14 



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 Chirono- 

 midae, the caddisfly Hydropsyche, the leech Helobdella, 

 the riffle beetle Psephenus, the chironomid midge Eukief- 

 feriella, 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 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 semi- 

 permanently 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. Ex- 

 posure 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 suc- 

 cessfully 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 temper- 

 ature, 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 com- 

 munities 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 adap- 

 tation 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 inverte- 

 brate 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. 



Cobble-Gravel. — The unconsolidated particles smaller 

 than stones are predominantly cobble and gravel, although 

 finer sediments may be intermixed. Examples of Domi- 

 nance Types for the Marine and Estuarine Systems are 

 the mussels Modiolus and Mytilus, the brittle star Am- 

 phipholis, 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. Ex- 

 amples 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. Organ- 

 isms 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, fresh- 

 water mollusks A nodon ta.Anodontoit les, and Elliptio, the 

 fingernail clams Pisidium and Sphaerium, and the midge 

 ' 7/ 1 ronomus. 



