from the feces of various organisms and by flocculation (clumping together) of 

 dissolved compounds. 



Fine particulate organic matter and the bacteria which colonize it are eaten 

 by invertebrates known as collectors, which either filter the particles from 

 the water or gather it from the sediments (figure 6-3). Typical Maine 

 collectors (table 6-4) include larval blackflies (filter feeders), and larval 

 chironomid midges (gatherers). Collectors generally select their food on the 

 basis of size and do not specifically select bacteria-enriched particles 

 (Cummins and Spengler 1978). 



Metabolic wastes of shredders, collectors, and other organisms, along with 

 material leached directly from aquatic debris and the terrestrial environment, 

 contribute to the supply of dissolved organic matter. This material is an 

 important source of nutrients, including nitrogen and phosphorus. In upper 

 perennial reaches, most of these nutrients support growth of bacteria and 

 fungi. Photosynthetic activity in streams is limited by forest canopy 

 shading. Invertebrates that are dependent upon the limited food supply 

 provided by aquatic plants in upper perennial regions are referred to as 

 scrapers (e.g., snails; table 6-4). 



Stream predators include vertebrates, primarily fish (table 6-5 ; e.g., 

 sculpins and brook trout) and invertebrates, such as some stoneflies and 

 caddisflies. These predators consume shredders, collectors, grazers, and 

 other predators. Terrestrial predators including many birds (e.g., waterfowl, 

 herons, kingfishers, and ospreys) and some mammals (e.g., otters) also prey on 

 stream organisms. People consume some fishes (i.e., resident sport fish). 

 Human consumption accounts for <l7o of the total biomass of the stream animals 

 in the upper reaches of the stream (Cummins and Spengler 1978). 



The biological community in the lower reaches (lower perennial, tidal) of 

 streams is less dependent on direct organic inputs from the terrestrial 

 environment and more dependent on instream photosynthesis and import of fine 

 organic particles from upstream (Marzolf 1978) 



Aquatic plants, including planktonic algae (e.g., the brown alga Dinobryon) 

 and rooted vegetation (e.g., the river bulrush Scirpus ) are more common in 

 lower reaches and require greater inputs of nutrients (e.g., nitrogen, 

 phosphorus) either directly from terrestrial sources (e.g., leaching) or from 

 dissolved material processed by the upstream community. Since plants are more 

 dominant in the lower perennial reaches of stream systems so are grazers, 

 (equivalent to upstream scrapers) which feed on them (figure 6-4). The lower 

 reaches of streams have a well-developed zooplankton community (primarily 

 collectors) that feed on planktonic algae. Most zooplankton are collectors, 

 although some of the larger copepods may be predatory. Fishes that feed on 

 zooplankton (e.g., alewives) and fish predators (e.g., chain pickerel) are 

 more important in the lower reaches of streams than in the upper reaches. 



In contrast to that of standing water systems much of the organic matter 

 (living and nonliving) in stream systems is exported downstream from its point 

 of origin for varying distances before being consumed. Very coarse particles, 

 such as leaves, are normally trapped by obstructions within 300 feet (100 m) 

 of the point where they enter the stream (Cummins and Spengler 1978). Benthic 

 macroinvertebrates , which are frequently swept up by the current, normally 



6-21 



10-80 



