CUMULATIVE EFFECTS OF RIPARIAN DISTURBANCES ALONG HIGH DESERT TROUT STREAMS OF THE JOHN DAY BASIN, OREGON

In a study of cumulative effects of riparian disturbance by grazing on the trophic structure of high desert trout streams, watersheds with g reater riparian canopy had higher standing crops of rainbow trout Onco rhynchus mykiss, lower daily maximum temperatures (range, 16-23-degree s-C compared with 26-31-degrees-C), and perennial flow. Watershed aspe ct influenced the response of trophic structure to grazing influences. Standing crops of rainbow trout were negatively correlated with solar radiation and maximum temperature in watersheds flowing northward. A different relationship was observed for a set of watersheds with a sou thern aspect, perhaps due to the presence of spring seeps and stream d esiccation in the heavily grazed stream. Trout biomass was negatively correlated with solar radiation, whereas positive relationships were f ound for discharge and depth. Algal biomass was positively correlated with solar insolation (r = 0.91), total invertebrate biomass (r = 0.77 ), and herbivorous invertebrate biomass (r = 0.79) in all watersheds. Invertebrate biomass was not significantly correlated with rainbow tro ut standing crop. High irradiance apparently resulted in increased alg al biomass and invertebrate abundance. However, temperature elevations to levels close to lethal may impose high metabolic costs on rainbow trout, which may offset higher food availability and affect the availa bility of prey.