Macroinvertebrates are animals which inhabit the substratum of streams, lakes, and other waters. They are considered by definition to be visible to the unaided eye and are retained by Standard No. 30 mesh netting. The composition density (number of individuals per unit area) and relative abundance of each macroinvertebrate taxa in streams are useful indicators of envirdnmental quality. Macroinvertebrate response to organic loading and substrate alteration have been well documented. Severe organic pollution usually results in a restriction of the diversity of macroinvertebrates to only the most pollution tolerant and a corresponding increase in the density of tolerant organisms. Siltation and chemical pollution may not only reduce but eliminate the entire community from affected areas. A common method of assessing the impact of a possible pollution source is to compare macroinvertebrate community structures from suspect and unaffected areas.
The latest comprehensive macroinvertebrate survey of Strawberry Creek was done by Stephens during October-November 1986. Four to fifteen samples were taken from midstream in both riffle and pool areas at each of eight locations (Figure 13). Substrate consisted primarily of mixed gravel, pebbles, and cobbles. Samples were taken using a one minute kick method. A triangular D-frame net was placed against the creek bottom and the substrate was subsequently disturbed three feet upstream for one minute to dislodge the macroinvertebrates.
The results of the survey me presented in Table 18. The three upper South Fork sites above the retention dam had the highest diversity (number of species), and were characterized predominantly by animals that occur in unstressed environments. The site above the Botanical Garden (#8) had the greatest abundance of organisms found in the survey and was dominated by the caddisfly Lepidostoma sp. The other two canyon sites (#6 and #7) were both dominated by the stonefly Amphinemura sp.
The lower South Fork sites (#4 and #5) on central campus exhibited much lower diversity and had the lowest abundance of any of the sites sampled on both forks of the creek. Less than half as many species and less than one-fifth the total number of organisms were observed here as compared to the canyon sites. More pollution tolerant organisms generally dominated these sites. These are organisms that can frequently tolerate mcxierate levels of organic pollution. The largest form of aquatic life observed by the author in the South Fork over the past year has been adult crayfish. It is evident from this analysis that the macroinvertebrate community is stressed in the South Fork on campus.
The North Fork was sampled both on campus and in the city below La Loma Avenue (#2 and #3, respectively). Diversity and abundance of macroinvertebrates were slightly better than the South Fork on campus, but still much lower than in the canyon. These communities were made up of facultative and intolerant forms. Orthocladius sp. and Argia sp. were the dominant organisms on campus and in the city, respectively. Oligochaetes, aquatic earthworms, were· commonly found in the North Fork, especially on campus. An abundance of Oligochaetes often indicates a stressed condition. Oligochaetes are favored by organic enrichment and are often the dominant invertebrates in severely polluted areas. The most highly stressed community was sampled in the main branch of Strawberry Creek above Oxford Street. Only four different kinds of organisms were observed here. Some of these organisms are indicators of organic contamination. This site had the lowest diversity and abundance of any of the sites sampled. Either discharges in the lower reaches of Strawberry Creek or habitat destruction, or possibly a combination of these two factors, could produce the highly stressed condition exhibited by the macroinvertebrate community upstream of Oxford Street.