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 (riparian vegetation, algae, macrophytes, plankton) and an invertebrate detritivore were examined in a regulated Rocky Mountain river (fourth order). The isotopic composition of seston varied among sites and seasons for 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. Multiple 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 third order tributary. Seston (> 53 mum) from 0.5 km below the dam was relatively C-13-depleted (-30 to -28 parts per thousand), reflecting the contribution of reservoir-derived plankton; seston from the site 8.7 km downstream was relatively C-13-enriched (-18 to -23 parts per thousand), reflecting autochthonous inputs. Tributary seston had an intermediate deltaC-13 value (-22 to -26 parts per thousand), reflecting allochthonous inputs (conifer litter) and macrophyte (Elodea) detrital inputs. deltaN-15 values of seston were generally similar among sites (4-8 parts per thousand) but were more depleted for the tributary (1-4 parts per thousand). Depleted deltaN-15 values (0-2.5 parts per thousand) 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 isotopic composition of stonefly larvae, Pteronarcys californica, varied among sites, and closely matched that of the local seston (especially FPOM), suggesting that with respect to detritus origin, larvae were opportunistic foragers. The origin of organic matter is influenced by flow regulation, resulting in compressed isotopic gradients.