water flow results in the deposition of 

 finer-grained, more stable sediments. On 

 a larger scale, coarser-grained sandy sed- 

 iments are found in channels, on beaches, 

 and near the mouths of inlets, while 

 finer-grained sediments are associated 

 with increasing distance from the mouths 

 of inlets and at higher intertidal eleva- 

 tions. Redfield (1S72) described these 

 sediment distribution patterns at Barn- 

 stable Harbor, Massachusetts, noting a 

 decrease in grain size proceeding from 

 the mouth of the harbor to the vegetated 

 salt marsh. 



Wind-generated waves and currents 

 also affect mixing and redistribution of 

 sediments on some tidal flats. The 

 magnitude of wind impact is largely 

 dependent upon the size and depth of the 

 waterbody over which the wind passes. 

 Large shallow embayments in some southern 

 states, for example, can be influenced 

 considerably by wind-generated waves 

 (Peterson and Peterson 1979). In New 

 England, embayments are comparatively 

 smaller and shallower; wind action is 

 generally less significant than tidal 

 action. Most wind effects on tidal flats 

 are probably concentrated in periods of 

 storm activity when resuspension and 

 redistribution of sediments occur. 



The New England coast has semi- 

 diurnal tides (e.g., two high and two low 

 tides per tidal day). Channel constric- 

 tions and bottom topography alter the 

 magnitude of the tidal range although the 

 mean tidal range south of Cape Cod is 

 about 1 to 1.5 m (3 to 5 ft) while mean 

 tides north of Cape Cod range 3 to 4 m (10 

 to 13 ft). The twice daily inundation and 

 exposure contributes in an important man- 

 ner to the spatial and temporal complexity 

 of the tidal flat habitat. When tidal 

 flats are submerged, they share many of 

 the same physical and chemical character- 

 istics of the water found in adjacent 

 coastal and/or estuarine systems. When 

 exposed, tidal flats are affected by cli- 

 matic variations of air temperature, pre- 

 cipitation, and wind. Organisms living in 

 these environments, therefore, must be 

 well adapted to the physically rigorous 

 environmental conditions. 



While the physical conditions of the 

 water over the tidal flats may change con- 

 siderably during a tidal cycle, physical 

 features of the sediments are less vari- 

 able. Even at low tide, small amounts of 

 water are retained in the sediments; this 

 helps prevent desiccation. Sediments also 

 tend to buffer temperature and salinity 

 fluctuations (Sanders et al. 1965; Johnson 

 1965, 1967). The net result is that 

 organisms living within tidal flat sedi- 

 ments are normally able to withstand 

 greater environmental fluctuation than 

 exposed organisms attached to or living on 

 the sediments (Alexander et al. 1955). 



Chemical properties of the sediments 

 vary vertically in tidal flats and it is 

 possible to view this stratification by 

 examining sediment samples in cross- 

 section. In muddy sediments, two or three 

 distinctly colored zones commonly exist. 

 The uppermost is light-brown, extending 1 

 to 5 mm below the sediment surface. This 

 is the zone of oxygenated sediment. Below 

 this thin layer is a black zone where oxy- 

 gen is absent and the sediments smell of 

 hydrogen sulfide ("rotten egg" gas). The 

 black color is due primarily to the pres- 

 ence of iron sulfides. In some muddy 

 sediments a third, gray-colored zone may 

 exist below the black zone due to the 

 presence of iron pyrite. 



The boundary between and position of 

 the oxygenated and black anoxic zone 

 (termed the redox potential discontinuity, 

 or redox zone) varies with depth, depend- 

 ing on the amount of organic matter in the 

 sediment, sediment grain size, and the 

 activities of organisms burrowing through 

 the sediment or disturbing the surface. 

 Oxygen diffusion may extend 10 to 20 cm 

 (4 to 8 inches) below the sediment-water 

 interface in sandy sediments due to 

 increased percolation of water through the 

 sediments and small amounts of organic 

 material. On many sandy flats it may be 

 difficult to find a black zone and the 

 sediments may not smell of hydrogen 

 sulfide. In muddy sediments containing 

 greater amounts of organic material, 

 the redox zone is usually within sev- 

 eral millimeters of the surface. Rhoads 

 (1974) noted that activities of burrowing 



