Laserfiche WebLink
The number of species of invertebrates in pools is usually limited by <br /> the fact that pools exhibit reduced algae production (decreased sunlight), <br />1 homogeneous silt-covered substrate and the possibility of local oxygen <br /> deficits (low current combined with decay of organic matter). The organisms <br /> that dominated the pools at Middle Creek thrive in this habitat. Both <br /> Chironomidae Form 8 and the tubificid worm Limnodrilus are adapted to pools. <br /> They prefer soft sediments and possess hemoglobin in their blood, which <br /> allows them to withstand short periods without oxygen. The elmid beetle, <br /> Dubiraphia quadrinotata, was abundant in the pools at Station 2. These <br /> I beetles are common in the slow moving sections of streams as well as ponds <br /> (Brown, 1972). <br />I Table 1 also presents the number of species and species diversity (D) <br /> for the stations on Middle Creek. The number of taxa in the riffles <br /> decreased from 22 at Station 1 to 15 at Stations 2 and 3 due primarily to <br /> fewer species of mayflies at the lower stations. Species diversity was <br /> calculated to measure the quality of the environment of Middle Creek. The <br /> EPA encourages the use of the Shannon-Wiener Index (D) to determine the <br /> effects of stress on the invertebrate community (Weber 1973). This index is <br />based upon the premise that undisturbed environments will support communities <br /> that possess large numbers of taxa with no one taxon dominating, i. e., few <br />4 <br />1 species with large numbers of individuals and many species with fewer indi- <br /> viduals. Increased stress generally eliminates some of the rare species, <br /> 1 decreases the abundance of intolerant species and leads to increased density <br /> of the tolerant forms no longer affected by interspecific competition (Mathis <br /> 1908). Diversity values from riffle areas in Middle Creek decreased slightly <br /> 1 at Station 2. However, values of 2.50 to 3.50 are typical in streams in the <br /> Rocky Mountain region. Seriously stressed ,aquatic environments would be <br /> expected to exhibit diversity values of less than 1.00 in riffle areas. <br /> Riffles tend to be "richer" sources of invertebrates in terms of numbers <br /> of species than pools (Hynes 1970). This is borne out in Middle Creek. <br /> Although pool densities are high at all three stations, numbers of taxa and <br />1 species diversity (D) are both much lower than those in the riffle sections. <br />• <br />-13- <br />