Marsh

• 1. Introduction

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  A marsh holds 0.1% of all the carbon locked away in the world's land. That is a tiny fraction for a kind of ground most people walk past without naming. Yet this low-lying, seasonally waterlogged terrain does something most landscapes cannot. It absorbs high tides and the surges of extreme weather, steadying the coastlines and waterways around it.

  In ecology, a marsh is a wetland ruled by herbaceous plants rather than woody ones. That single distinction separates it from the swamp, with its trees, and from the mire, with its acidic peat. But the word stretches further than ecology. In Europe and in agricultural writing, drained low-lying meadows and embanked polderlands carry the name too.

  What lives in ground that is half water and half land? How does soft, flooded soil end up filtering pollution and slowing floods? And why, after all this usefulness, are most natural marshlands now under threat? The answers run through grasses and rushes, through salt and freshwater, and through ground that humans have spent centuries draining away.

• 2. Breathing In The Mud

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  Aerenchyma is the trick that makes marsh plants possible. These are channels inside the stem that carry air down from the leaves into the rooting zone. Without them, a plant could not survive in wet mud where oxygen runs low. The cattails, sedges, papyrus, and sawgrass of a marsh all live by moving air through their own bodies.

  Rhizomes give these same plants a second advantage underground. The rhizome stores energy and lets the plant reproduce below the surface, anchored in saturated soil. This is why grasses, rushes, and reeds tend to dominate a marsh, and why any woody plants present stay low, as shrubs. When such shrubs do appear, the marsh is sometimes called a carr.

  The animals face the same shortage of oxygen and have their own solutions. Fish, salamanders, and other aquatic creatures generally tolerate low oxygen in the water. Some draw oxygen from the air instead, while others can live indefinitely in oxygen-poor conditions. The chemistry around them tends toward neutral or alkaline, unlike a bog, where peat builds up under acid conditions. Firmer clumps of vegetation in this boggy ground have a name of their own: hassocks, or tussocks.

• 3. The Working Wetland

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  Some of the highest levels of biological production in the world happen in a marsh. That raw productivity is why marshes matter to fisheries, feeding the invertebrates, fish, amphibians, waterfowl, and aquatic mammals that shelter there. A patch of waterlogged ground turns out to be one of the most fertile systems on the planet.

  Filtering is the marsh's quieter service. Acting as a sink, it traps pollutants and sediment from the water flowing through it, consuming nutrients and pollution before they pass downstream. During heavy rainfall, the same ground absorbs water and releases it slowly into waterways, which lowers the magnitude of flooding. People draw value from marshes in other ways too, through tourism, recreation, education, and research.

• 4. Salt And Tide

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  Mid to high latitudes, wherever a coastline is protected, are where salt marshes form around the world. They sit close enough to shore that the tides reach them, and from time to time the water covers them completely. They flourish in one specific condition: when sediment builds up faster than the land beneath sinks. Salt-tolerant grasses, specially adapted and rooted, dominate the scene.

  Lagoons, estuaries, and the sheltered side of a shingle or sandspit are the usual settings. Currents carry fine particles around to the quiet side of the spit, where sediment begins to gather. In that calm, the marsh absorbs excess nutrients from the water before it reaches the oceans and estuaries. These marshes are slowly declining, with coastal development and urban sprawl driving significant loss of the habitat.

  Freshwater tidal marshes share the pull of the ocean tides but escape the stress of salinity. Free of that pressure, they carry a much higher diversity of plants and animals than their saltwater counterparts. Their gravest threats come from the growing size and pollution of the cities that surround them.

• 5. Freshwater Variety

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  North America's most common form of wetland is the freshwater marsh, and it is also the most diverse of the three marsh types. Within that single category sit a range of distinct landscapes, each shaped by where it lies and how long it holds water. Some are wet year-round, others vanish in summer.

  Wet meadows form in shallow lake basins, low-lying depressions, and the band of land between shallow marshes and upland. They also gather on the edges of large lakes and rivers. These meadows often hold very high plant diversity and dense reserves of buried seed. Regularly flooded, they are frequently dry by summer.

  Vernal pools appear only seasonally, in shallow depressions that may sit under thin water and then go completely dry by summer and fall. In western North America they tend to open in grasslands, while in the east they often lie within forests, and further south they form in pine savannas and flatwoods. Because these ponds hold no fish to eat eggs and young, many amphibians depend on them for spring breeding, among them the endangered gopher frog. Similar temporary ponds exist in ecosystems worldwide under local names, but the term vernal pool can cover them all.

  Playa lakes are shallow freshwater marshes in the southern high plains of the United States, present only at certain times of year and generally circular in shape. As a playa dries through summer, a conspicuous zonation of plants develops along its shoreline. Prairie potholes tell a different origin: glaciers once covered northern North America, such as the Prairie Pothole Region, and left countless shallow depressions behind. Filling with water in spring, they become important breeding habitat for waterfowl, with some pools seasonal and others holding water all year.

• 6. Marsh By Many Names

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  Along the fringes of large rivers, many kinds of marsh take shape, sorted by water level, nutrients, ice scour, and waves. The same river that floods a meadow can carve out a dozen different wetland forms depending on these forces. Riverine wetlands are as varied as the rivers that feed them.

  Embanked and artificially drained, large tracts of tidal marsh carry a roll call of regional names. The Dutch call them polders. In Northern Germany and Scandinavia they are Marschland, Marsch, or marsk; in France, marais maritime. The Netherlands and Belgium designate them marine clay districts. In East Anglia, in the East of England, the embanked marshes are known as the Fens.

• 7. What Is Lost And Returned

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  Ninety percent of the wetlands in some areas, marshes included, are already gone. They were drained for agricultural land or filled to make room for urban sprawl. The loss is the reason restoration exists: returning marshes to the landscape to replace what disappeared.

  Restoration works at very different scales. On the large scale, it can mean letting rivers flood naturally in the spring, the way they once did before embankments held them back. On the small scale, it can mean bringing wetlands back into urban landscapes, one patch at a time. The same fate that threatens natural marshlands today, sea level rise and the erosion that follows, is what gives this slow work of return its urgency, even as some marshes are expected to migrate upland.