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. Exposure 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 successfully 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 temperature, 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 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 behavioral 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 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 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-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. Examples 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. 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 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, freshwater mollusks Anodonta, Anodontoides, and Elliiotio, the fingernail clams Pisidium 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 Estuarine 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 oligochaete worm Limnodrilus.