STABLE CARBON AND NITROGEN ISOTOPE TRACERS OF TROPHIC DYNAMICS IN NATURAL-POPULATIONS AND FISHERIES OF THE LAHONTAN LAKE SYSTEM, NEVADA

The influence of combined inorganic N on the growth of N2-fixing blue-green algae (Aphanizomenon fos-aquae) in Lahontan Reservoir was detected with N isotopic compositions (.delta.15N = +1.2 to +7.0) that were enriched in 15N. In Pyramid Lake, however, the .delta.15N of the entire food web was influenced by a contribution of isotopically light N released from N2-fixing blue-green algae (Nodularia spumigena). Carbon isotope ratios (.delta.13C) of these unplatable, blue-green algae from both lakes were different enough from the zooplankton and higher animals to preclude a direct trophic link. An enrichment in 13C with trophic level in the food chain was measured in both lakes. Carbon isotope ratio measurements clearly illustrated the isotopic similarity between hatchery-raised Lahontan cutthroat trout (Salmo clarki henshawi) and cui-ui (Chasmites cujus) and the artifical diet, but differed from the .delta.13C of their wild counterparts consuming natural foods. There is a consistent isotope fractionation between the .delta.13C of scales and the .delta.13C of muscle from fish that is species specific. This finding demonstrates that fish scales, as well as muscle, can be used to determine diet.