Taxonomic & Functional Organization of an Eastern Highland Rim Stream, A Comparison of Macroinvertebrate Assemblages on Riffle & Wood Habitats

Aquatic insects were collected from coarse woody debris (CWD) and riffle habitats in Trammel Fork Drakes Creek, a fifth-order south-central Kentucky stream between March 2000 and April 2001. This research tested the following hypotheses: 1) there were no significant differences in the physical and chemical environments between riffle and CWD habitats; 2) aquatic insect densities and functional feeding group ratios were greater on CWD than on riffle habitats because CWD represented a more stable habitat; and 3) hydrologic disturbances influenced the distribution of aquatic insect densities between the two habitats by causing a shift from the less stable riffle habitats to the more stable CWD habitat. Plecoptera species size frequency histograms were analyzed to determine if any species undergo habitat shifts between CWD and riffle habitats during their life cycle. There were no significant differences in pH (p=0.23), Conductivity (p=0.84), Temperature (p=0.67) or Dissolved oxygen (p=0.91) concentrations between CWD and riffle habitats. In contrast, CWD and riffle habitats were significantly different according to the depth and velocity where samples were taken. A total of 47,310 individuals were collected from 116 samples. Total aquatic insect density was significantly higher on CWD A (aerial coverage of stream bed) and significantly lower on CWD SA (CWD surface area). The densities of Diptera and Plecoptera were significantly higher on CWD A and CWD SA than riffle habitat. Densities of Trichoptera and a grouping of other orders collected (Hemiptera, Megaloptera, Odonata and Lepidoptera) were not significantly different for either CWD A or CWD SA habitats. Ephemeroptera densities were not significantly different on CWD A but were significantly lower on CWD SA. Of the thirteen Ephemeroptera genera collected, four had significantly higher densities on riffle habitats and one genera was significantly higher on CWD. Coleoptera showed the opposite trend with CWD A densities significantly different than riffle and CWD SA densities not significantly different than riffle densities. Mean substrate stability calculated as the Functional Feeding Group (FFG) ratio of scrapers (SCR) plus filtering collectors (FC) to shredders (SCR) plus gathering collectors (GC) was significantly higher on CWD than riffle habitats. This study highlights the importance of CWD in a middle order stream ecosystem in the highland Rim Level III Ecoregion. CWD has long been known to be an important component of smaller streams but was thought to be relatively unimportant in larger streams. Larger streams were generally thought to contain enough stream power to wash out CWD, especially large streams with higher gradients. However, in coastal blackwater streams CWD was found to be an important component of the stream ecosystem by providing stable substrate for aquatic insect production (Benke et al 1984). This research extends the importance of CWD to low gradient streams in other ecoregions