generalization occured in sand flats with unstable sediment surfaces, which 

 were characterized by pronounced sand waves. At Heal Eddy, a sand flat in 

 Georgetown (region 2), Larsen and Doggett (1978a) found that sand wave areas 

 were dominated by the amphipods Acanthohaustorius millsi , a suspension feeder, 

 and Psammonyx nobilis , an omnivore (Croker 1977). Deposit feeders are 

 uncommon in areas with ripples, because the instability of the sediment 

 prevents the establishment of unicellular algae (Sanders et al. 1962). 



Predators on Maine's intertidal flats include the green crab, the moon snail, 

 the sand shrimp, and the sandworm. At high tide the flats are utilized by 

 bottom-feeding fish. Shorebirds feed on the exposed flats (low tide) 

 especially during fall migration. 



The primary waterbird groups feeding on intertidal flats include shorebirds, 

 gulls, wading birds, and waterfowl (dabbling ducks, geese, and brant). 

 Herring gulls, great black-backed gulls, and black ducks use flats throughout 

 the year. Estuarine flats are especially important habitat for migratory and 

 wintering black ducks. Shorebirds, Bonapartes gulls, and ring-billed gulls, 

 are more abundant on intertidal flats in fall rather than in spring. In 

 contrast, brant are most abundant during spring migration. Wading birds use 

 mudflats throughout spring, fall, and summer. 



Flats are critically important for shorebirds because of their almost obligate 

 dependence on the coastal habitats as refueling and resting areas during their 

 long migration from the Arctic to South America. Shorebirds occur there in 

 very large numbers in relatively small areas, which makes them susceptible to 

 environmental contaminants and habitat deterioration. 



Mud and sand flats along the southwest New Brunswick and eastern Maine 

 (regions 5 and 6) coasts are especially important to migrating semipalmated 

 plovers. Numbers between 1000 and 2400 have been reported in many areas in 

 this region. These same flats are important feeding areas for semipalmated 

 sandpipers migrating in fall (see sand beaches above for a discussion of their 

 preferred roosting areas). Large numbers of short-billed dowitchers and 

 black-bellied plovers feed on Maine flats. 



Sand flats usually occur seaward of sand or gravel beaches in which active 

 erosion of shore deposits supplies sand to the intertidal zone. Isolated, 

 irregularly shaped sand deposits may occur as flats adjacent to tidal 

 channels, as levee deposits, or as bar deposits in the insides of channel 

 bends . 



The composition of sand flats ranges from medium sand to silty sand (Timson, 

 unpublished ) , but some areas also may have some pebbly sands (gravel) and 

 coarse sands toward the shoreward end of the sand flat. Sediments of sand 

 flats are generally poorly sorted. Muds are periodically deposited over the 

 sand during low wave activity, or coarser particles are moved seaward over 

 fine-grained deposits during storms. Intertidal organisms also mix sediment 

 layers after deposition. Under conditions of a rising sea level, such as 

 exists on the Maine coast, sand flats migrate shoreward as beaches recede and 

 mud deposition covers the seaward margins of the sand flat. 



Sand flats of the bar variety occur adjacent to tidal channels in which 

 current velocities are strong enough to transport sand as bedload. The sand 



5-101 



10-80 



