Description. Aquatic Beds represent a diverse group of plant communities that requires surface water for optimum growth and reproduction. They are best developed in relatively permanent water or under conditions of repeated flooding. The plants are either attached to the substrate or float freely in the water above the bottom or on the surface.

Subclasses and Dominance Types.

• Algal. -- Algal Beds are widespread and diverse in the Marine and Estuarine Systems, where they occupy substrates characterized by a wide range of sediment depths and textures. They occur in both the Subtidal and Intertidal Subsystems and may grow to depths of 30 m (98 feet). Coastal Algal Beds are most luxuriant along the rocky shores of the Northeast and West. Kelp (Macrocystis) beds are especially well developed on the rocky substrates of the Pacific Coast. Dominance Types such as the rockweeds Fucus and Ascophyllum and the kelp Laminaria are common along both coasts. In tropical regions, green algae, including forms containing calcareous particles, are more characteristic; Halimeda and Penicillus are common examples. The red alga Laurencia, and the green algae Caulerpa, Enteromorpha, and Ulva are also common Estuarine and Marine dominance types; Enteromorpha and Ulva are tolerant of fresh water and flourish near the upper end of some estuaries. The stonewort Chara. is also found in estuaries.
  Inland, the stoneworts Chara, Nitella, and Tolypella are examples of algae that look much like vascular plants and may grow in similar situations. However, meadows of Chara may be found in Lacustrine water as deep as 40 m (131 feet) (Zhadin and Gerd 1963), where hydrostatic pressure limits the survival of vascular submergents (phanaerogams) (Welch 1952). Other algae bearing less resemblance to vascular plants are also common. Mats of filamentous algae may cover the bottom in dense blankets, may rise to the surface under certain conditions, or may become stranded on Unconsolidated or Rocky Shores.

• Aquatic Moss. -- Aquatic mosses are far less abundant than algae or vascular plants. They occur primarily in the Riverine System and in permanently flooded and intermittently exposed parts of some Lacustrine systems. The most important Dominance Types include genera such as Fissidens, Drepanocladus, and Fontinalis. Fontinalis may grow to depths as great as 120 m (394 feet) (Hutchinson 1975). For simplicity, aquatic liverworts of the genus Marsupella are included in this Subclass.

• Rooted Vascular. -- Rooted Vascular Reds include a large array of vascular species in the Marine and Estuarine Systems. They have been referred to by others as temperate grass flats (Phillips 1974); tropical marine meadows (Odum 1974); and eelgrass beds, turtlegrass beds, and seagrass beds (Akins and Jefferson 1973; Eleuterius 1973; Phillips 1974). The greatest number of species occur in shallow, clear tropical, or subtropical waters of moderate current strength in the Caribbean and along the Florida and Gulf Coasts. Principal Dominance Types in these areas include turtlgrass (Thalassia testudinum), shoalgrass (Halodule writghtii), manatee grass (Cyrnodocea filiformis), widgeon grass (Ruppia martima), sea grasses (Halophila spp.), and wild celery (Vallisneria americana).
  Five major vascular species dominate along the temperate coasts of North America: shoalgrass, surf grasses (Phyllospadix scoulleri, P. torreyi), widgeon grass, and eelgrass (Zostera marina). Eelgrass beds have the most extensive distribution, but they are limited primarily to the more sheltered estuarine environment. In the lower salinity zones of estuaries, stands of widgeon grass, pondweed (Potamogeton), and wild celery often occur, along with naiads (Najas) and water milfoil (Myriophyllum).
  In the Riverine, Lacustrine, and Palustrine Systems, rooted vascular aquatic plants occur at all depths within the photic zone. They often occur in sheltered areas where there is little water movement (Wetzel 1975): however, they also occur in the flowing water of the Riverine System, where they may be streamlined or flattened in response to high water velocities. Typical inland genera include pondweeds, horned pondweed (Zannichellia palustris), ditch grasses (Ruppia), wild celery, and waterweed (Elodea). The riverweed (Postostemum ceratophyllum) is included in this class despite its lack of truly recognizable roots (Sculthorpe l967).
  Some of the rooted vascular species are characterized by floating leaves. Typical dominants include water lilies (Nymphaea, Nuphar), floating-leaf pondweed (Potamogeton natans), and water shield (Brasenia schreberi). Plants such as yellow water lily (Nuphar luteum) and water smartweed (Polygonum amphibium), which may stand erect above the water surface or substrate, may be considered either emergents or rooted vascular aquatic plants, depending on the life form adopted at a particular site.

• Floating Vascular. -- Beds of floating vascular plants occur mainly in the Lacustrine, Palustrine, and Riverine Systems and in the fresher waters of the Estuarine System. The plants float freely either in the water or on its surface. Dominant plants that float on the surface include the duckweeds (Lemna, Spirodela), water lettuce (Pistia stratiotes), water hyacinth (Eichhornia crassipes), water nut (Trapa natans), water ferns (Salvinia spp.), and mosquito ferns (Azolla). These plants are found primarily in protected portions of slow-flowing rivers and in the Lacustrine and Palustrine Systems. They are easily moved about by wind or water currents and cover a large area of water in some parts of the country, particularly the Southeast. Dominance Types for beds floating below the surface include bladderworts (Utricularia), coontails (Ceratophyllum), and watermeals (Wolffia) (Sculthorpe 1967; Hutchinson 1975).