STABLE CARBON AND NITROGEN ISOTOPE ANALYSIS OF SESTON IN A REGULATED ROCKY-MOUNTAIN RIVER, USA

The deltaC-13 and deltaN-15 of seston, organic matter sources (riparia n vegetation, algae, macrophytes, plankton) and an invertebrate detrit ivore were examined in a regulated Rocky Mountain river (fourth order) . The isotopic composition of seston varied among sites and seasons fo r all size fractions; among-site variation was weakest for ultra-fine particulate organic matter (< 53 mum). Comminution of seston to ultra- fine size partially obscured isotopic differences among sites. Multipl e isotope signatures of seston were distinct among sites at different distances from a dam (0.5, 5.3, 8.7 km) and between the river and a th ird order tributary. Seston (> 53 mum) from 0.5 km below the dam was r elatively C-13-depleted (-30 to -28 parts per thousand), reflecting th e contribution of reservoir-derived plankton; seston from the site 8.7 km downstream was relatively C-13-enriched (-18 to -23 parts per thou sand), reflecting autochthonous inputs. Tributary seston had an interm ediate deltaC-13 value (-22 to -26 parts per thousand), reflecting all ochthonous inputs (conifer litter) and macrophyte (Elodea) detrital in puts. deltaN-15 values of seston were generally similar among sites (4 -8 parts per thousand) but were more depleted for the tributary (1-4 p arts per thousand). Depleted deltaN-15 values (0-2.5 parts per thousan d) for seston from below the dam in late summer corresponded to a blue -green algae bloom in the reservoir. Isotopic shifts of organic matter incubated for 60-90 days in situ were small (< 2 parts per thousand) and varied in direction depending on isotope and litter type. The isot opic composition of stonefly larvae, Pteronarcys californica, varied a mong sites, and closely matched that of the local seston (especially F POM), suggesting that with respect to detritus origin, larvae were opp ortunistic foragers. The origin of organic matter is influenced by flo w regulation, resulting in compressed isotopic gradients.