Document: DOU-3-73-14

Longitudinal variation in riverine trophic structure: Linkage between flow, sediment and biota.

OSMUNDSON, D.B.* ¹, R.J.RYEL ², V.LAMARRA ³ and J.PITLICK ⁴

U. S. Fish and Wildlife Service, Grand Junction CO 81506 USA ¹
Utah State University, Logan, UT 84322 USA ²
Ecosystems Research Institute, Logan, UT 84321 USA ³
Colorado State University, Boulder CO 80309 USA ⁴

Abstract:
Regulation of rivers often results in an alteration of the natural flow regime. How flow regime affects fish distribution and assemblage structure is poorly understood. The upper Colorado River population of the endangered Colorado pikeminnow Ptychocheilus lucius suffers from low rates of recruitment and reduced carrying capacity. We hypothesized that availability of prey fish for this native piscivore may in part be limited by reduced primary and secondary productivity resulting from accumulation of fine sediment in the bed of the river. We stratified the 300-km-long study area into 11 strata and sampled various physical and biological parameters in runs and riffles of 3 randomly selected 1-3-km long study reaches in each stratum during base flows of spring and fall, 1994-1995. Significant correlations were found between chlorophyll-a and invertebrate biomass and various physical metrics that described the degree of fine sediment accumulation in gravel-cobble substrates. Substrates of riffles were relatively free of fine sediment throughout the study area but substrates in runs contained progressively more fines with distance downstream. There was a corresponding longitudinal structuring of biota along the river continuum with greatest fish, invertebrate and algal biomass occurring in upstream strata that declined with distance downstream. Adult Colorado pikeminnow were concentrated in upstream strata where potential food fishes were in greatest abundance. In some downstream strata, steep banks, deep runs and few riffles have probably always limited periphyton and invertebrate production; however, in middle strata, fine sediment in the bed of runs contributes to low algal and invertebrate biomass. Historically, high spring flows from snowmelt runoff frequently flushed the bed of silt and sand. Following regulation, the mean recurrence interval of spring-runoff flows of magnitude sufficient to mobilize the gravel-cobble substrate and thereby winnow silt and sand from the bed of run habitats lengthened from 1.3-2.7 yr (depending on the stratum) to 2.7-13.5 yr. Our results describe and help explain the spatial distribution of the Colorado River fish community and establish a link between flow, sediment, and the riverine food web supporting the community's top predator, the endangered Colorado pikeminnow.

Keywords: river regulation, Colorado pikeminnow, fine sediment, riverine productivity

This abstract is being presented at: 10:30 AM in session:
RIPARIAN ECOLOGY