extruding mucus threads. The extent of 

 movement is variable and species-specific, 

 ranging from diurnal ly migrating forms 

 such as Hantzschia to relatively immobile 

 forms such as Amphora (Round 1979). Ver- 

 tical movements are thought to be depend- 

 ent upon cycles of illumination with 

 diatoms appearing at the sediment surface 

 at low tide and burrowing into the sedi- 

 ment at flood tide (Palmer and Round 

 1967). The downward migration into the 

 sediments is considered to be either an 

 active response to compensate for dis- 

 placement by tidal action or a mechanism 

 for increasing nutrient availability 

 (Pomeroy 1959). While the non-migratory 

 forms are most commonly attached to sand 

 grains, some species are capable of 

 limited mobility. 



Although episammic forms are not 

 as intensively studied as the epipelic 



diatoms because they become more easily 

 buried in unstable tidal flat sediments 

 (Williams 1962; Sullivan 1975; Pace et al. 

 1979), these forms may be important 

 benthic primary producers. Riznyk (1973) 

 found that when sampling methods were used 

 to collect both motile and non-motile 

 forms, the latter group was more abundant 

 on an Oregon tidal flat. 



Occasionally algal mats are present 

 in the higher elevations of tidal flat 

 habitats. The mats consist of tightly 

 intertwined groups of species of green and 

 blue-green algae. The mats form a dark- 

 green or blue-black crust on the sediment 

 surface and are found in protected areas. 

 The principle species found in a Massa- 

 chusetts salt marsh by Brenner et al. 

 (1976) were Lyngbya aestuari , Microcoleus 

 chthonoplastes , and Calothrix contarenii . 

 In cross-section, many of the mats form 



Epipelic pennate diatoms (this specimen is approximately 0.2 mm long) are commonly seen 

 in the upper several centimeters of tidal flat sediments. When very abundant, benthic 

 diatoms form brownish films on the sediment surface. Photo by R.B. Whitlatch, Univer- 

 sity of Connecticut. 



10 



