water. In these areas significant depletion of oxygen may occur if winter ice 

 blocks gas exchange between the water and the air and if snowcover on the ice 

 interferes with light penetration and, thus, reduces photosynthesis. Low 

 rates of flow may reduce oxygen replenishment from upstream and exacerbate the 

 depletion problem. In addition, oxygen depletion may occur in bottom waters 

 when large amounts of decaying vegetation or other organic matter (e.g., 

 domestic or industrial wastes) are present and decomposer organism activity 

 consumes oxygen. Saturation levels of oxygen in fresh water at sea level 

 range from 14.6 ppm at 320F (QOC) to 7 . 6 ppm at 86°F (30°C) (temperatures in 

 excess of 86°F, or 30°C, are unusual in Maine streams). 



Carbon dioxide from deep rock formations enters the stream via groundwater 

 discharge. Decomposition of organic matter in the soil produces carbon 

 dioxide that enters the stream via surface runoff or throughflow. Organic 

 decomposition and plant and animal respiration within the stream itself also 

 yields CO2 (Golterman 1975). Dissolved carbon dioxide is essential for 

 photosynthetic activity in submerged plants. In combination with carbonates 

 it plays a major role in regulating the pH (acidity) of streams. In addition 

 to direct diffusion into the atmosphere, buffering and photosynthesis are the 

 primary means through which carbon dioxide is removed from the aquatic system. 



ENERGY FLOWS 



In forested regions such as the State of Maine, shading by the forest canopy 

 limits the growth of aquatic vegetation (primarily attached algae and aquatic 

 mosses) to the extent that only about 10% of the organic matter supply of 

 headwater (upper perennial, intermittent) streams is derived from instream 

 photosynthesis (Cummins and Spengler 1978). The major energy source in these 

 streams is coarse particulate organic material (e.g., leaves, twij,s) derived 

 from trees and other streamside vegetation. The use and breakdown of this 

 coarse material into fine, particulate, and dissolved material involves 

 considerable biological activity. The role of an organism in the structure of 

 the stream community is largely determined by its role in organic matter 

 processing (table 6-4, figure 6-3). 



The first step in the processing of coarse organic material is a primarily 

 physical process, in which leaves or other material loses dissolved components 

 through leaching. Thirty to 40% of the dry weight of some types of leaves may 

 be lost in this manner (Cummins and Spengler 1978). These dissolved organic 

 compounds are a source of nutrients for stream bacteria. The leached leaves 

 then are colonized by stream bacteria and fungi. Hyphomycete fungi are 

 particularly important, since they are capable of breaking down cellulose and, 

 in some cases, lignin, which are major components of leached leaves and woody 

 debris (Cummins and Spengler 1978). Different types of organic debris are 

 colonized at different rates. Material from deciduous trees usually is 

 colonized more rapidly than that from conifers (Cummins and Spengler 1978; and 

 Marzolf 1978). Invertebrates that feed on leafy or woody debris are referred 

 to as shredders. These animals frequently select the most heavily colonized 

 debris as food, presumably because the bacteria and fungi add to the available 

 prote-:p (Cummins and Spengler 1978; and Marzolf 1978). Typical shredders in 

 Maine streams include cranefly larvae, caddisfly larvae, and stonefly nymphs. 

 Microbial metabolism, shredder feeding, and mechanical breakage serve to 

 convert coarse particulate organic matter into fine particles and dissolved 

 compounds (Cummins and Spengler 1978). Fine particles also may be derived 



6-16 



