Intertidal flats. Intertidal flats are the most characteristic habitat of 

 the estuarine system in Maine. The preliminary NWI data show 66% (41,650 

 acres; 16,862 ha) of the intertidal estuarine subsystem of the 

 characterization area is composed of flats. In Maine, intertidal flats are 

 principally sand or mud, with some cobble/gravel. 



Mud flats develop in relatively sheltered waters where suspended sediments are 

 deposited during slack (high or low) tide periods. Most intertidal flat 

 sediments originate from offshore, from coastal river sediment loads, or from 

 other flats (Schnitker 1974; see "Geology" above.) 



Sand flats are found in areas that are exposed to greater tidal and wave 

 energy than mud flats. Sand flats are caused by deposition of sand carried by 

 tidal or wave-generated currents. The sand flats at Sagadahoc Bay and Heal 

 Eddy, Georgetown (region 2) are good examples of areas exposed to wave and 

 current action. Areas just a short distance away in the Kennebec Estuary are 

 less subject to waves and currents and are composed of mud. 



Stable macroalgal communities do not develop on intertidal flats because of 

 the lack of suitable substratum. Ephemeral algae, however, such as the sea 

 lettuce, Ulva, and the green algae, Enteromorpha , sometimes bloom on flats, 

 but their contribution to the total production of the system is unknown. 



The principal difference between sand and mud flats is that mud is more 

 anaerobic than sand. The production of "new" organic matter usually is 

 limited to benthic diatom productivity on the flats, yet the influx of organic 

 matter from the water column or from shore and its modification by benthic 

 fauna is relatively large. Metabolic wastes of the fauna and buried plant 

 material provide a rich substratum for microheterotrophic growth. In fact, 

 certain species of diatoms found in surface layers of the sediment appear to 

 have acquired partial heterotrophic capabilities. During their growth, 

 microheterotrophs (certain diatoms, bacteria, and fungi) may cause aggregation 

 of particles, which helps to stabilize bottom sediments. Through 

 decomposition by bacteria and fungi, much of the organic matter in the flats 

 is broken down and remineralized. In areas that do not receive much organic 

 matter, microbial activity may be at a level high enough to prevent the 

 accu-mulation of organic matter. More frequently, however, an excess of 

 organic matter is present on the flats and the vigorous microbial consumption 

 of oxygen often leads to the formation of anaerobic zones where fermentation 

 as well as desulfurication occur. The latter reaction has a lasting effect on 

 sediment diagenesis, that is, the hydrogen sulfide released during sulfate 

 reduction combines with iron to form ferrous sulfide, and black "sulfide mud" 

 is produced. Anaerobic decomposition of proteins also leads to the production 

 of ammonia. This ammonia may diffuse out of the sediments and contribute a 

 nutrient source to the water column or may be oxidized to form N 

 (denitrification) which cannot be used by producer organisms. In regions 

 bordering on aerobic zones sulfur-oxidizing bacteria again may contribute to 

 the oxidation of sulfides. 



Flats are especially important environments, not only because of the rich and 

 diverse fauna that they support but also because several of the invertebrate 

 species that live there are utilized commercially. The harvest of clams, 

 sandworms, and bloodworms, among the State's most valuable fisheries, is 

 centered on flats (see chapter 12, "Commercially Important Invertebrates" for 



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