Definition: The Class Rock Bottom includes all wetlands and deepwater habitats with substrates having an areal cover of stones, boulders, or bedrock 75 percent or greater and vegetative cover of less than 30 percent. Water Regimes are restricted to Subtidal, Permanently Flooded, Intermittently Exposed, Semipermanently Flooded, Permanently Flooded-Tidal Fresh, and Semipermanently Flooded-Tidal Fresh. 5 Percent areal cover is seldom measured in the application of the WCS, but the term must be defined in terms of area. We suggest 2 m2 for herbaceous and moss layers, 16 m2 for shrub layers, and 100 m2 for tree layers (Mueller-Dombois and Ellenberg 1974). When percent areal cover is the key for establishing boundaries between units of the classification, it may occasionally be necessary to measure cover on plots, in order to maintain uniformity of ocular estimates made in the field or interpretations made from aerial imagery. 


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:


  • Bedrock -  Bottoms in which bedrock covers 75 percent or more of the surface.
     
  • Rubble - Bottoms with less than 75 percent areal cover of bedrock, but stones and boulders alone, or in combination with bedrock, cover 75 percent or more of the surface. 


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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