MACROINVERTEBRATE COMMUNITY STRUCTURE ALONG THE LONGITUDINAL GRADIENTOF AN AGRICULTURALLY IMPACTED STREAM

Lapwai Creek, an agriculturally impacted stream in northern Idaho, was sampled seasonally over a two-year period to determine if macroinvert ebrate community composition changed along the longitudinal gradient a nd ii changes followed predictions of the river continuum concept. Pos sible relationships between changes in food resource availability and community structure were also examined. Benthic invertebrates were col lected at eight locations along the longitudinal gradient of Lapwai Cr eek using a Hess sampler. Random skewer analysis suggested there was n o longitudinal gradient for either number of individuals or functional feeding group composition. Cluster analysis revealed that all locatio ns, excluding a site receiving outflow from a small, eutrophic reservo ir, had a similar community structure, further suggesting that inverte brate community composition remained consistent along the longitudinal gradient of the stream. The community was dominated at all sites, exc luding the site below the reservoir, by functional grazers. Shredders were rare throughout Lapwai Creek, even in areas where healthy riparia n vegetation stili remained. Studies of other streams within the drain age basin show that many species found in the upper reaches of these s treams; where agricultural impacts are low, were absent throughout the length of Lapwai Creek. Data collected concurrently with macroinverte brates indicated that the input, storage, and transport of particulate organic matter was low throughout the stream, whereas periphyton abun dance was high. The absence of longitudinal changes, despite flowing t hrough three distinct geomorphological regions, and the grouping of ai l sites except one by cluster analysis for both dominant taxa and func tional feeding groups suggest that agricultural alteration has influen ced community structure of Lapwai Creek, resulting in a relatively hom ogeneous assemblage of macroinvertebrates capable of tolerating agricu ltural nonpoint source pollution. Additional support for this hypothes is is the high abundance of one food source, periphyton, and the small quantities of terrestrially derived organic matter. The abundance of the former and the rarity of the latter can be attributed to alteratio n of the drainage basin resulting from agricultural activities through inputs of fertilizers that generated high nutrient concentrations and the removal of riparian vegetation to clear more land for agriculture and provide increase access to the stream.