of Penobscot 

 species are 

 distribution 



local environmental features, members of 

 both groups may occupy the same habitat 

 reproducing at different times of the year 

 at water temperatures appropriate for each 

 species (Whitlatch 1977). It has been 

 hypothesized that a third biogeographic 

 boundary exists northeast 

 Bay, Maine, where boreal 

 limited in their southern 

 by warm summer water temperatures (Bous- 

 field and Laubitz 1972 cited in Fefer and 

 Schettig 1980). 



On a more local scale, the structure 

 of New England tidal flat macrofaunal 

 communities is also determined by temporal 

 and spatial variations in temperature. 

 Green and Hobson (1970) found that small 

 differences in tidal range influenced the 

 density of several species of infauna and 

 affected the growth rate of the small bi- 

 valve, Gemma gemma . Since tidal flats are 

 gently sloping habitats, zonation patterns 

 are not as pronounced as those observed in 

 rocky intertidal areas. Figure 8 shows an 

 example of infaunal zonation on a muddy- 

 sand flat in Massachusetts. Broadly de- 

 fined, species-specific patterns are prob- 

 ably related to physiological tolerances, 

 desiccation, and temperature as well as 

 certain biological interactions (e.g., 

 competition and predation). Larsen (1979) 

 suggested the importance of temporally and 



spatially variable hydrographic features 

 affecting nearshore zonation of infauna. 

 In northern New England regions, winter 

 ice and spring thaw can alter patterns of 

 salinity for brief periods. In areas with 

 restricted water flow (e.g., glacially- 

 incised estuaries), this yearly event may 

 have profound effects on infaunal distri- 

 bution patterns (Larsen 1979). 



New England tidal flat macrofauna 

 display high temporal and spatial varia- 

 bility; numbers of species and total num- 

 bers of organisms may vary by several 

 orders of magnitude within and between 

 years. This high degree of variability, 

 coupled with the effects of latitudinal 

 variation in physical properties of the 

 region, make it difficult to describe a 

 "typical" tidal flat infaunal association. 

 Figures 6 and 7 and Appendix I illustrate 

 some of the more common macrofaunal organ- 

 isms found in sand flats and mud flats. 

 Not all species will always occur together 

 in any one particular habitat. Rather, 

 the species are representative of those 

 associated with the two different sediment 

 types. 



Most macrofauna live in the upper 

 layers of the sediment, probably reflect- 

 ing the greater amount of food and oxygen 

 in this zone (Figure 9). Amphipods and 



20 bO 



BURROWING 

 ^i POLYCHAETES 



BIVALVES 



Figure 9. Vertical distributions of major groups of tidal flat macroinvertebrates 

 (Whitlatch unpublished data, Barnstable Harbor, MA, 1974 to 1977). 



31 



