they are larger than the western regions and they encompass bays. Casco Bay 

 (region 1) has the next highest area, 11,625 (4706 ha; 19% of the total area 

 of the intertidal marine subsystem), among the six regions. Regions 2 and 3 

 account for only 3% (1540 acres; 623 ha) and 6% (3442 acres; 1394 ha), 

 respectively, of the total intertidal marine acreage. 



Acreages of the habitats of the intertidal marine subsystem are given in table 

 4-8. The NWl classification system grouped some of the 11 types of habitats 

 listed above. The term "beach/bar" in the NWI system includes sand beach, 

 gravel beach, and cobble beach. "Flats" encompass mud flats, sand flats, and 

 stream bed. Boulder beaches and exposed and protected rocky shores are 

 included in the NWI category of "rocky shore." 



Flats and rocky shores are the dominant habitats in the characterization area 

 and much of the area (867o and 87% respectively) of these habitats is in the 

 eastern regions (4, 5, and 6). Aquatic beds (probably eelgrass) are most 

 dominant in region 1. Region 2, which has only 3% of the total intertidal 

 marine area, has 14% of the total beach/bar habitat. Most of the marine 

 intertidal habitat in this area is sand beach. 



Organisms inhabiting the intertidal zone live within biological boundaries; 

 the area between these boundaries is called the littoral zone (see 

 "Introduction to the Subtidal and Intertidal Subsystem" above) . The permanent 

 residents of this zone are principally macroalgae and benthic invertebrates. 

 At high tide, fishes from the subtidal zone move in to forage and at low tide 

 the zone is utilized by shorebirds, waterfowl, wading birds, and mammals. The 

 permanent intertidal residents are mainly of marine origin and can be 

 considered to be displaced from the subtidal zone. 



Physical factors affecting the distribution of organisms of the intertidal 

 zone are: tide, temperature, waves, nature and angle of substratum, available 

 sunlight, fog, and size of the intertidal zone (see 

 "Physical/Biogeochemical/Biological Interactions" above). 



The intertidal zone can be considered a stress gradient, running from low 

 stress near extreme low water spring tides to very high stress as extreme high 

 water spring tides are approached. The distance that plants and animals 

 penetrate upwards in the intertidal zone is determined to a significant degree 

 by their resistance to the stresses there, which include extreme cold and 

 heat, extreme drying by wind, and osmotic stress from heavy rain (i.e., fresh 

 water) . 



Resistance to stresses of the intertidal zone varies among species. Few 

 species can tolerate the upper levels of the intertidal zone. The ribbed 

 mussel is an example of such a tolerant species in Maine. Some individuals of 

 this species may be reached by the sea only during a few tides each month. 



The lower limit of a species' distribution in the intertidal zone is set by 

 biological factors, such as competition or predation. Although intertidal 

 animals have adapted to stressful physical conditions in the intertidal zone, 

 many are not able to avoid being preyed upon or to compete for space and food 

 with the less adaptable but more specialized subtidal animals. 



4-76 



