsh at the Herring River

    By

    WELLFLEET — Duck Harbor, enjoyed by visitors as a bayside beach, is returning to its 19th-century roots as a salt marsh, according to Tim Smith, a restoration ecologist at the Cape Cod National Seashore who is studying the ecological transition here. And Smith doesn’t mean “roots” as a metaphor: the seeds of saltwater plants that have lay dormant for more than 100 years are sprouting there, including glasswort, seablite, and seaside orach, he said.

    Tommy Tucker, a marine fisheries research assistant with the Center for Coastal Studies, lies on the ground after attempting to retrieve a particularly difficult sediment core in Duck Harbor last year. A colleague, geologist Katie Castagno, stands nearby. (Photo by Jim Lynch)

    After seawater repeatedly breached a coastal dune and poured into the freshwater confines of Duck Harbor in 2021, trees behind the bank turned gray and died. With those so-called overwash events continuing since then, bursts of green saltmarsh plants have started to emerge — near the beach at first, but then farther and farther upstream.

    These overwashes are not purely a function of rising seas but involve other coastal processes including sand movement, according to Katie Castagno, a geologist and director of the Land-Sea Interaction Program at the Center for Coastal Studies who is also studying the evolution of Duck Harbor. Winter storms could bring enough sand to fill the saltwater pathway inland, she said, which would put a stop to the intrusions, at least for a while.

    In the meantime, research on the ecological transition at Duck Harbor can, according to Smith, provide a preview of “what the whole Herring River system will do once tides are restored.”

    The decades-long $70-million Herring River Restoration Project, a collaboration between the town and the National Park Service, involves the replacement of a dike that blocks salt water from the mouth of the Herring River with a bridge and gates that allow for the gradual restoration of tidal flow there. The return of salt water to the estuary means a transition in the kinds of vegetation and wildlife that will thrive in the area.

    Tommy Tucker pulls a ground-penetrating radar transmitter across Duck Harbor to generate images of underground sediment layers. (Photo by Katie Castagno)

    Duck Harbor and the Herring River are part of the same system, Smith said, so what happens at Duck Harbor can help predict what will occur in the restored estuary. “The data we’re collecting are all related,” he said.

    Unlike the larger restoration project, the Duck Harbor overwashes were not deliberate. The U.S. Fish and Wildlife Service did, however, spend $2 million to have 125 acres of dead trees in Duck Harbor turned into mulch in 2023 to help facilitate the transition to saltwater species.

    Soil and Species

    Castagno said her team from the Center for Coastal Studies is taking sediment samples in hopes of learning how the area has changed “over potentially thousands of years” — a timeframe that should soon be nailed down by lab tests on their oldest samples.

    “We were taking sediment cores all last summer,” said Castagno. To get them, her team had to plunge tubes deep into the muck to bring up what lies beneath: layers of peat, clay, silt, and sand that tell researchers about the continuing flux of landscapes and ecosystems here. Her team also used ground-penetrating radar to generate images of underground sediment layers.

    The cores don’t only describe the past, Castagno said. “If we’re anticipating erosion in different areas” from sea-level rise, “it will be useful to know what our substrate is that might erode,” she said.

    Park Service research in Duck Harbor has included surveys of fish and plant species and measurements of changing soil chemistry. “Last summer we did fish sampling,” said Smith, “trying to quantify the composition and distribution of fish species” in the brackish waters.

    The Park Service also established 40 one-meter-square plots in Duck Harbor to record the plant species that are appearing and disappearing in them. Smith is measuring the geochemical response of soils as they are rewetted with salt water.

    So far, Duck Harbor is undergoing a “textbook” saltmarsh recovery, Smith said — in the first year after the overwash, salt-tolerant species such as spear saltbush and pickleweed began to flourish.

    “When the salt water comes in, it kills these other plants, and it gives these plants a space to survive and to spread,” Smith said. Over the long term, these herbaceous annuals do not typically fill a mature salt marsh, however; on the Outer Cape, that niche belongs to grasses such as the intensely green spartina grass.

    Researchers are beginning to see sparse patches of that grass in Duck Harbor, Smith said, indicating that the next stage of the transition to a salt marsh has already begun.

    Knowledge about Duck Harbor’s transition has relevance beyond Cape Cod, Smith said. There are many places where humans have restricted tidal flow around or in wetlands, whether with dikes and berms or simply with roads, and people involved in their restoration will be watching this project closely.

    The freshwater marshes that often result from the construction of dikes as happened here and other human-made development will be increasingly vulnerable as sea levels and high-tide lines rise, Smith said. Saltwater inundations, and therefore saltmarsh transitions, will likely become more common, whether they happen as part of managed processes or on their own.

    BLOODSUCKERS

    Winter Floods Set the Stage for More Mosquitoes

    Flood waters abate and larvicide is sprayed, but a lot depends on the weather

    By

    WELLFLEET — Barton Morris stands in the Duck Harbor parking lot and surveys the floodplain to the north. It’s a cool April day, and it’s been about two weeks since the last in a series of strong storms hit this beach. The storms pushed salt water over the dunes into this floodplain, creating puddles in the field of newly cut wood chips.

    Barton Morris, assistant superintendent at the Cape Cod Mosquito Control Project, sprays stagnant puddles near Merrick Island in Wellfleet. (Photo by William von Herff)

    Morris is here to investigate those puddles — or, more accurately, what’s in them. He puts on his boots, gets out his dipper — a low-tech cup on the end of a pole — and marches out into the muddy plain to look for mosquito larvae.

    Wellfleet is wary about the bloodsuckers. In a January 2021 storm, the coastal bank at Duck Harbor was overwashed, storm waves pushing salt water into the pine forest here, killing it. Then, the water pooled, trapped on one side by the bank and on the other side by mosquito ditches dug long ago to keep water flowing but long since abandoned and clogged. These pools of stagnant water were the perfect breeding ground for mosquitoes.

    In the summer that followed, the mosquito population in Wellfleet exploded to unprecedented levels. Morris, who is the assistant superintendent of the quasi-governmental Cape Cod Mosquito Control Project (CCMCP), said that traps in the area were capturing thousands of mosquitoes every day in summer 2021 (the one-day record was 5,642), when any number more than 200 is considered a problem by the agency.

    Fortunately, Morris says, Duck Harbor isn’t much of an area of concern anymore. The trees have been felled, the ditches are clear, and the gap in the dunes has widened, allowing water to flow out more easily. Plus, he notes, the larger puddles have fish in them, and they eat mosquito larvae.

    He lowers his dipper into a few puddles, and they all come up empty. Besides a small second overwash creating a new stagnant wetland south of the parking lot, which Morris says could pose a problem without regular treatment, Duck Harbor is now a secondary concern. Instead of working here, he packs up his gear and heads inland, where the real problems lie.

     

    New Floods at Bound Brook

    All winter and spring, regular flooding has pushed salt water farther up the Herring River, inundating roads and often cutting off residents of Bound Brook Island from the rest of town. These floods have mostly been driven by strong storms out of the east coinciding with full or new Moons when high tides are higher than usual.

    While the roads are clear for now and the town and Cape Cod National Seashore are working on plans to ensure the roads stay clear though the summer, CCMCP is worrying about the fact that the floods have created perfect mosquito breeding grounds — pockets of stagnant salt water much farther inland than ever before. These are particularly prevalent around the confluence of the Herring River and Bound Brook near the Atwood-Higgins House and around High Toss Road, Morris said.

    If we get lucky and have a dry, storm-free spring, the flood water could all dry up before too many mosquitoes hatch. But both Morris and his boss, Gabrielle Sakolsky, the superintendent of CCMCP, said the conditions indicate another big outbreak of mosquitoes is possible this summer.

    If this flood water sticks around, the saltwater mosquitoes getting a boost are active during the day, according to Morris, making them hard to avoid. They can also fly upwards of five miles from where they hatch, meaning an outbreak along the Herring River could affect a large area from Truro to South Wellfleet.

    Morris parks his truck along Bound Brook Island Road and walks up a trail near Merrick Island. The forest is full of puddles, some nearly knee-deep. “Until Duck Harbor, there was never really water here,” he says. He sticks his dipper into the first puddle he sees and scoops up a cupful of water. In the tea-brown water are squirming mosquito larvae. Morris returns to his truck, where he fills a plastic backpack connected to a hose with a brown, milky liquid — a larvicide.

    During the winter, CCMCP crews are mostly focused on water management, digging and clearing ditches to allow stagnant water to drain. Starting in mid-April, they shift their control measures to larvicide application. They use a strain of Bti (Bacillus thuringiensis subspecies israelensis), which contains spores that produce toxins that specifically target mosquitoes. Bti is found naturally in Cape Cod soil and is harmless to the vast majority of animals, including humans: besides mosquitoes, it kills only a handful of other fly larvae. Morris doesn’t wear protective gear while applying it.

    Morris begins wading through the puddles, spraying larvicide in each one. Across the Cape, 10 crews of CCMCP workers apply larvicide this way, spraying each water source listed on a map of thousands of mosquito breeding sites. They repeat their applications regularly, as the larvicide wears off after a few days. Morris, however, has mostly been out here by himself. The crews are tied up with their regular routes, so the duty of spraying the flood zone has fallen almost entirely to him. Puddle by puddle, he empties his four-gallon backpack into the marsh before returning to his truck.

    Even here, relatively close to the road, there are areas Morris can’t reach. He points across a channel to a thorny, overgrown patch of brush in the marsh. He has no idea whether there is water pooling there, he said, and the brush is so dense it would be extremely difficult to check.

    “I was hoping they would clear that,” he said, referring to the brush clearing that the Herring River Restoration Project has been doing in the area.

     

    Much Depends on the Weather

    In the long term, the Herring River Restoration Project will allow salt water to drain from this area more easily, Morris said, hopefully making mosquitoes less of a concern. But for this year, the big unknown is whether flooding will happen again, and if not, whether the current flood water will manage to drain from the system before the mosquito population explodes.

    Sakolsky said there’s reason to be hopeful: the water levels in the area are already going down, and many puddles are drying up; since the last storm in early April, there has been no further water accumulation. Also, our spring has been cold so far, which slows mosquito development. The forecast for early May looks dry, too.

    Sakolsky said her team will “just have to be regularly on it, checking in, and treating what we can get to.” As the summer heats up and mosquitoes develop more rapidly, the team will apply larvicide to this area more regularly. Morris said he will likely bring in additional workers then.

    A lot depends on the weather over the next few months. Sakolsky said she is hoping for limited rainfall and calm weather during the kinds of high Moon tides that caused the flooding previously.

    “Nice and cool, no rain, no storms,” she said. “I could use a little bit of that.”

    ENVIRONMENT

    Salt Hay Rebounds at Duck Harbor

    By

    WELLFLEET — As you drive down Griffins Island Road toward Duck Harbor, your eyes are drawn to what looks at first like a treeless wasteland.

    Spartina patens, commonly known as saltmarsh hay, makes a comeback behind Duck Harbor where two storm overwashes flooded the area with salt water. It grows at mid-tide to high-tide elevations in undisturbed salt marshes, and besides providing habitat for estuarine animals, it stores lots of carbon in its roots and rhizomes underground. (Photo by Benjamin Siegel)

    On closer inspection, you notice masses of green vegetation sprouting. “That’s all pioneering saltmarsh species,” says Geoff Sanders, chief of natural resource management and science at the Cape Cod National Seashore. Glasswort, saltmarsh spikegrass, salt hay, and sea blite are among the plants making a comeback, he says.

    The freshwater vegetation that had colonized the Duck Harbor basin after the Herring River was diked in 1909 was almost entirely wiped out when salt water rushed in during storm tide overwashes in 2020 and 2021.

    After the removal of 80 acres of that dead vegetation this past winter, the saltwater plants restoration experts had hoped would flourish here have made an impressive rebound. With regular saltwater flooding, the western portion of Duck Harbor should revert to salt-marsh grasses, says John Portnoy, co-chair of the science advisory group at Friends of Herring River. The eastern portion is expected to receive less water until the later stages of the Herring River Restoration Project.

    Dale Rheault, chair of Friends of Herring River, says that although the removal of vegetation at Duck Harbor will enhance the work of the Herring River Restoration Project, the timing is just a coincidence.

    For scientists working on the restoration, the Duck Harbor recolonization has provided an opportunity for experiments with transplantation and seeding. Sanders says that Park ecologists have transplanted plugs of saltmarsh cordgrass without great success, so now they tentatively plan to reseed the Duck Harbor basin after 40 more acres of vegetation are removed in November.

    Since the groundbreaking ceremony on March 1, the restoration timeline has held steady, according to Sanders. In March, two houses on Way 672 in Wellfleet that the Seashore acquired in 2017 and 2019 to facilitate the restoration were taken down due to concerns about their vulnerability in the face of tidal flow.

    The MIG Corp. of Acton, the contractor hired by the town of Wellfleet, is expected to complete a temporary bypass bridge next to the dike by October. Once the bypass bridge, a single lane with a signal on either end, is done, people will be able to access Griffin Island, Duck Harbor, and Great Island while work on a larger bridge takes place.

    Jacqualyn Fouse, who lives in the last house on the right as you near the dike, says she has not been affected by the construction. “I think they are doing a really great job not being very disruptive to everything out here,” she says. Fouse is the treasurer for Friends of Herring River.

    In this first phase of the project to restore tidal flow in the river, low-lying roads will be raised, culverts will be replaced, a few freshwater wells will be relocated, and the Chequessett Club golf course will be elevated.

    The Park Service is also planning the removal of soil berms that were created when the river was channelized decades ago, but the timeframe for that work hasn’t been determined.

    Once the larger bridge is completed, 165 feet of tidal sluice gates will be opened a bit to increase tidal flow in the system. Using adaptive management techniques, scientists will begin by letting in small amounts of water. That’s when, Sanders says, “you will start to see changes in vegetation. You will start to see improved fish passage.”

    Currently, the marsh edge is artificially about a meter below sea level because of the diking; the sediment has subsided due to limited tidal flow. With the restoration of the flow, sediment will move into the area and rebuild the marsh.

    At that point, Rheault says, the sluice gates “will not be opened any further for at least two years.” That way, “scientists can monitor how tidal flow impacts the estuary.”

    Ronald Gabel, a Yarmouth resident who is among those who have been critical of the project, says he remains wary of it. Gabel and others say the project may have significant unintended consequences that Wellfleet taxpayers could be held liable for.

    Sanders believes that “there really is a preponderance of evidence that the restoration is going to be a huge benefit, and it’s not going to result in a lot of these issues people are worried about.”

    Portnoy agrees, adding that “the Herring River Project will protect upland shoreline development around the marsh edge.” As saltwater grasses recolonize, he says, they impose significant friction on water flow, which should help protect against the higher storm surges that are likely to be increasingly frequent with rising sea levels.

    “A lot of work, science, research, and monitoring has gone into preparing for this,” says Sanders. “If, by some chance, something unexpected happens, you can always manage the water level with the tide gates.”

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