16 



can successfully root if wave action and currents are 

 not too strong. Most animals in unconsolidated sedi- 

 ments live within the substrate, e.g., Macoma and the 

 amphjpod 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, Uncon- 

 solidated 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 charac- 

 teristic 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 beha- 

 vioral 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 Lacus- 

 trine 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 sup- 

 ports 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 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-Eu- 

 kiefferiella (Slack et al. 1977), chironomid midge- 

 caddisfly- snail Chironomus-Hy dropsy che-Physa 

 (Krecker and Lancaster 1933), the pond snail Lym- 

 naea, the mayfly Baetis, the freshwater sponge Eunap- 

 ius, the oligochaete worm Lumbriculus, the scud Gam- 

 marus, 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 Lacus- 

 trine, 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 inter- 

 mixed. 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 Platv- 

 odon, the deep-sea scallop Placopecten, the quahog 

 Mercenaria, the macoma Macoma, the echiurid worm 

 Urechis, the mud snail Nassarius, and the sea cucum- 

 ber Thyone. Examples of dominance types for the 

 Lacustrine, Palustrine, and Riverine systems are the 

 sewage worm Tubifex, freshwater mollusks Anodonta, 

 Anodontoides, and Elliptio, the fingernail clams Pisi- 

 dium and Sphaerium, and the midge Chironomus. 



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 Estua- 

 rine 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 oligo- 

 chaete worm Limnodrilus. 



Dominance types for Unconsolidated Bottoms in the 

 Marine and Estuarine systems were taken predomi- 

 nantly 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). 



Aquatic Bed 



Definition. The class Aquatic Bed includes wetlands 

 and deepwater habitats dominated by plants that 

 grow principally on or below the surface of the water 

 for most of the growing season in most years. Water 

 regimes include subtidal, irregularly exposed, regu- 

 larly flooded, permanently flooded, intermittently 

 exposed, semipermanently flooded, and seasonally 

 flooded. 



Description. Aquatic Beds represent a diverse group 

 of plant communities that require s lrface water for 

 optimum growth and reproduction. They are best 

 developed in relatively permanent water or under 

 conditions of repeated flooding. The plants are either 



