Crowley Island’s salt marshes can migrate inland as sea levels rise

Climate Change and Sea Level Rise

Although the causes may be disputed it is a fact that the entire world is in a period of warming climates. In Maine the direct consequence is that we are experiencing increasing sea water temperatures in the Gulf of Maine. Some scientists claim that the Gulf’s waters are heating up faster than 99 per cent of the world’s oceans. Even if that is an overstatement Maine’s fishermen are already noting the effects in the changing distribution and abundance of lobsters and in a reduction in mussel populations along the Downeast coast.

A major result of rising temperatures is that glaciers and polar ice caps are melting and sea levels are rising worldwide. It’s easy to comprehend the often dramatic effects of rising sea levels when seeing pictures of flooding in Venice’s Piazza San Marco, or even the flooding that takes place in southern Maine towns during major storms.

While it may be possible to slow down the rate of global warming by addressing some of the man-made causes this will not stop the trend of rising sea levels anytime soon so we need to figure out how to live with the results. Venice is building structures to keep out the sea water. Other coastal cities will need to follow suit. Eventually, people who have summer cottages or year-round residences built close to the shore will have to move or fortify their property.

The last Pleistocene glaciers retreated from Maine’s coast over 13,000 years ago and extensive coastal salt marshes began to be established. During the most recent 6-7,000 years Maine has experienced a period of gradually rising sea levels at the rate of around four inches per century. According to water levels are currently rising at the rate of one inch every 8 years, about three times as fast. Climatologists and oceanographers forecast even more rapid rises over the next 100 years.

As the waters rise, not only cities, but our shorelines, tidal flats and salt marshes are being threatened with inundation. This is important because the health of the entire Gulf of Maine is dependent on preserving the photosynthetic productivity of salt marshes. As with Rockweed, the detrital contribution of organic nutrients produced photosynthetically and shed annually by salt marshes is at the base of the marine food web and healthy estuarine wetlands are essential to local marine resource based livelihoods like commercial fishing, clamming, worming and lobstering. Marshes and their adjacent mudflats are also critical to the life cycles of many shorebird and waterfowl species as essential seasonal foraging and roosting areas.

Sadly, ever since the colonial period we have seen the steady loss of Maine’s marshes and flats through drainage and diking to manage for salt hay, build airports or for commercial development. On the positive side coastal wetlands have long been a conservation focus of PRWF, other coastal Maine land trusts and of the USFWS and MDIFW and these organizations have a culture of wetlands protection.

In Downeast Maine much of our shorefront is fringed by highly productive salt marsh and mudflats. As sea level rises these wetlands and their essential wildlife habitat will also be flooded. Today there are over 10,000 acres of salt marsh and intertidal mudflats in the estuaries of the Pleasant and Indian Rivers and Mason Bay, some of the largest and least disturbed in the state. PRWF created Project Areas in all three of these areas, selected in part because of the opportunity to save their salt marshes. This isn’t done solely by preserving existing marshes, some of which will inevitably be flooded, but especially by protecting marshes which can recreate themselves as sea level rises.

Where the dry land adjacent to the salt marshes is relatively low lying and free of barriers the marshes can shift gradually inland with sea level rise creating new marshes, an ages old process known as marsh migration. New shorebird and waterfowl habitat will be established thus mitigating some of the effects of climate change.

The second mechanism by which coastal wetlands survive rising waters is soil accretion, the process of growing vertically by building layers of new soil, called peat, from shoreline minerals eroding into the marshes and organic material accumulating from the annual crop of salt grasses that falls each winter. Many of northeastern Maine’s estuaries have shorelines of glacial clay, silt, sand and gravel that erode easily into adjacent marshes and mudflats during unusually high tides and coastal storms. In Maine peat beds, in some places over 20 feet deep, have formed as the marsh floors have risen. Mason Bay is a prime example of growth by accretion; its marshes are almost entirely bordered by a broad buffer of glacial soils.

Clay Point in Mason Bay Clay Point in Mason Bay is comprised of glacial till which is slowly eroding away and building up the adjacent marsh.

Both marsh migration and vertical growth through accretion may be blocked when lands bordering marshes are developed for housing, blueberry farming, businesses or roadways. Shoreline hardening with seawalls or riprap and stabilization of easily eroded shorelines often ensues in an effort to protect valuable assets from rising sea levels. Marshes then cannot expand inland and their source of eroded minerals is also eliminated.

Thus hemmed in, our marshes would be drowned, flooded too deeply to survive. Thus it is an urgent conservation priority not only to preserve the marshes themselves but also a deep inland buffer of low lying undeveloped land as free as possible of private and community assets. Here marshes can grow inland as their outer parts are drowned. Similarly, we need to protect shoreline buffers built of the glacial soils that are essential for accretion and vertical marsh and mudflat growth.

Land Trusts in the Downeast area (PRWF, DCC, DSF, MCHT and TNC) are working together in a climate change initiative to mitigate the long-term biological and societal impacts of climate change. They are identifying and protecting carefully selected wetlands and buffer land that offer the best adaptive potential when threatened by rising sea levels. They are also working to address tidal restrictions that impede fish passage and tidal water flows that are natural responses when sea levels rise. These organizations understand that success in their efforts depends on engaging directly with local communities and landowners, building a deeper understanding of the benefits to everyone of healthy estuaries.