amounts of organic pollutants, communities must be capable of withstanding 

 chronic oxygen shortage. Many of the species composing such communities are 

 typical of standing water systems (lacustrine and palustrine) , where low 

 oxygen conditions are more common (Hynes 1970a). Some stream invertebrates 

 are capable of tolerating low and varying oxygen conditions. Typical of this 

 type of organism in Maine are some chironomid midge larvae and some aquatic 

 worms (e.g., Tubif icidae ) . 



Low dissolved-nutrient levels may limit the growth of aquatic plants and 

 animals. Submerged plants (e.g., aquatic mosses) and floating plants (e.g., 

 duckweed, Potamageton ) are frequently dependent on dissolved nitrogen, 

 phosphorus, and potassium, and low levels may limit growth. Effects of 

 nutrient supply on aquatic plants are difficult to document (Westlake 1975) 

 and quantitative information on these effects in running water is extremely 

 rare (Hynes 1970a). Available information differs on the levels at which 

 nutrient supply may become limiting in any given running water plant 

 community. Animals that use calcium carbonates in their exoskeletons , 

 including crustaceans (e.g., amphipods and crayfish) and molluscs (e.g., 

 snails and clams), are sensitive to deficiencies in these nutrients and are 

 generally less common in soft water, where these nutrients are less abundant 

 (Hynes 1970a). At least one freshwater snail (Ancylus fluviatilus ) , cannot 

 tolerate waters with less than 2 mg/1 of calcium (Maitland 1966). The notable 

 absence of amphipods from many apparently suitable Maine streams (personal 

 communication from C. F. Rabeni , University of Maine, Orono, ME; March, 1979) 

 may be related to the low level of calcium carbonate in Maine waters. Acidity 

 (pH) may influence plant growth. Acid waters tend to favor the growth of 

 algae such as the conjugales (e.g., Spirogyra ) and mosses (Westlake 1975). 

 Very acidic water may endanger animal populations through interference with 

 feeding and reproduction. Extremely high acidities may be directly lethal. 

 Exact levels at which these problems occur vary according to species and local 

 conditions. However, in Maine, problems are most likely to occur at pH <5 . A 

 discussion of the effects of acid rain is presented in chapter 3, "Human 

 Impacts on the Ecosystem." 



Light and Temperature 



Light has a direct effect on the type and amount of vegetation present in any 

 given stream community. Aquatic mosses, for example, prefer low light levels 

 and are frequently the predominant vegetation (along with encrusting algae) in 

 shaded streams. Emergent vegetation is usually more common in streams in 

 summer. The total amount of vegetation present generally increases with the 

 amount of light reaching the stream. The type and amount of vegetation, in 

 turn, determines the relative dominance of scraper-grazer types of organisms 

 in the animal community. 



The amount of light penetrating water influences its temperature. Stream 

 sources (e.g., spring or pond) also influence water temperature. Many stream 

 organisms have definite temperature requirements. Stoneflies, an important 

 food source for game fish in many Maine streams, are generally limited to cool 

 headwaters (Hynes 1970a). 



6-29 



10-80 



