STREAM ECOSYSTEM DYNAMICS OF THE SALMON RIVER, IDAHO - AN 8TH-ORDER SYSTEM

Studies were conducted at three locations in the Lower Salmon River, Idaho (USA) and at one Upper Salmon "calibration" site to extend results of previous research (headwater through 5-6th-order [1-814 links] streams) to an 8th-order (15,000 links) river. Transported organic matter (TOM) generally increased downstream. Coarse TOM was prevalent only in the upper 50 km of the system, and ultrafine was the predominant particulate component throughout the system. Stored (benthic) organic matter (OM) was highest in the headwaters and showed a progressive downstream decrease. The headwaters were highly retentive of coarse particulate organic matter (CPOM) but much less so of the fine and ultrafine fractions. Transported and stored OM approached a balance only in the lowermost (deeper) reaches of the river system. Analyses of the components of ecosystem metabolism revealed that gross primary production and respiration (R) both showed a general downstream increase. Overall, the net metabolism measurements showed that R was predominant in the upstream reaches, autotrophy prevailed in the middle reaches, and R was again predominant at the farthest downstream site. Insect functional feeding-group patterns apparently were altered in their relative abundances from that predicted by the River Continuum Concept by lower than expected deciduous CPOM-loading in the headwaters and high sediment transport into the mid-reaches from a major tributary (East Fork). Although shredders were prevalent in headwater reaches, they were less abundant than predicted. Gatherers were dominant in the upper reaches and filterers in the mid-sized reaches above East Fork. Scrapers increased progressively downstream in relative abundance. Spiraling and retention measurements revealed a general downstream increase in the rate of movement of OM; turnover length of carbon increased progressively downstream. The ratio of mean water velocity: rate of downstream movement of OM indicated that the export of organic carbon was reduced 3500-fold in headwaters relative to that of mean water velocity whereas the lower reaches behaved more like a conduit with carbon atoms passing through at a rate comparable to water velocity. In spite of the cold and arid climate in which the Salmon River Basin is located and the low allochthonous input from deciduous plants, the data generally support the basic premises of the River-Continuum Concept.