Mud flats are the dominant intertidal habitat in the estuaries of coastal 

 Maine. They are the fine-grained equivalent of sand flats, originating from 

 the deposition in the intertidal zone of sand, silt, and clay particles by 

 rivers and offshore waters (Schnitker 1974) , and through the erosion of 

 adjacent shorelines. 



Mud flats consist primarily of silt particles with some fine sand and clay 

 intermixed. The sediments and their clay mineral compositions are derived 

 from subaerial and subtidal glacio-marine clays and silts of the Presumpscot 

 Formation (Bloom 1963; and Timson, in preparation ) . Mud flats also contain a 

 small amount of organic detritus but generally <5% by weight (Timson, 

 unpublished ) . The organic fractions of a mud flat are usually bound to fecal 

 pellets, agglomerates, and flocculates. Agglomerates are clay particles, 

 irregular in shape, that are bound by organic material. Flocculates are clay 

 particles bound primarily by electrostatic forces (Zabawa 1978) . 



Grain size over the mud flat surface generally decreases landward, unless 

 significant volumes of sediment have been contributed by shoreline erosion. 

 Grain size decreases from tidal channels toward the periphery of the flat. 

 This distribution is caused by the movement of water traveling over tidal 

 flats during a tidal cycle (Postma 1967; and Van Straatan and Kuenan 1957). 



Transport by suspension is the primary process of sedimentation on the flats 

 and much of the suspended sediments are derived directly from the flats 

 themselves (Anderson 1973; and Schnitker 1974). What is eroded from the flats 

 at high water usually is redeposited elsewhere on the flat by the ensuing ebb 

 tide. Currently, mud flats in Maine probably are accreting at a rate of 1.9 

 to 2.8 cm/year (Schnitker 1972). 



Sediment transport and deposition within tidal channels that border or drain 

 mud flats occurs through suspension and bedload transport. The most coarse 

 sediments (fine to very fine sand) in mud flats are found on the floor of 

 tidal channels. Coarser sediments also are found in association with mussel 

 banks flanking large tidal channels that border on mud flats. Significant 

 quantities of sand and shell particles compose sediments that act as a 

 substratum for the banks (Timson, unpublished ) . 



The dominant natural processes affecting mud flats are tidal currents, local 

 wave conditions, and biological activities such as the burrowing of 

 invertebrates. Tidal currents and waves suspend and redistribute particulate 

 matter over the flat surface. The activities of organisms on the sediment 

 surface mix sediment layers directly on the flats. 



Mud flats do not provide a stable substratum for the attachment of the large 

 macroalgae, but protected waters often may permit the seasonal development of 

 highly productive small plants, including green algae such as Enteromorpha . 

 Dense populations of dinof lagellates and diatoms also may appear. Stable 

 substrata permit the occasional development of isolated larger plants, but in 

 general and excepting the benthic microflora the habitat depends on imported 

 organic material to support its rich fauna. 



Mud flats support a diverse population of invertebrates and many species 

 develop large populations only in this habitat (Larsen and Doggett 1978a). 



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