Classification of Wetlands and Deepwater Habitats of the United States
Definition. The Class Unconsolidated Bottom includes all wetland
and deepwater habitats with at least 25% cover of particles
smaller than stones, and a vegetative cover less than 30%. Water
regimes are restricted to subtidal, permanently flooded,
intermittently exposed, and semipermanently flooded.
Description. Unconsolidated Bottoms are characterized by the
lack of large stable surfaces for plant and animal attachment.
They are usually found in areas with lower energy than Rock
Bottoms, and may be very unstable. Exposure to wave and current
action, temperature, salinity, and light penetration determines
the composition and distribution of organisms.
Most macroalgae attach to the substrate by means of basal hold-fast
cells or discs; in sand and mud, however, algae penetrate the
substrate and higher plants can successfully root if wave action
and currents are not too strong. Most animals in unconsolidated
sediments live within the substrate, e.g., Macoma and the amphipod
Melita. Some, such as the polychaete worm Chaetopterus, maintain
permanent burrows, and others may live on the surface, especially
in coarse-grained sediments.
In the Marine and Estuarine Systems, Unconsolidated Bottom
communities are relatively stable. They vary from the Arctic to
the tropics, depending largely on temperature, and from the open
ocean to the upper end of the estuary, depending on salinity.
Thorson (1957) summarized and described characteristic types of
level-bottom communities in detail.
In the Riverine System, the substrate type is largely
determined by current velocity, and plants and animals exhibit a
high degree of morphologic and behavioral adaptation to flowing
water. Certain species are confined to specific substrates and
some are at least more abundant in one type of substrate than in
others. According to Hynes (1970:208), "The larger the stones, and
hence the more complex the substratum, the more diverse is the
invertebrate fauna." In the Lacustrine and Palustrine Systems,
there is usually a high correlation, within a given water body,
between the nature of the substrate and the number of species and
individuals. For example, in the profundal bottom of eutrophic
lakes where light is absent, oxygen content is low, and carbon
dioxide concentration is high, the sediments are ooze-like organic
materials and species diversity is low. Each substrate type
typically supports a relatively distinct community of organisms
(Reid and Wood 1976:262).
Subclasses and Dominance Types.
Dominance Types for Unconsolidated Bottoms in the Marine and
Estuarine Systems were taken predominantly from Miner (1950),
Smith (1964), Abbott (1968), and Ricketts and Calvin (1968).
Dominance Types for Unconsolidated Bottoms in the Lacustrine,
Riverine, and Palustrine Systems were taken predominantly from
Krecker and Lancaster (1933), Stehr and Branson (1938), Johnson
(1970), Brinkhurst and Jamieson (1972), Clarke (1973), Hart and
Fuller (1974), Ward (1975), and Pennak (1978).
Cobble-Gravel. -- The unconsolidated particles smaller than
stones are predominantly cobble and gravel, although finer
sediments may be intermixed. Examples of Dominance Types for the
Marine and Estuarine Systems are the mussels Modiolus and Mytilus,
the brittle star Amphipholis, the soft-shell clam Mya, and the
Venus clam Saxidomus. Examples for the Lacustrine, Palustrine, and
Riverine Systems are the midge Diamesa, stonefly-midge Nemoura-Eukiefferiella
(Slack et al. 1977), chironomid midge-caddisfly-snail
Chironomus-Hydropsyche-Physa (Krecker and Lancaster 1933),
the pond snail Lymnaea, the mayfly Baetis, the freshwater sponge
Eunapius, the oligochaete worm Lumbriculus, the scud Gammarus, and
the freshwater mollusks Anodonta, Elliptio, and Lampsilis.
- Sand. -- The unconsolidated particles smaller than stones are
predominantly sand, although finer or coarser sediments may be
intermixed. Examples of Dominance Types in the Marine and
Estuarine Systems are the wedge shell Donax, the scallop Pecten,
the tellin shell Tellina, the heart urchin Echinocardium, the
lugworm Arenicola, the sand dollar Dendraster, and the sea pansy
Renilla. Examples for the Lacustrine, Palustrine, and Riverine
Systems are the snail Physa, the scud Gammarus, the oligochaete
worm Limnodrilus, the mayfly Ephemerella, the freshwater mollusks
Elliptio and Anodonta, and the fingernail clam Sphaerium.
- Mud. -- The unconsolidated particles smaller than stones are
predominantly silt and clay, although coarser sediments or
organic material may be intermixed. Organisms living in mud must
be able to adapt to low oxygen concentrations. Examples of
Dominance Types for the Marine and Estuarine Systems include the
terebellid worm Amphitrite, the boring clam Platyodon, the deep-sea
scallop Placopecten, the quahog Mercenaria, the macoma
Macoma, the echiurid worm Urechis, the mud snail Nassarius, and
the sea cucumber Thyone. Examples of Dominance Types for the
Lacustrine, Palustrine, and Riverine Systems are the sewage worm
Tubifex, freshwater mollusks Anodonta, Anodontoides, and
Elliiotio, the fingernail clams Pisidium and Sphaerium, and the
- Organic. -- The unconsolidated material smaller than stones is
predominantly organic. The number of species is limited and faunal
productivity is very low (Welch 1952). Examples of Dominance Types
for Estuarine and Marine Systems are the soft-shell clam Mya, the
false angel wing Petricola pholadiformis, the clam worm Nereis,
and the mud snail Nassarius. Examples for the Lacustrine,
Palustrine, and Riverine Systems are the sewage worm Tubifex, the
snail Physa, the harpacticoid copepod Canthocamptus, and the
oligochaete worm Limnodrilus.
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