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Classification of Wetlands and Deepwater Habitats of the United States

Rock Bottom


Definition. The Class Rock Bottom includes all wetlands and deepwater habitats with substrates having an areal cover of stones, boulders, or bedrock 75% or greater and vegetative cover of less than 30%. Water regimes are restricted to subtidal, permanently flooded, intermittently exposed, and semipermanently flooded.

Description. The rock substrate of the rocky benthic or bottom zone is one of the most important factors in determining the abundance, variety, and distribution of organisms. The stability of the bottom allows a rich assemblage of plants and animals to develop. Rock Bottoms are usually high-energy habitats with well-aerated waters. Temperature, salinity, current, and light penetration are also important factors in determining the composition of the benthic community. Animals that live on the rocky surface are generally firmly attached by hooking or sucking devices, although they may occasionally move about over the substrate. Some may be permanently attached by cement. A few animals hide in rocky crevices and under rocks, some move rapidly enough to avoid being swept away, and others burrow into the finer substrates between boulders. Plants are also firmly attached (e.g., by hold-fasts), and in the Riverine System both plants and animals are commonly streamlined or flattened in response to high water velocities.

Subclasses and Dominance Types.

Examples of Dominance Types for these two Subclasses in the Marine and Estuarine Systems are the encrusting sponges Hippospongia, the tunicate Cnemidocarpa, the sea urchin Strongylocentrotus, the sea star Pisaster, the sea whip Muricea, and the American lobster Homarus americanus. Examples of Lacustrine, Palustrine, and Riverine Dominance Types are the freshwater sponges Spongilla and Heteromeyenia, the pond snail Lymnaea, the mayfly Ephemerella, various midges of the Chironomidae, the caddisfly Hydropsyche, the leech Helobdella, the riffle beetle Psephenus, the chironomid midge Eukiefferiella, the crayfish Procambarus, and the black fly Simulium.

Dominance Types for Rock Bottoms in the Marine and Estuarine Systems were taken primarily from Smith (1964) and Ricketts and Calvin (1968), and those for Rock Bottoms in the Lacustrine, Riverine, and Palustrine Systems from Krecker and Lancaster (1933), Stehr and Branson (1938), Ward and Whipple (1959), Clarke (1973), Hart and Fuller (1974), Ward (1975), Slack et al. (1977), and Pennak (1978).


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