relationships between stream order (Leopold et. al, 1964, Hynes 1970), size 

 of organic matter, and production-respiration (P/R) ratios. Stream order 

 employs an ordinal scale to describe stream characteristics. Streams of orders 

 1, 2, or 3, for example, are headwaters streams with few or no tributaries 

 (figure 17). 



Headwater streams characteristically receive substantial terrestrial 

 contributions (allochthonous) of organic matter, especially coarse particulate 

 organic matter (CPOM) such as leaf litter, with little or no photosynthetic 

 production of organic matter. The two categories of dominant macroconsumers 

 are detritivores (collectors) feeding on fine particulate organic matter (FPOM) 

 and CPOM-feeding invertebrates (shredders). Thus, a headwaters food chain can 

 be described as: CPOM--fungi--shredders--FPOM--bacteria--collectors 

 (figures 17 and 18). 



Food chains in intermediate-sized rivers are less dependent upon allochthonous 

 inputs and more on organic production by producer organisms along with input 

 of FPOM from upstream. The ratio of photosynthetic production to community 

 respiration is often greater than one (P/R>1) in contrast to headwater and 

 large rivers where P/R < 1 (figure 17). 



Large rivers tend to be turbid with heavy sediment loads, the culmination 

 of all upstream processes. These systems, which possess plankton communities, 

 could be characterized by their food chains: FPOM--bacteria--collectors (figure 

 17). 



Fish populations generally show a downstream transition from cold-water 

 invertivores to warm-water invertivores and from piscivores to planktivores. 



A more autecological approach to distribution of aquatic invertebrates 

 in aquatic ecosystems investigates the distribution and abundance of stream- 

 dwelling invertebrates as regulated by such factors as current speed, temp- 

 erature, substrata, vegetation, and dissolved substances (Hynes 1970); others 

 are competition, zoogeography, and food. 



Temperature and water chemistry usually exert the greatest influence on 

 the composition of living communities considered over large areas, but because 

 of feeding and respiratory requirements, it is largely current that determines 

 how local communities actually are composed (Jaag and Ambuhl 1964, Chutter 1969). 

 In fact, some macroinvertebrate species are confined to fairly narrow ranges 

 of current speed. As an example, in the case of the net-building caddisflies 

 (e.g., Hydropsyche, Cheumatopsyohe, Parapsyche) , the nets require a definite 

 current in order for them to function properly (Philipson 1954). Many organisms 

 must function in proximity to a specific current but cannot tolerate being 

 actually in it. There is often great variation in current velocity for an 

 insect living on top of a rock compared with one living under that rock, yet 

 both may have current requirements. Because of the impossibility of taking 

 measurements at most places macroinvertebrates inhabit (such as under rocks), 

 current velocity is usually measured at some reproducible depth, e.g., mid-depth, 

 six-tenths of total depth, or near the bottom (Hynes 1970). 



There are unmistakable high-current specialists (e.g., Baetis, Simulium, 

 and Hydropsyche), while some organisms find optimum habitat at low velocities 

 (e.g., Gammarus, Hyalella, Trioorythodes) . Each species prefers a certain range 

 of current velocity. 



27 



