EFFECTS OF FLUCTUATING FLOW RATES 153 



washout by high discharge rates. High recruitment from hatching 

 eggs in May and June, followed by late summer emergence of other 

 cohorts of chironomids, would be characteristic of this pattern of 

 changing density. The two downstream tributary stations (P-2 and 

 C-2) appear to have a combination of both genera typical of the 

 larger, slower flowing Grand River and genera typical of small, 

 fast-flowing streams. 



The relatively high biomass and density of chironomid larvae in 

 most of our study area apparently result from a combination of 

 relatively nutrient-rich, stable substrate and slow-flowing waters. It is 

 difficult to distinguish natural fluctuations of flow rates and water 

 levels from managed fluctuations in this reservoir system. Other 

 investigators have noted significant stresses on chironomid popula- 

 tions as a result of both natural and regulated flow regimes. For 

 example, Spence and Hynes (1971) concluded that, relative to 

 stations farther downstream or upstream, increased numbers of 

 chironomids result from greater availability of detritus and uniform 

 flow below Shand Dam in the Grand River in Ontario. Unregulated 

 streams may also be affected strongly by flow-rate and water-level 

 fluctuations that occur after rapid runoff in foothill streams 

 (Siegfried and Knight, 1977) or during flooding after heavy rainfall 

 (Hoopes, 1974). 



Various types of adaptations give distinct advantages to partic- 

 ular species in habitats with fluctuating waters. The end result is a 

 high standing-crop biomass of only a few dominant species and a 

 general pattern of low diversity. These frequent disruptions in the 

 physical and chemical nature of the habitat are analogous to 

 observations in plankton communities that high species diversity 

 results from a "contemporgineous disequilibrium" of the open waters 

 (Richerson, Armstrong, and Goldman, 1970). Benthic habitats 

 exposed to intensely fluctuating waters are apparently characterized 

 by low diversity, however, because of the high degree of seasonal and 

 daily unpredictability in flow and because of the relative newness of 

 these types of frequently exposed and inundated substrates. Fisher 

 and LaVoy (1972) aptly termed these substrates freshwater "inter- 

 tidal" biotopes and pointed out that a high degree of "pulse 

 stability" is potentially attainable in some new habitats. Currently 

 the managed freshwater "intertidal beaches" appear unstable in 

 comparison with the naturally regulated streams and are dominated 

 by a very restricted number of taxa. The dominant chironomids 

 generally have high fecundity and are well adapted for dispersal both 

 as larvae and as airborne adults. Thus they can be viewed as typical 

 "r-selected" species (Baxter, 1977). 



