Classification of Wetlands and Deepwater Habitats of the United States
Aquatic Bed
Definition. The Class Aquatic Bed includes wetlands and deepwater
habitats dominated by plants that grow principally on or below the
surface of the water for most of the growing season in most years.
Water regimes include subtidal, irregularly exposed, regularly
flooded, permanently flooded, intermittently exposed,
semipermanently flooded, and seasonally flooded.
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).
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