18 



restricted to subtidal, irregularly exposed, regularly 

 flooded, and irregularly flooded. 



Description. Reefs are characterized by their ele- 

 vation above the surrounding substrate and their 

 interference with normal wave flow; they are primarily 

 subtidal, but parts of some Reefs may be intertidal as 

 well. Although corals, oysters, and tube worms are the 

 most visible organisms and are mainly responsible for 

 Reef formation, other mollusks, foraminifera, coralline 

 algae, and other forms of life also contribute sub- 

 stantially to Reef growth. Frequently, Reefs contain 

 far more dead skeletal material and shell fragments 

 than living matter. 



Subclasses and Dominance Types. 

 Coral.— Coral Reefs are widely distributed in 

 shallow waters of warm seas, in Hawaii, Puerto Rico, 

 the Virgin Islands, and southern Florida. They were 

 characterized by Odum (1971) as stable, well-adapted, 

 highly diverse, and highly productive ecosystems with 

 a great degree of internal symbiosis. Coral Reefs lie 

 almost entirely within the Subtidal Subsystem of the 

 Marine System, although the upper part of certain 

 reefs may be exposed. Examples of dominance types 

 are the corals Pontes, Acropora, and Montipora. The 

 distribution of these types reflects primarily their ele- 

 vation, wave exposure, the age of the Reef, and its 

 exposure to waves. 



Mollusk.— This subclass occurs in both the Inter- 

 tidal and Subtidal subsystems of the Estuarine 

 System. These Reefs are found on the Pacific, 

 Atlantic, and Gulf coasts and in Hawaii and the Carib- 

 bean. Mollusk Reefs may become extensive, affording 

 a substrate for sedentary and boring organisms and a 

 shelter for many others. Reef mollusks are adapted to 

 great variations in water level, salinity, and tempera- 

 ture, and these same factors control their distribution. 

 Examples of dominance types for this subclass are the 

 oysters Ostrea and Crassostrea (Smith 1964; Abbot 

 1968; Ricketts and Calvin 1968). 



Worm.— Worm Reefs are constructed by large 

 colonies of Sabellariid worms living in individual tubes 

 constructed from cemented sand grains. Although 

 they do not support as diverse a biota as do Coral and 

 Mollusk reefs, they provide a distinct habitat which 

 may cover large areas. Worm Reefs are generally 

 confined to tropical waters, and are most common 

 along the coasts of Florida, Puerto Rico, and the 

 Virgin Islands. They occur in both the Marine and 

 Estuarine systems where the salinity approximates 

 that of sea water. The reef worm Sabellaria is an 

 example of a dominance type for this subclass 

 (Ricketts and Calvin 1968). 



Streambed 



Definition. The class Streambed includes all wetland 

 contained within the Intermittent Subsystem of the 

 Riverine System and all channels of the Estuarine 



System or of the Tidal Subsystem of the Riverine 

 System that are completely dewatered at low tide. 

 Water regimes are restricted to irregularly exposed, 

 regularly flooded, irregularly flooded, seasonally 

 flooded, temporarily flooded, and intermittently 

 flooded. 



Description. Streambeds vary greatly in substrate 

 and form depending on the gradient of the channel, the 

 velocity of the water, and the sediment load. The 

 substrate material frequently changes abruptly be- 

 tween riffles and pools, and complex patterns of bars 

 may form on the convex side of single channels or be 

 included as islands within the bed of braided streams 

 (Crickmay 1974). In mountainous areas the entire 

 channel may be cut through bedrock. In most cases 

 streambeds are not vegetated because of the scouring 

 effect of moving water, but, like Unconsolidated 

 Shores, they may be colonized by "pioneering" an- 

 nuals or perennials during periods of low flow or they 

 may have perennial emergents and shrubs that are too 

 scattered to qualify the area for classification as 

 Emergent Wetland or Scrub-Shrub Wetland. 



Subclasses and Dominance Types. 



Bedrock. —This subclass is characterized by a 

 bedrock substrate covering 75% or more of the stream 

 channel. It occurs most commonly in the Riverine 

 System in high mountain areas or in glaciated areas 

 where bedrock is exposed. Examples of dominance 

 types are the mollusk Ancylus, the oligochaete worm 

 Limnodrilus, the snail Physa, the fingernail clam 

 Pisidium. and the mayflies Caenis and Ephemerella. 



Rubble. — This subclass is characterized by stones, 

 boulders, and bedrock that in combination cover more 

 than 75% of the channel. Like Bedrock Streambeds, 

 Rubble Streambeds are most common in mountainous 

 areas and the dominant organisms are similar to those 

 of bedrock and are often forms capable of attachment 

 to rocks in flowing water. 



Cobble-Gravel— In this subclass at least 25% of 

 the substrate is covered by unconsolidated particles 

 smaller than stones; cobbles or gravel predominate. 

 The subclass occurs in riffle areas or in the channels of 

 braided streams. Examples of dominance types in the 

 Intermittent Subsystem of the Riverine System are 

 the snail Physa, the oligochaete worm Limnodrilus, 

 the mayfly Caenis, the midge Chironomus, and the 

 mosquito Anopheles. Examples of dominance type in 

 the Estuarine System or Tidal Subsystem of the Riv- 

 erine System are the mussels Modiolus and Mytilus. 



Sand.— In this subclass, sand-sized particles 

 predominate among the particles smaller than stones. 

 Sand Streambed often contains bars and beaches 

 interspersed with Mud Streambed or it may be inter- 

 spersed with Cobble-Gravel streambed in areas of fast 

 flow or heavy sediment load. Examples of dominance 

 types in the Riverine System are the scud Gammarus, 

 the snails Physa and Lymnaea, and the midge Chiron- 



