certain Maine rivers are listed in table 6-6. Certain fishes may occur in 

 streams included in table 6-6 but are not listed as their occurrence has not 

 been reported (see also "Fishes," chapter 11). 



Waterbirds, mammals, reptiles, and amphibians also inhabit the riverine 

 system. Several species of waterbirds, including waterfowl, shorebirds, and 

 wading birds utilize riverine systems for feeding, resting, breeding, and/or 

 rearing habitats (see chapter 14, "Waterbirds," and chapter 15, "Waterfowl"). 

 Mammals, including mink, otter, muskrat, beaver, raccoon, fox, deer, and moose 

 also utilize riverine habitats for their feeding (see "Terrestrial Mammals," 

 chapter 17). Birds, mammals, reptiles, and amphibians that inhabit riverine 

 systems also utilize palustrine habitats as these systems are often 

 interspersed. The occurence of these species in coastal Maine is given in the 

 appendices to chapter 8, "The Palustrine System". 



Decomposers 



The significance of detritivores in the riverine system has been discussed 

 under "Energy Flow," in this chapter. The abundance of allochthonous material 

 supports a significant cycle within this group. Organic matter is consumed, 

 excreted, re-colonized by microbes, re-consumed, and so on. Breakdown of the 

 detritus is especially important to filter feeders because they are dependent 

 on the limited supplies of plankton and microparticles of detritus for their 

 food. The existing decomposer fauna of riverine systems in coastal Maine is 

 not precisely known; however, assumptions may be made regarding those present 

 in some substrate habitats (see table 6-4) . 



NATURAL FACTORS AFFECTING DISTRIBUTION 



Many factors influence the distribution of aquatic organisms. The overriding 

 factors that determine the types of organisms that compose the biological 

 stream community include water chemistry, light and related temperature, 

 current speed, and related substrate factors (including detritus and 

 sedimentation) . 



Water Chemistry 



Dissolved oxygen levels can be limiting to aerobic organisms if they become 

 extremely low, although this is a rare occurrence in most Maine streams. 

 Reduced levels of dissolved oxygen can be a problem primarily in lower 

 perennial and tidal reaches of streams. The U.S. Environmental Protection 

 Agency (EPA) has encountered difficulty in setting water quality standards for 

 dissolved oxygen, because of lack of definitive data on fish oxygen 

 requirements (Everhart et al. 1975). It is generally agreed that dissolved 

 oxygen levels of less than 5 to 6 ppm would be too low for long-term survival 

 of coldwater fishes (e.g., salmonids). Many factors could cause fish oxygen 

 requirements to be higher than this level (Everhart et al. 1975), however. 

 Factors involved in determining actual oxygen requirements of fishes at any 

 specific site include current velocity, water temperature, age of fishes, 

 growth rate of fishes, spawning condition, migration, predation pressure, 

 exposure to pollutants or disease organisms, and exposure to varying water 

 chemistry (e.g., acidity). Under conditions where moderately low oxygen 

 levels recur frequently, as in shallow bays, areas with sluggish current where 

 large amounts of vegetation collect and decay, or waters receiving large 



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