The Outside Story: Freshwater marshes are biodiversity hotspots

Sunlight glittered on the water as we paddled our canoe down a winding canal that cut through a swamp of tall grasses and wild rice. Two white, long-legged birds—great egrets—strutted through the shallows, ready to spear fish with their sharp beaks. A bald eagle landed in a tree and screeched as it joined its mate. After four miles of canoeing down the Missisquoi River in northwest Vermont, we had reached the point where the river meets Lake Champlain.

Along the river we had passed a series of marshes—flat-bottomed marshes, cattail marshes, deep rush marshes, wild rice marshes—a vast mosaic that makes up much of the Missisquoi National Wildlife Refuge. Swamps are open wetlands of soft-stemmed vegetation where saturated soil or standing water prevents most shrubs and trees from growing. These wetlands are rich in biodiversity.

Swamp plants have special adaptations that allow them to survive the humid conditions. For example, cattails and arrowheads can exchange gases between their emerging leaves and submerged roots. The type of vegetation that grows in a given swamp depends on the hydrology and soil. In shallow swamps, the water level varies from just a few inches to a foot deep. The soil can always be saturated or periodically flooded. Deeper swamps are constantly flooded, with large areas of open water. Swamp soils range from decomposed mud to highly organic mineral soils.

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At the edges of lakes, ponds and rivers, swamp vegetation often grows in distinct bands, influenced by water depth and exposure. For example, sedges grow in moist to saturated soil. Cattails and pickerelweed, with their distinctive stalks of purple flowers, prefer stagnant water for most of the growing season. Water butt and wild rice are found in deeper waters.

These and several other plants, such as duckweed and arrowroot, form the basis of the wetland food web. Waterfowl and other birds feed on the seeds, fruits, and vegetation of the plants, and the decomposed remains of the plants feed on a variety of invertebrates such as snails, worms, crustaceans, and insects. The invertebrates, in turn, provide food for frogs, fish, turtles and songbirds, which feed on water snakes, raccoons, herons, osprey and bald eagles, among others. Common swamp dwellers, muskrats eat the rhizomes (roots) of cattails and water lilies, and build their domed winter huts out of cattail leaves. Mink glide through the lodges’ underwater entrances to hunt muskrats. Many birds, including hard-to-see bitterns, nest in swamps, and red-winged blackbirds often attach their nests to old cattail stalks.

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As we journeyed down the Missisquoi, we were amazed at the sheer abundance of wildlife, some of which was nurtured by the refuge’s swamps. Around every bend we found gray herons chasing prey in the shallow water. When frightened, these large birds would take off, flapping their huge wings clumsily. Kingfishers flew out of the trees and made their rattling calls. Freshwater mussels were visible underwater, sticking out of the silt at the bottom. A green heron, long neck slung over shoulders, darted across the river ahead of our bow.

In addition to providing excellent habitat for wildlife, freshwater marshes serve several important ecological functions. Marsh plants trap sediment that runs off roads, settlements, and agricultural fields, and filter out excess nutrients that would otherwise degrade water quality. These wetlands store floodwaters, control erosion, and replenish groundwater supplies. Swamps also offer recreational value and are popular spots for paddling, bird watching, hunting and fishing.

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Unfortunately, it’s only in the last few decades that people have recognized the value of swamps and other wetlands, and some have protected them legally. Since European settlement, many swamps have been filled in for agriculture or development, polluted by industrial effluents, or converted into ponds or lakes by dams. Restoration efforts have been made in some locations, but it is difficult to replicate a natural swamp, although beaver activity may create new swamps or alter them to create other forms of wetlands.

Late summer is a great time to explore a swamp, especially as early migratory birds use this habitat. Easily accessible marshes worth visiting include The Nature Conservancy’s LaPlatte River Marsh Preserve in Shelburne, Vermont, the Chaffee Wildlife Sanctuary in Lyme, New Hampshire, which has nature trails, and the McDaniels Marsh Wildlife Management Area in Grafton, New Hampshire. accessible by boat.

Susan Shea is a Vermont-based naturalist, writer, and conservationist. Illustration by Adelaide Murphy Tyrol. The Outside Story is edited and published by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation:

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