Current and Substrate 



Many aquatic organisms have distinct current-speed requirements. Planktonic 

 algae and zooplankton require relatively still waters, since they have little 

 or no capacity for resisting currents. Other organisms require water that 

 moves at higher velocities, because of food-capturing or respiratory 

 requirements. Net-spinning caddisflies, important collectors, depend on water 

 currents to carry food into their nets. Many insects (stoneflies and 

 mayflies) and fishes depend primarily on water currents to move water over 

 their gills to allow exchange of respiratory gases. 



Current also affects the distribution of stream organisms through its effect 

 on substrate. Substrates of larger particles (e.g., stones) are normally 

 associated with more rapid current. Smaller particles (e.g., sand and silt) 

 are carried away and deposited in areas with slower currents. Among stream 

 plants, mosses and encrusting algae are associated primarily with stony 

 substrates, while emergent vegetation may require softer substrates for 

 rooting. Many benthic invertebrates exhibit substrate preferences (table 6- 

 4). Rabeni and Gibbs (1977) found that silt and detritus concentrations, 

 along with current (which controls the deposition of this material), are major 

 factors controlling the distribution of benthic invertebrates in the Penobscot 

 River. Most stream fishes show little direct preference for substrate types 

 during most of the year, although some (e.g., chain pickerel; table 6-5) may 

 be associated with vegetation, which is limited to certain substrate types. 

 Many fishes do have specific requirements for spawning substrates, however. 

 For example, salmonids require clean gravel and rapid current for spawning 

 beds (table 6-5 and chapter 11, "Fishes"). 



RESEARCH NEEDS 



Virtually all aspects of Maine riverine systems require more research for 

 management and impact assessment. Water quality is of primary concern. Data 

 are needed on the amounts of toxic materials (heavy metals, pesticides, 

 herbicides, oil, and PCB's) present in Maine streams and their effects (short 

 and long term) on populations of organisms. 



6-30 



