An estuary is where fresh water from streams and rivers mixes with salt water from the ocean in bays, salt marshes and mangrove swamps. Plants have to endure varying levels of salt in the water, strong currents, storm waves, wind, unpredictable sunlight and low levels of oxygen in muddy soils.
Sea grass is a flowering plant with small, dark green, ribbon-like leaves that roots in sandy silts and tidal flats. Sea grass is exposed at low tide and covered with three to six feet of water at high tide. Its stems slip a few inches under the mud and tangle with those of adjacent plants to form a mat, anchoring the plant. As sediments collect on the mat, other plants begin to grow.
Tropical estuaries are frequently dominated by intertidal forests of mangrove, a small tree or shrub that can live in brackish water or pure sea water. The mangrove has a gray, twisted trunk and leathery, olive-colored leaves. Its vertical roots take in air at low tide. The roots need to be exposed to air for half the tidal cycle for the plant to get enough air. Sediments and mud collect around the roots in the slow-moving sheltered water. The tree seeds germinate while still on the tree, and then drop young plants. Plants that hit the mud grow quickly; plants that hit tide water float away to grow somewhere else.
Salt Marsh Plants
Plants in salt marshes live in soil that is waterlogged and low in oxygen. Plants that can live in salt water and salty soil are called "halophytes." Since they can survive while submerged in water part of the time, they are also called "hydrophytes."
Rushes, grasses and sedges prosper in cool, temperate climates. They thrive in areas that are in the path of tidal waters. They have special cells that concentrate the salt, yielding saltwater that the plant can use. Marsh grass filters and traps silt. They have strong roots that anchor them in storm surges
Estuaries on the Gulf Coast are often dominated by cord grass that lives in low-lying areas of estuaries that are regularly flooded by the tides. Cord grass has thick, tough stalks and roots evolved to anchor it in the mud. Its narrow, tough blades have special glands that excrete excess salt.
Ellgrass is a rooted, submerged plant that lives in water below low tide. It has long, thin, ribbon-like leaves three to four feet long. Since these leaves resemble that of celery, it is sometimes called wild celery. It lives in dense, tangled colonies.