TROPHIC LINKAGES IN THE LOWER COLORADO RIVER - MULTIPLE STABLE-ISOTOPE EVIDENCE

Trophic linkages in Glen and Grand Canyons of the lower Colorado River downstream from Glen Canyon Dam were examined using multiple stable i sotope analysis. The deltaC-13 values of dissolved inorganic carbon (D IC), and the deltaC-13, deltaN-15, and deltaS-34 values of seston, aqu atic and terrestrial plants, and aquatic animals were determined. The deltaC-13 value of DIC varied among sites. DIC from the epilimnion of the reservoir (Lake Powell) and from a tributary was more C-13-enriche d than DIC in the Colorado River, probably as a result of variation in aquatic primary production and dissolution of carbonate among sites. Four potential bases of aquatic secondary production: upland vegetatio n, riparian vegetation, reservoir plankton, and benthic algae were iso topically (deltaC-13 and deltaN-15) distinct from each other. Analysis of deltaC-13, deltaN-15, and deltaS-34 showed that seston from the da m tailwater (Glen Canyon) consisted of lotic algae and zooplankton fro m Lake Powell, except for the ultra fine fraction (<0.053 mm) which wa s derived from Lake Powell particulate organic matter. Longitudinal va riation in the composition of Glen Canyon seston was generally small. Seston from a tributary (the Paria River) was derived from a mixture o f upland and riparian vegetation and was isotopically distinct from Co lorado River seston. Isotope analysis revealed three trophic levels in Glen Canyon: algae (Cladophora glomerata and diatoms), macroinvertebr ates (e.g., Gammarus lacustris and chironomids), and fish (primarily r ainbow trout, Oncorhynchus mykiss). Trout also consumed zooplankton ex ported from Lake Powell. Direct assimilation of algal N by trout was n ot indicated despite the high incidence of algae in trout stomachs. Is otope values of fishes (trout and speckled dace, Rhinichthys osculus) from Grand Canyon tributaries reflected variation in the trophic basis of fish production; one tributary fish population appeared to be supp orted by tributary autochthonous production or mainstem organic matter sources, and others were linked to riparian or upland organic matter inputs.