Estuaries and salt marshes once covered nearly the entire eastern seaboard from Nova Scotia to Florida. They are the wetlands between the ocean beaches and the freshwater rivers. Salinity varies from about 35 parts per thousand (ppt), which is ocean salinity, to 6 ppt up in the Hackensack River, which is almost fresh water. Yet, these nearly fresh waters exhibit the full tidal range and so support a whole suite of plants different from the ocean and inland lakes, rivers and streams.
In the case of estuaries, the floor can build up quickly. Some rivers of the mid-Atlantic carry a million tons of sediment a year to the mouth, and in fierce storms, many carry that much in a day. When the river slows, it drops the sediment in a wide fan, a delta. Likewise, when incoming ocean tides reach the quiet waters behind barriers, they too drop fine suspended particles. Thus, the sand or mud flat gradually builds up and spreads out.
During part of the day, the flat is covered by water. When the tide drops, the flat is exposed to open air. The remains of algae and other marine organisms begin to decay in the sun, fertilizing the flat and making it ripe for a special class of plants called "halophytes" that can tolerate tidal flooding, waterlogged soil, and a high salty source. The first halophyte to colonize the flat and transform it into a marsh is saltmarsh cordgrass, Spartina alterniflora.
The tallest cordgrass is along the bay's edge and along the tidal creeks that braid their way through the marsh. Here, the pulse of tides flushes away debris from the plant's base and brings nutrients and oxygen that stimulate growth. Behind these 6-foot blades, a shorter form of the same species grows to just 1 ½ feet tall.
Farther back from the incoming tides, in the "high marsh" zone, saltmeadow cordgrass, or Spartina patens grows. These high meadows are flooded on an irregular basis, on the full and new moons, and during storm events. You can recognize a high marsh by S. patens tendency to lie flat in swirls called "cowlicks". Because it is not constantly cleansed by tides, old growth remains like a mulch at the base of the plant, keeping the ground moist and providing protected runways for many small rodents.
Shallow dips in the marsh often fill with water when tides are extremely high. When the tide falls, water is stranded in these pools. As the water evaporates, the salt level climbs very high. The fleshy-stemmed glassworts and saltworts colonize these saltpans. Also look for widgeon grass in the more permanent pools.
Farther up, where only the highest tides lick, black grass (Juncus gerardii) and marsh elder (Iva frutescens) forms the official inland boundary of the marsh. Farther back still is an area only affected by salt spray. Flowering plants such as swamp rose mallow, saltmarsh pink, marsh thistle, and groundsel bush (Baccharis halimifolia) lend splashes of color to the scene. Beyond this, Black, northern red, and "scrub" oaks get established, along with pitch pine, beach plum and bayberries.
The dominant theme in all salt marsh life is the regular waxing and waning of tides. "Spring" tides occur twice a month, when the earth, the moon and the sun are all in line. These highest tides cover even the upper marsh. All marsh life is adapted to the twice-daily tide cycle. The air-breathing coffee-bean snail, for instance, climbs the stem of the cordgrass twice a day, just ahead of the rising tide. Birds that dine on the snails use the shells to help grind up food in their stomach. Flying insects that are in the "grounded" stage between nymph and adult
Salt marshes produce 10 tons of organic matter per acre per year, which is twice as productive as most farms.