Kevin P. Wilson

AQUATIC FOOD BASE RESPONSE TO THE 1996 TEST FLOOD BELOW GLEN CANYON DAM, COLORADO RIVER, ARIZONA

We examined the impact of the 1996 test flood released from Glen Canyon Dam (GCD) on the aquatic food base in the Colorado River through Grand Canyon National Park, Arizona, USA. Benthic scour and entrainment of both primary and secondary producers occurred at all study sites along the 385-km river corridor. The majority of the organic drift occurred within the first 48 h of the test flood with the arrival of the hydrostatic wave. Recent macrophyte colonizers (Chara, Potamogeton, and Elodea) of fine sediment in the tailwaters were scoured from the channel bottom, with recovery to pre-flood estimates within 1–7 months depending on taxa. Macroinvertebrates and filamentous algae recovered within three months depending on taxa. The test flood removed suspended particles from the water column and increased water clarity, which enhanced benthic recovery. The test-flood hydrograph was designed primarily as an experiment in sand transport and occurred during a period of sustained high releases from GCD starting i...

ALGAL ECOLOGY IN TAILWATER STREAM COMMUNITIES : THE COLORADO RIVER BELOW GLEN CANYON DAM, ARIZONA

Water discharges from hydropower dams exert major influences on the standing crop and structure of downstream algal communities. Dams typically reset physicochemical conditions in downstream tailwaters to those of lower order streams, which results in critical changes throughout the food web (Ward and Stanford 1979, Lillehammer and Saltveit 1984, Blinn and Cole 1991, Stevens et al. 1997). Those physicochemical and hydrologic changes that have the greatest influence on tailwater algal communities include: (1) diel and seasonal constancy of water temperatures, (2) modifications in nutrient regimes, (3) removal of sediment and clarification of water, (4) formation of stable armored substrates, (5) unnatural fluctuations in water levels that subject biota to diel drying and wetting, and (6) reductions in seasonal flow variability and alterations in the timing or occurrence of extreme flows. The greatest changes in biotic communities result from alterations in timing or occurrence of discharge patterns that have produced community interactions over evolutionary time and result in the invasion of exotic species (Resh et al. 1988). Channel geomorphology, discharge volume, and relative tributary size influence the recovery from these altered conditions over distance downstream from the dam (Ward and Stanford 1983, Stevens et al. 1997). This minireview focuses on the influences that regulated discharges have placed on the algal community below Glen Canyon Dam (GCD), Arizona, compares these alterations to findings below other hydropower dams, and addresses how algae can be used in environmental management below dams. Lowe (1979) and Biggs (1987) provided earlier reviews on the ecology of algae in regulated rivers. We also experimentally measured: (1) primary productivity of the phytobenthos in submerged and varial zones (that shoreline region in the tailwaters of dams that enlarges and contracts in response to dam

Effects of suspended sediment and desiccation on the benthic tailwater community in the Colorado River, USA

We demonstrated that differences in habitatrequirements by C. glomerata and Oscillatoria havea profound bottom-up influence on the foodweb in thetailwaters below Glen Canyon Dam in the Colorado Riverthrough Grand Canyon National Park, USA. We examinedthe effects of suspended sediment and desiccation onthe colonization sequence of Cladophora glomerata andOscillatoria spp. and the consequent effects onmacroinvertebrates in each algal community in a seriesof reciprocal transplants in the regulated ColoradoRiver, AZ. Our experiments showed that C. glomeratagrows best in continuously submerged, clear-water,stable habitats, whereas Oscillatoria forms densemat-like matrices of trichomes and sand in varialzones and habitats with high suspended sedimentstypical of many southwestern USA streams. Varial zoneconditions have a stronger influence on communitystructure than habitats with high suspended sediments. Recruitment by chironomid larvae was less dependent onC. glomerata and less affected by suspended sedimentand periodic desiccation than Gammarus lacustris. Weestimated the energy from macroinvertebrate biomassassociated with tufts of C. glomerata to be an orderof magnitude higher than that in Oscillatoriamatrices. Therefore, loss of C. glomerata andreplacement of habitat more suitable for Oscillatoriaas a result of regulated flows indirectly reducespotential energy flow in the Colorado Riverfoodweb.

FOOD WEB STRUCTURE, ENERGETICS, AND IMPORTANCE OF ALLOCHTHONOUS CARBON IN A DESERT CAVERNOUS LIMNOCRENE: DEVILS HOLE, NEVADA

Abstract We tested the hypothesis that allochthonous carbon is important in the diet of the endemic pupfish Cyprinodon diabolis over a 3-year period in Devils Hole, Nevada. Devils Hole is a cavernous limnocrene located in the Mojave Desert approximately 15 m below the land surface and receives direct solar radiation <7% of the total time during the year. The system is dominated by filamentous cyanobacteria in the summer and supports about 15 invertebrate species and the endemic pupfish C. diabolis. We examined food web structure and measured autochthonous carbon production and entry of allochthonous carbon with funnel traps. Stable isotopes were used to compare contributions of each carbon source. Allochthonous carbon contributed 2971 kj ·yr−1 (60%) of the total available energy to the food web compared to only 2000 kj ·yr−1 autochthonous production. A major stochastic rain event delivered more terrestrial carbon to Devils Hole in l hour than annual allochthonous carbon, estimated from data collected with funnel traps. Mixing models with δ15N, δ13C, and δ34S showed a seasonal shift in diet for C. diabolis from filamentous cyanobacteria in the summer to the collector/gathering insect Stenelmis calida in the winter. Stenelmis tissue had high proportions of allochthonous plant carbon in the winter.

Food Web Implications of δ13C and δ15N Variability over 370 km of the Regulated Colorado River USA

Abstract Dual stable isotope analysis in the regulated Colorado River through Grand Canyon National Park, USA, revealed a food web that varied spatially through this arid biome. Down-river enrichment of δ13C data was detected across three trophic levels resulting in shifted food webs. Humpack chub δ13C and δ15N values from muscle plugs and fin clips did not differ significantly. Humpback chub and rainbow trout trophic position is positively correlated with standard length indicating an increase in piscivory by larger fishes. Recovery of the aquatic community from impoundment by Glen Canyon Dam and collecting refinements for stable isotope analysis within large rivers are discussed.

Driftwood: an alternative habitat for macroinvertebrates in a large desert river

Driftwood was an important substratum for macroinvertebrates in the Colorado River, a desert river in southwestern U.S.A. with high suspended sediments and limited cobble substrata. Higher light availability and reduced abrasion on driftwood substrata resulted in a significantly higher ash-free dry mass (AFDM) of biofilm when compared to cobbles. Overall mean mass of biofilm on driftwood was 3.76 g m-2 AFDM (SE ±0.19) compared to 2.45 g m-2 AFDM (±0.15) on cobbles. Total macroinvertebrate AFDM was not significantly different between cobble and driftwood substrata. However, there were differences in the Ephemeroptera assemblage on the two substrata which were attributed to the type of food resources available in each habitat. Driftwood was dominated by the scraper/collector Heptagenia elegantula (Eaton), while the filterer/collector, Traverella albertana (McDunnough) dominated cobbles. Twenty taxa were found on driftwood substrata. This habitat expands the types of niches available to macroinvertebrates in lotic systems with high suspended sediments. We calculated that 4.4 m2 of driftwood substratum passed our sample station each minute during a 12-h sampling period on the rising limb of the hydrograph. River impoundments limit the supply and transport of driftwood, which may have negative implications on macroinvertebrate communities in desert rivers with high suspended sediment. Studies in turbid desert rivers that do not sample driftwood may underestimate both the total standing mass/energy of the system and taxon richness.

Life in a fishbowl: Prospects for the endangered Devils Hole pupfish (Cyprinodon diabolis) in a changing climate

The Devils Hole pupfish (Cyprinodon diabolis) is a federally listed endangered species living solely within the confines of Devils Hole, a geothermal pool ecosystem in the Mojave Desert of the American Southwest. This unique species has suffered a significant, yet unexplained, population decline in the past two decades, with a record low survey of 35 individuals in early 2013. The species survives on a highly variable seasonal input of nutrients and has evolved in a thermal regime lethal to other pupfish species. The short lifespan of the species (approximately 1 year) makes annual recruitment in Devils Hole critical to the persistence of the species, and elevated temperatures on the shallow shelf that comprises the optimal spawning habitat in the ecosystem can significantly reduce egg viability and increase larval mortality. Here we combine computational fluid dynamic modeling and ecological analysis to investigate the timing of thresholds in the seasonal cycles of food supply and temperature. Numerical results indicate a warming climate most impacts the heat loss from the water column, resulting in warming temperatures and reduced buoyancy-driven circulation. Observed climate change is shown to have already warmed the shallow shelf, and climate change by 2050 is shown to shorten the window of optimum conditions for recruitment by as much as 2 weeks. While there are many possible reasons for the precipitous decline of this species, the changing climate of the Mojave region is shown to produce thermal and nutrient conditions likely to reduce the success of annual recruitment of young C. diabolis in the future, leading to continued threats to the survival of this unique and enigmatic species.

Interference Competition between the Net-Building Caddisfly Ceratopsyche oslari and the Amphipod Gammarus lacustris

ABSTRACT We investigated the interspecific interactions between the amphipod, Gammarus lacustris (Sars) and the cosmopolitan caddisfly, Ceratopsyche oslari (Banks). We believe this is an example of interference competition and may be one of several factors that limits the distribution of C. oslari from the Colorado River below Glen Canyon Dam. Field observations indicated that C. oslari was associated with fewer food capture nets in the Colorado River than in the Paria River tributary. Laboratory experiments indicated the amphipod Gammarus lacustris, which is abundant in the Colorado River but absent from the Paria River, had a detrimental effect on net-building success by C. oslari. Net numbers in laboratory chambers declined significantly after 6 d due to destruction by the swimming and feeding activities of G. lacustris. Competition with the introduced, non-native G. lacustris for interstitial spaces along with several abiotic factors, may limit the distribution of C. oslari in the Colorado River.

Benthic community structure of the green and Colorado Rivers through Canyonlands National Park, Utah, USA

Abstract We sampled the aquatic benthos at 6 remote sites on the Colorado and Green rivers through Canyonlands National Park, Utah, USA. This study provides the first published description of benthic standing mass, invertebrate community composition, and primary carbon source for this portion of the Colorado River system. High suspended sediment concentrations prohibited growth of primary producers. The primary carbon source for benthic invertebrates was terrestrial organic matter. The invertebrate community was composed of 49 taxa, mostly mayflies, caddisflies, and diptera, which were dominated by filterer/collector species. A smaller portion of the community was made up of predatory stoneflies and odonates. Standing mass of invertebrates on cobble substrates within a given site was stable over the multiyear sample period (1993 through 1996) and was comparable with other southwestern streams (overall mean = 0.41 g/m2 ash-free dry mass ± 0.01 SE). Invertebrate standing mass at each site was controlled by the availability of primary carbon. Primary carbon availability was controlled by supply to the site and retention within the site. Both aspects might be influenced by anthropogenic alteration of the river basin and discharge patterns upstream of the study site.

The shallow thermal regime of Devils Hole, Death Valley National Park

Devils Hole, a fracture in the carbonate aquifer underlying the Death Valley Regional Groundwater Flow system, is home to the only extant population of Devils Hole pupfish (Cyprinodon diabolis). Since 1995, the population of C. diabolis has shown an unexplained decline, and a number of hypotheses have been advanced to explain this. Here, we examine the thermal regime of Devils Hole and its influence on the pupfish population. We present a computational fluid dynamic (CFD) model of thermal convection on the shallow shelf of Devils Hole, which provides critical habitat for C. diabolis to spawn and forage for food. Driven by meteorological data collected at Devils Hole, the model is calibrated with temperature data recorded in the summer of 2010 and validated against temperatures observed on the shallow shelf between 1999 and 2001.The shallow shelf experiences both seasonal and diel variations in water temperature, and the model results reflect these changes. A sensitivity analysis shows that the water temperatures respond to relatively small changes in the ambient air temperature (on the order of 1 °C), and a review of local climate data shows that average annual air temperatures in the Mojave Desert have increased by up to 2 °C over the past 30 years. The CFD simulations and local climate data show that climate change may be partially responsible for the observed decline in the population of C. diabolis that began in 1995.