08 June, 2021

Oyster Reefs and Salt Marsh Resilience

 On an island like Nantucket it’s hard lately to not read something about climate change, sea level rise, coastal resilience, erosion, etc. These buzz words can be heard all around the island from the flash flooding on Easy St to storm erosion near Hummock Pond. From south shore to harbor – Nantucket is starting to feel the impacts of climate change but we are also starting to figure out how to respond to this change.

At the Nantucket Conservation Foundation, we recognize the wealth of resources we have in our acres of permanently protected open space and in the past few years we have focused our coastal research on resilience, particularly in our salt marshes. Of approximately 1600 acres of salt marsh on Nantucket, NCF owns approximately 1200 acres, so we have a lot of area to work with!

Medouie Creek salt marsh in eastern Polpis Harbor, future home of a coastal resilience improving oyster reef?!

The Medouie Creek salt marsh in eastern Polpis Harbor has been the focus of over 15 years of restoration and research on salt marsh ecology from salt marsh restoration to spotted turtle habitat to salt marsh dieback and purple crab invasions. In 2020 we started preliminary research to see if the intertidal shore of Medouie would make good habitat for an oyster reef.

Why an oyster reef? There are so many amazing ecological reasons to have an oyster reef! One of the biggest being the potential to increase water quality. This summer we collected water samples to document how much nitrogen and phosphorus was in the intertidal flats near Medouie Creek, both to make sure oysters would be happy here but also track if and how a reef improves water quality over time.

An oyster reef can provide coastal resilience, decrease erosion, stabilize the shore, reduce wave impacts all while the oysters work to improve water quality!

Oyster reefs can slow down normal and moderate waves and storm surges, reducing shoreline erosion. Oyster reefs placed near a salt marsh can also help sediment drop out of the water and settle, building up soil and potentially allowing a salt marsh to migrate outwards over time!

Oyster reefs can potentially capture sediments, allowing salt marsh vegetation to migrate seaward and increasing resilience

In 2020, we began the work to find out if Medouie Creek would be a good spot to place an oyster reef and this involved the collection of ALOT of data, in partnership with the Town of Nantucket Natural Resources Department. Data like water depth over the whole summer is important, we don’t want oysters out of water for too long! How much light gets down through the water, how warm is it in the summer? Does anything else important live there?

Snorkel surveys with TON NRD showed very few other things living where we want to place a reef: bad for Polpis Harbor right now but very good for our plans to establish a reef! No eelgrass, shellfish or other aquatic plants were found in the reef area.

Seine netting sweeps found a few different fish, crabs and shrimp (so many shrimp!) but nothing unusual or unexpected.

One of the more exciting monitoring pieces was the placement of a tilt current meter courtesy of Lowell Instruments! Few oyster reef restoration project have actually documented how a reef can change tidal patterns in a near shore area. To demonstrate how a reef can improve coastal resilience near a salt marsh, we placed a tilt current meter out on the site and will leave it out after the reef is in place!

Now, it doesn’t look like much but this meter is constantly measuring water velocity – both direction and speed of water moving around it! The logger is buoyant and we anchor it to the harbor floor on a short, flexible tether. As water moves around the logger, it tilts in the flow direction and the logger can record tilt direction and angle, allowing us to calculation velocity. Larger storms and increased wind lead to increased velocity. If the oyster reef slows the velocity and associated erosion potential, this lovely meter will help us track that!

Now that NCF understands the current habitat near Medouie, we will begin the planning and permitting process and hope to install an oyster reef in the summer of 2021. Stay tuned!

Thank you to the Nantucket Shellfish Association for supporting purchase of pre-restoration monitoring equipment.

Questions? email Dr. Jen Karberg jkarberg@nantucketconservation.org

Harmful Algal Blooms on Nantucket

 As we dive into the warm steamy months of summer on Nantucket, we are also approaching the season of harmful algal blooms in our freshwater ponds. Algal blooms have made headlines in recent years as concerns have increased over harmful impacts to pets and humans. What should you look for when visiting a pond in the summer?

The first rule of thumb is to check the water before diving in or letting your pup dive in! Warm summer days provide ripe conditions for active algal blooms and erring of the side of caution is always encouraged.

Green-tinged Harmful algal bloom at the Head of Hummock Pond

To help, in 2020, the Town of Nantucket Natural Resources Department has organized an island-wide weekly monitoring program, partnering with the Nantucket Conservation Foundation, the Nantucket Land Bank and the Nantucket Land Council. Together we visit ponds around the island weekly, conducting visual surveys for HABs. When a HAB is seen, signs, like those below, are posted.

Look for these HAB warning signs around the island and proceed with caution!

The Town is also maintaining an informational website updated weekly with pond HAB status. Check in every Thursday for updated Pond Status!

Report a HAB! Have you visited a local pond lately and seen a green color in the water or blue scum near the shore? Make a report on the Town website and someone will check it out!

So – what is a HAB? Algae itself is a natural part of the ecosystem and the base of our freshwater pond food chains. But certain algae types thrive in nutrient rich, warm water and multiply rapidly, producing harmful algal bloom. Cyanobacteria or blue-green algae are responsible for the HABs seen in our freshwater ponds. These extensive blooms are harmful to the pond ecosystem because they use up mass amounts of oxygen in the water, decreasing health for fish and other aquatic organisms.

Some types of cyanobacteria produce toxins that can be dangerous for mammals including dogs and humans. Unfortunately, its very difficult to visually tell the difference between a toxin producing algal bloom and a non-toxin producing bloom. When a bloom is present on a pond – best advice is to stay out of the water!

Turquoise HAB at the southern end of Clark’s Cove

Conservation organizations on island have long been concerned with tracking the conditions that encourage algal blooms. The Nantucket Land Council, a long time advocate, has facilitated research in water quality and pond conditions around the island related to HABs. The Nantucket Biodiversity Initiative’s Aquatic Threats Committee provides even more information about local HABs and comes guides for identification. has pulled together education about harmful algal blooms on island.

So as summer progresses, keep your eye open for signs of algal blooms, keep you pups out of the ponds and report anything unusual! Check back weekly to the Town’s HAB website for updated information!

Restoring Salt Marsh through Human Crab Predation

 In many conservation and restoration projects, ecologists work hard to protect native things or bring native things back to a place. But this last year, Nantucket Conservation Foundation Staff acted as predators and aggressively removed a native crab from a few salt marshes in Polpis Harbor. Our prey? The purple marsh crab (Sesarma reticulatum).

The purple marsh crab (Sesarma reticulatum)

The purple marsh crab is a beautiful crab and native to Nantucket and other New England salt marshes. Unfortunately, purple crab population numbers have spiked in recent years and increased grazing on salt marshes grasses by these crabs has dramatically destroyed salt marshes.

Salt marsh dieback along a creek bank in Polpis Harbor, Nantucket

The term is salt marsh dieback and we’ve written about how devastating these purple marsh crabs can be for salt marsh health and even coastal resilience. With a reduction in natural predators, the purple crab population climbs and as the crabs eat the smooth cordgrass (Spartina alterniflora, the dominate low salt marsh grass), large areas of soil are laid bare with no plants or roots to hold that soil in place. This is risky for salt marsh resilience, causing increase erosion and loss of salt marsh to regular incoming tides and large storm events. And even riskier for coastal resilience as these valuable salt marshes are lost to erosion.

Salt marsh dieback as seen at Medouie Creek on Polpis Harbor, Nantucket (left) alongside a diagram showing the progression of salt marsh dieback and Sesarma impacts on marsh health (right).

While some areas in New England may be seeing a reduction in purple marsh crab populations, any potential recovery comes after over 10 years of extensive dieback and marsh erosion.

On Nantucket we want to jump ahead of this problem and reduce salt marsh dieback while protecting the marsh from erosion now. In 2019, NCF decided to become the predators in two salt marshes in Polpis Harbor, the area on island most dramatically impacted by salt marsh dieback.

Checking crab traps along the edge of salt marsh dieback in Polpis Harbor.

Tennis ball cans, sunk into the soil along the edge of smooth cordgrass dieback, act as crab traps and were placed throughout two marshes. These traps, checked 2-3 times a week, captured 439 purple marsh crabs (see the graph below). As an ecological bonus, we also trapped large numbers of the non-native, invasive European green crabs which were also removed from the marshes.

Total crabs trapped in 2019. Purple marsh crab (purple bars) and Asian green crabs (green bars) from May – October.

We trapped and removed a lot of crabs from the marshes, what does that mean for salt marsh health? Removing the crabs seemed to give the smooth cordgrass a chance to recover. By August, both salt marshes saw regrowth of the grass near the crab traps and even recolonizing in marsh areas bare for the last 5 years!

August 2019 – Smooth cordgrass regrowing in the middle of extensive salt marsh dieback at NCF’s Medouie Creek.

This isn’t a finished project though. Crab trapping will continue at both marshes in 2020 to determine if we really decreased crab populations and/or if they continue to migrate in from other salt marshes. Hopefully continuing to trap crabs will give the smooth cordgrass more of a chance to reestablish and stabilize the creek banks.

In 2020 we hope to plant out smooth cordgrass into the dieback areas to continue jump-starting the recovery process. Combining crab predation and vegetation restoration into the dieback areas, we hope to give the salt marsh a chance to recover and increase longer-term coastal resilience at these sites. We’ve just gotten back out in the marshes the last few weeks and so far – no purple crabs! It’s still early in the season but stay tuned!

Preparing Nantucket for Climate Change

 The impacts of climate change: increased sea level rise, increased occurrence of strong storm events, increased and more variable temperature and rainfall, temporal ecological disjuncts (ie plants flowering before their pollinators are around), and changes to natural communities (to name just a few impacts) all feel very immediate and important when you live on an island in the Atlantic Ocean.

Last week, I attended a week long conference in Portland ME: Gulf of Maine 2050 International Symposium with the purpose of:

bringing together leaders across the Gulf of Maine with perspectives on environment, economy, society and institutions to talk about climate resilience

Watersheds of the Gulf of Maine taken from the University of New Hampshire Gulf of Maine Watershed Information and Characterization System.

Did you know that Nantucket is mostly (but sometimes not!) considered a part of the Gulf of Maine? Many of the climate change issues facing island communities in Maine and Nova Scotia are just as important down here south of the Cape.

Researchers, politicians, students, policy makers and more met to talk about the issues facing our communities and what solutions are working thinking about Sea Level Rise, Ocean Acidification, and Ocean Temperatures. All of these coastal communities in the Gulf of Maine will be negatively impacted by Climate Change and everyone is looking for solutions today.

Why did we look at the year 2050? Because this is the year where the climate models diverge between moderate temperatures increases to extreme temperature increases (see the figure below). Under the various climate change models predicted in the IPCC (Intergovernmental Panel on Climate Change) reports, predicted climate temperatures begin to diverge dramatically by 2050 – the life of a mortgage today. In the figure below, continuing to live life with our current high fossil fuel emissions (business as usual) results in dramatic increases in temperature (the top pink line). Choosing now to make dramatic reductions in our use of fossil fuels (low emissions) is predicted to result in the blue line – an increase in average temperature over today but much reduced and hopefully a level we can adapt to.

So, 2050, the life of a mortgage today. How do we go about reaching that lower prediction? We have to make conscious and purposeful change today.

Rachael Cleetus from the Union of Concerned Scientists said that change “needs to be about the Science, needs to be about the Policy, and needs to touch people in their Hearts.”

On Nantucket, we see the need for change evidenced in increased flooding downtown and the erosion of our coastlines – our hearts have been touched. Nantucket Island has spent the last year and a half increasing our focus on climate change issues. Last January, Nantucket became one of the communities in Massachusetts to complete Municipal Vulnerability Planning. One of the outcomes of the MVP process was the creation of the Town’s Coastal Resiliency Advisory Committee (CRAC – I know!) and the hiring of a dedicated Coastal Resilience Advisory Coordinator. The government of the State of Massachusetts is moving towards making state 100% renewable energy by 2050 – so it’s time to move forward!

We have scientists on island who are researching the impacts of climate change on island and are making changes (and this is not an exhaustive list):

  • My salt marsh health and restoration research at the Nantucket Conservation Foundation
  • Storm surge and storm tide pathways analysis contracted through the Center for Coastal Studies
  • Keeping History Above Water and visualizing water rise in town by the Preservation Institute of Nantucket
  • Water quality and oyster restoration at the Town of Nantucket Natural Resources Department
  • Eelgrass restoration by the Nantucket Land Council
  • Natural park buffers being created by the Nantucket Islands Land Bank in Town
  • Ecological phenology by the Linda Loring Nature Foundation

We have businesses offering services based on that science and we have the Town-level groups working to make change.

  • ACK Smart solar installation on Nantucket and their renewable-energy, water-smart showroom in Town
  • Living shoreline installations by Wilkinson Ecological
  • Nantucket’s single use plastic ban #ACKWithoutPlastic

We need the policy that backs up the work of these scientists and entrepreneurs and helps implement the changes we know in our hearts need to happen. Change is hard no matter how much we know we need it. Well thought out, science-based policy can help us effect positive change. Another message from the conference that resonated is to move past policy that grandfathers in negative practices and behaviors:

Our grandfathers got us into this, we need to be thinking about our grandchildren.

Resources exist to learn about the science and the policies and steps other islands and coastal communities have already taken. Nantucket’s CRACommittee will be working on pulling all of this together while helping oversee a Coastal Resilience Plan for the island. The meetings are OPEN to the public and take place the 1st and 3rd Tuesday’s of each month, noon, upstairs in the Public Safety Building. Come learn, come speak, and come help make change.

What changes are you making to reduce your contribution to climate change? Let us know!

Resources: