If, 



1959:29). When the subclass is based on life form, we 

 name the dominance type for the dominant species or 

 combination of species (codominants) in the same layer 

 of vegetation used to determine the subclass. 6 For 

 example, a Needle-leaved Evergreen Forested Wetland 

 with 70% areal cover of black spruce and 30% areal 

 cover of tamarack (Larix laricina) would be designated 

 as a Picea mariana Dominance Type. When the rela- 

 tive abundance of codominant species is nearly equal, 

 the dominance type consists of a combination of 

 species names. For example, an Emergent Wetland 

 with about equal areal cover of common cattail (Typha 

 latifolia) and hardstem bulrush (Scirpus acutus) would 

 be designated as Typha latifolia-Scirpus acutus Domi- 

 nance Type. 



When the subclass is based on substrate material, 

 the dominance type is named for the predominant 

 plant or sedentary or sessile macroinvertebrate 

 species, without regard for life form. In the Marine and 

 Estuarine systems, sponges, alcyonarians, mollusks, 

 crustaceans, worms, ascidians, and echinoderms may 

 all be part of the community represented by the 

 Macoma balthica Dominance Type. Sometimes it is 

 necessary to designate two or more codominant 

 species as a dominance type. Thorson (1957) recom- 

 mended guidelines and suggested definitions for estab- 

 lishing community types and dominants on level 

 bottoms. 



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 deter- 

 mining the composition of the benthic community. 



"Percent areal cover is seldom measured in the application of 

 this system, but the term must be defined in terms of area. 

 We suggest 2 m 2 for herbaceous and moss layers, 16 m ! for 

 shrub layers, and 100 m 2 for tree layers (Mueller-Dombois 

 and EUenberg 1974:74). When percent areal cover is the key 

 for establishing boundaries between units of the classifi- 

 cation, 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 photo- 

 graphs. 



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 sub- 

 strates between boulders. Plants are also firmly at- 

 tached (e.g., by holdfasts), 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% 

 or more of the surface. 



Rubble.— Bottoms with less than 75% areal cover 

 of bedrock, but stones and boulders alone, or in combi- 

 nation with bedrock, cover 75% or more of the surface. 



Examples of dominance types for these two sub- 

 classes in the Marine and Estuarine systems are the 

 encrusting sponges Hippospongia, the tunicate Cnemi- 

 docarpa, the sea urchin Strongylocentrotus, the sea 

 star Pisaster, the sea whip Muricea, and the American 

 lobster, Homarus americanus. Examples of Lacus- 

 trine, 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). 



Unconsolidated Bottom 



Definition. The class Unconsolidated Bottom in- 

 cludes all wetland and deepwater habitats with at least 

 25% cover of particles smaller than stones, and a vege- 

 tative cover less than 30%. Water regimes are re- 

 stricted to subtidal, permanently flooded, intermit- 

 tently exposed, and semipermanently flooded. 



Description. Unconsolidated Bottoms are charac- 

 terized 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 ac- 

 tion, temperature, salinity, and light penetration 

 determine 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, how- 

 ever, algae penetrate the substrate and higher plants 



