Interactions Between Physical and Biological Processes in Riverine Landscapes VI: Ecohydraulics B
Presiding: J M Buffington, USDA Forest Service; C V Baxter, Colorado State University; A E Rosenberger, University of Idaho and USDA Forest Service
B51A-01 08:30h
Ecohydrology of Sandhill Crane Riverine Roost Sites
Each spring the Platte River in central Nebraska provides critical nocturnal roosting habitat for approximately 400,000 -500,000 migrating sandhill cranes. Changes in the flow regime of the Platte River over the last century have led to vegetation encroachment and channel narrowing, which has in turn influenced the spatial distribution of crane roost sites. Evaluating the success of management actions such as vegetation clearing or streamflow modification taken to improve habitat for cranes requires an understanding of the physical and biological factors that influence roost site selection. We examined the relationships between flow, geomorphology and roosting habitat for sandhill cranes on the Platte River, Nebraska at two spatial scales. At the sub-reach scale, the locations of roosts are influenced by the flow patterns over and around riverine sandbars. We investigated the relationships at this scale along a 1-km study reach by comparing the pattern of roosts identified with aerial thermal infrared imagery to output from a depth-averaged two-dimensional hydraulic model. Cranes selected roosting habitat characterized by 0.01 - 0.4 m depths and 0.01 - 0.70 m/s velocities. The roost patterns changed with flow, and with the associated distributions of subaerial sandbar area, depth and velocities in the study reach. The amount of roost area in the study reach generally decreased as river flows fell below 23 cubic meters per second and also decreased as flows rose above 48 cubic meters per second. However, for all bed morphologies and river flows modeled the area roosting cranes used was much less than the area available. At the reach scale we examined how the amount of roost area in reaches of different channel type changed for a range of river flows. In the braided channel reaches carrying a portion of the flow, the greatest amount of roost area was observed at the highest river flow. In the single channel reaches carrying the total flow, the greatest amount of roost area was observed at the lowest flow. Taken together these observations may indicate that while cranes show fidelity for certain reaches, there is some threshold at which they abandon these sites in favor of alternate locations that may offer a greater proportion of suitable habitat.
B51A-02 08:45h
Effects of Patch Context on two Species of Invertivorous Fishes in a Riffle-Pool Stream
Lotic systems with riffle-pool geomorphology can be conceptualized as chains of habitat patches connected by an aquatic medium. Patches receive input from upstream, as well as from the adjacent riparian zone. For organisms dependent on such input for food, context of an occupied patch may be a better determinant of patch quality than within-patch factors. I tested this hypothesis for two invertivorous stream fishes in a prairie-margin, riffle-pool stream. Notropis boops is a drift-feeder, trophically dependent on input of aquatic invertebrate drift and terrestrial invertebrate input from outside an occupied patch. Etheostoma spectabile is a benthic feeder, trophically dependent on the within-patch standing crop of invertebrates. I measured resource levels by quantifying benthic invertebrate density, invertebrate drift input, and terrestrial invertebrate input for ten pools riffles. I determined the number of N. boops in each pool, and E. spectabile in each riffle, and the number of predators. I asked if resource levels differed among patches and if these differences were a function of upstream or riparian patch characteristics. Effects of resource levels on fishes were determined by comparing fish distribution among patches to that of resources, and examining fish diets in patches with different resource levels.
B51A-03 09:00h
Effects of Spatial Variability in Flow and Sediment Transport on Benthic Invertebrates During Runoff Events: Patch and Reach-Scale Challenges
The short-term impact of streamflow increases on benthic populations depends on spatial patterns of organism dispersal and mortality that are difficult to observe and quantify in the field. Laboratory and field experiments suggest that the size distribution, structure, and stability of streambed sediment play critical roles in mediating the effects of flow increases on dispersal and mortality of organisms. I present the results of laboratory flume experiments in which flow and sediment transport were progressively increased and the resulting displacement of aquatic insect larvae was quantified. These and other experiments demonstrate that the displacement and mortality of benthic organisms scales with streambed entrainment and sediment transport, but that bed structure and the physical and behavioral traits of the biota can strongly influence this relationship. Application of these patch-scale experimental results to understanding the hydrogeomorphic determinants of reach-scale flood impacts involves important scientific challenges and uncertainties. Reliable estimation of the spatial variability of streambed mobilization and sediment transport as a function of channel and substrate characteristics, flow history and sediment supply is necessary to compare the effects of different events or among different stream reaches. Also needed is a better appreciation of the spatial scales of organism dispersal during flow events and the physical and biological controls on patterns of dispersal at various scales.
B51A-04 09:15h
A Multiscale Analysis of the Relationship Between Current Velocity and Invertebrate Drift: Do Faster Currents Transport More Food?
Salmonids may select feeding stations to maximize their net energy intake (NEI). Variation in NEI is based on the assumption that there is a positive linear relationship between food availability and current velocity. Three to four drift samples were collected from each of three riffles nested in three reach types nested in three Rocky Mountain streams for 4 to 6 weeks. We calculated regressions between current velocity at the net, Reynolds number for a riffle, and slope for a reach against the biomass of invertebrates collected for 10 hours during the day and again at night at each scale. We expected positive correlations between drift and current velocity within the same riffle because the effects of other factors on drift should be similar within the same riffle (e.g. benthic densities). Ninety percent of the regression slopes for nets within riffles were not different from zero. However, the number of positive slopes increased as scale increased when we compared hydraulically rough to smooth riffles and steep to low-gradient reaches. Rougher riffles and steeper reaches tended to produce more drift than smooth riffles and low-gradient reaches. Drift density was not correlated with current velocity within riffles, but became more predictable at intermediate scales.
B51A-05 09:30h
The Influence of Flood Disturbances on Interspecific Competition Between Stream Invertebrates
We investigated the influence of repeated abiotic disturbances on interspecific competition between stream invertebrates. Artificial substrates (tiles) were exposed in 2 German streams with different flow regimes (floodprone versus stable). Half the tiles were disturbed (once every 2 weeks) by scrubbing to simulate a bed-moving flood, the other half remained undisturbed. We also manipulated densities of 2 of the most common invertebrate taxa in each stream (floodprone: Baetis and Heptageniidae; stable: Simulium and Brachycentrus montanus). Three invertebrate removal treatments (removal of taxon 1, removal of taxon 2, and unmanipulated controls) were applied, and invertebrates were removed every 3 days. After 2, 4 and 6 weeks each, we sampled 8 tiles from each treatment and determined invertebrate faunal composition, epilithic algal biomass (Chl a) and biofilm biomass (AFDM) for each sample. Both disturbance and invertebrate removal affected the stream biota. Disturbance reduced algal and biofilm biomasses in both streams, and invertebrate density in the floodprone stream. Brachycentrus removal resulted in higher flatworm density. Disturbance and invertebrate removal also interacted with each other. Density of the "non-target" mayfly Baetis in the stable stream was highest in Simulium removal treatments on disturbed tiles, but lowest in Simulium removal treatments on undisturbed tiles.
B51A-06 09:45h
Biotic and Abiotic Patch Dynamics in Two Macrophyte-Dominated Headwater Streams in Lowland UK
UK groundwater-fed river systems have characteristically stable temperature and flow regimes with low turbidity. Unshaded shallow reaches support extensive growths of submerged macrophytes (e.g. aquatic Ranunculus spp.). The plant stands create habitat patch mosaics at the meso- and micro-habitat scale by modifying water velocity and depth and trapping fine sediment. The structure, dynamics and functioning of these discrete habitat patches has attracted comparatively few studies. This research investigated spatial and temporal variation in the biotic and abiotic components of such patches for two headwater streams. For both headwater streams, the habitat mosaic of patches at the reach scale incorporated vegetated patches (in-channel and marginal) and non-vegetated patches. However, the two reaches differed in terms of their in-channel and marginal vegetation growth patterns and species diversity and the numbers of dominant associated biota (Oligochaeta). Lower peak flows recorded at one site reduced disturbance and scour and promoted the growth of denser in-channel and marginal vegetation that enhanced sediment trapping and retention. The increased plant biomass caused heightened water levels during the summer stimulating further marginal vegetation growth and augmenting the sediment and nutrient retention of the marginal zone. Our results will be discussed in the context of the patch dynamics concept.