French Broad River Macroinvertebrates 35 



(1973), because tabulation of percent abundance of major taxonomic 

 groups requires identification only to the level of order. 



Many differences in abundance can be related to watershed size 

 and stream gradient. These factors were grouped and included in Table 

 6 when available. High percentages of Plecoptera (> 20%) were observed 

 only in streams with very small watersheds (SW-1 and BR-1). This phe- 

 nomenon was not evident at BC-1 (tributary to Beech Creek), a site that 

 was not sampled in the fall when most Plecoptera would probably be 

 present. In these very small first order streams, low flow rates allow 

 large accumulations of leaves. Such accumulations ("leaf packs") are the 

 preferred habitat of "shredder" Plecoptera genera, especially Allocap- 

 nia, Pel toper la and Taeniopteryx. 



High gradient streams with watersheds of from 2 to 4 km 2 (PC-1, 

 BL-1, and Cox-1) were strongly dominated by Ephemeroptera, espe- 

 cially Epeorus {Iron) and Rithrogenia. These streams are characterized 

 by many small waterfalls. Water velocity is sufficient to quickly remove 

 leaf accumulations and the substrate often has a very "clean" appear- 

 ance. We suspect that these watershed characteristics were also true for 

 those studied by Talak (1977). 



Larger streams (CF-1, TM-1, DR-1; and Tebo and Hassler 1961) 

 were dominated by a combination of Ephemeroptera and Trichoptera. 

 These streams are characterized by somewhat less canopy cover, greater 

 amounts of fine particulate organic matter, and higher flow. The com- 

 position of the benthic fauna for this group of streams is similar to that 

 described for mountain rivers in Table 3. 



Seasonal Trends 



Abundance: River Stations. — Abundance values showed large 

 between-year and between-station variability (Fig. 3). However, several 

 distinct trends may be observed. 



Organism abundance was usually greatest at station 4 and can be 

 correlated with substrate characteristics. Station 4 is located in an area 

 of heavy Podostemon (riverweed) growth. According to Anderson and 

 Sedell (1979), macrophytes may increase the density of benthic 

 macroinvertebrates although not serving as food. Macrophytes structur- 

 ally modify the river bed by increasing the available benthic area and 

 reducing flow. A reduction in flow in turn results in both less scour and 

 greater retention of detritus. This hypothesis is supported by the con- 

 vergence of abundance values (Fig. 3) at all stations during winter and 

 early spring, a period of Podostemon dieback. 



All stations appeared to have a "refuge" from scouring. Following 

 the 1979 spring flood (Fig. 1), the number of benthic invertebrates did 

 not appear much lower than those observed in the spring of 1978, de- 

 spite a lower 1978 flow. Extremely high flows appeared to have minimal 



