Russell B. Rader

Interspecific leaf interactions during decomposition in aquatic and floodplain ecosystems

An experiment was designed to test the importance of the potential interaction (inhibition or enhancement) between slow and fast decaying leaf species on processing rates in a stream and its floodplain. The decomposition of water oak (Quercus nigra) and sweetgum (Liquidambar styraciflua) in single-species packs was compared with water oak plus sweetgum in mixed-species packs within three habitats (stream snags, floodplain pools, and elevated floodplain surfaces) at three sites in coastal plain streams. Fast-decaying sweetgum leaves did not enhance the rate of oak decomposition. Sweetgum leaves in mixed packs decomposed more slowly than single species packs in seven out of nine comparisons. Increases in bacterial density on leaves were depressed in mixed-species packs relative to single-species sweetgum packs. Fungal hyphae could not be observed in mixed or single-species packs. The effect of oak leaves on sweetgum leaves was affected by frequency and period of inundation. Macroinvertebrate shredders were rare or absent from most leaf packs collected from stream snags and floodplain pools. Over 40% of leaf packs placed in the stream contained no shredders, while another 28% contained <0.001 g shredders/g leaf dry weight. Therefore, shredders were too rare to influence overall leaf processing rates. These studies suggest that microbial processing accounts for most leaf decomposition and oak leaf leachate is shown to be inhibitory to microbial processing of sweetgum leaves.

Mechanisms influencing community composition and succession in mountain stream periphyton: interactions between scouring history, grazing, and irradiance

A field experiment with a factorial design was used to examine changes in benthic periphyton exposed to different regimes of scouring (scoured/non-scoured), light (open/shaded), and grazing (grazed/non-grazed) to investigate the role of environmental heterogeneity in shaping assemblages of periphyton in Rocky Mountain streams. Clay tile substrata were positioned at the start of the experiment either in the main channel (to receive bedload scouring) or a protected side channel (where scouring did not occur) of a 3rd-order, subalpine stream (elevation 3000 m asl). After 45 d, tiles were transferred to in situ stream channels where light intensity and densities of a mayfly grazer (Rhithrogena robusta) were manipulated. Scoured tiles carried ∼2× greater algal biovolume than non-scoured tiles 1 wk after high flows receded. The chrysophyte Hydrurus foetidus and the cyanobacterium Chamaesiphon incrustans predominated on scoured tiles, whereas diatoms, which were rare on scoured tiles, were the most abundant on non-scoured tiles. Non-scoured tiles also had 3× greater algal species richness than scoured tiles. Thus, seasonal high flows resulted in higher algal biomass but lower algal diversity than base flows at the spatial scale of individual cobbles. Scouring history, light, and grazer manipulations influenced the final periphyton assemblage structure in post-scour experiments. Scouring effects persisted throughout the experiment, in that scoured tiles had greater periphytic ash-free dry mass (AFDM) and lower algal biovolume than non-scoured tiles at the end of the experiment. Light had little direct influence on periphyton. Tiles in open channels receiving full sunlight (irradiance = 1696 μmol m−2 s−1) supported less periphytic AFDM than shaded tiles (506 μmol m−2 s−1), but light had no effect on total algal biovolume and influenced the abundance of only 2 of 5 algal taxa. Grazers depressed periphytic AFDM, algal biovolume, and the abundance of all major taxa except H. foetidus. Differences in community composition resulting from scoured and non-scoured treatments influenced periphytic and algal response to grazers and light. No single factor drove periphytic heterogeneity. Post-scouring community structure and interactions between grazers and light were important processes producing a mosaic of patches.

Light and a grazing mayfly shape periphyton in a Rocky Mountain stream

To examine the combined effects of light and grazers in structuring periphyton in a subalpine stream, we conducted a field experiment in St. Louis Creek (elevation 3000 m), a 2nd-order Rocky Mountain stream (Colorado, USA). Quarry tiles were placed in the stream for 60 d to colonize with periphyton and then moved into 36 stream channels in which light and grazers were manipulated. Light treatments were high (100% of ambient), intermediate (∼40% of ambient), and low light (∼5% of ambient). A grazing mayfly, Rhithrogena robusta, was maintained inside channels at high (288/m2), low (96/m2), or zero densities. After 22-23 d of exposure, mayfly grazing significantly reduced algal biovolume under all light regimes, although species assemblages differed between light treatments. As light levels changed, some algae showed different responses to grazing. Hydrurus foetidus (Chrysophyta), for example, was more abundant on grazed substrata relative to non-grazed controls under high light, but it declined in abundance when grazed under intermediate light. Light and grazers also had an interactive effect on periphytic biomass; as light increased, grazers caused greater depletions in periphytic ash-free dry mass. Results suggest that, in subalpine streams, light may be instrumental in establishing periphytic structure and in modifying the impact that grazers have on algae.

Influence of impoundments on mayfly diets, life histories, and production

We investigated the influence of stream regulation on the dietary composition, life histories, and production of four mayfly species (Drunella grandis, Ephemerella infrequens, Paraleptophlebia heteronea, and Baetis tricaudatus) at three sites in the Upper Colorado River. Sites (unregulated, intensely regulated, and recovery) were selected based upon degree of regulation (i.e., upstream, 0.4 km downstream, and 4.0 km downstream from a deep-release reservoir). Although levels of food abundance (diatom density and sedimentary detritus) varied between sites, the dietary compositions of B. tricaudatus (primarily diatoms) and P. heteronea (detritus) did not change. However, D. grandis and E. infrequens consumed diatoms downstream from the reservoir whereas their diet was strictly detritivorous at the upstream site. The number of degree days and mean annual water temperatures at the downstream sites were greater than at the free-flowing unregulated site. Baetis tricaudatus responded to regulated conditions by higher rates of development, altered voltinism, and extended emergence. Ephemerella infrequens and P. heteronea were univoltine at each site. However, development of E. infrequens and P. heteronea was poorly synchronized and emergence was extended at both downstream sites. Drunella grandis was univoltine and well synchronized with a short emergence period at each site. Life history flexibility, in rates of egg development, growth, maturation, and emergence, at the population level may be determined by a lack of dependence by individuals upon temperature or photoperiod cues to synchronize the developmental process. Annual production estimates at downstream sites were higher for all four species than at the upstream site. Annual production of D. grandis (15.38 g/m2) at the recovery site and of E. infrequens (15.89 g/m2) at the regulated site are the highest production estimates recorded for mayflies.

Resource utilization, overlap and temporal dynamics in a guild of mountain stream insects

SUMMARY 1. Resource utilization was quantified for six mayfly (Ephemeroptera) and one caddis (Trichoptera) species comprising a lotic scraper/collector‐gatherer guild across three niche dimensions (temporal, trophic and spatial). Based on trophic differences and inferred microspatial utilization, the members of this guild separated into two groups: (1) cryptic detritivores and (2) exposed algivores. 2. Each species demonstrated a slow seasonal univoltine life cycle except for Epeorus longimanus (Eaton) and Baetis iricaudatus (Dodds) which were fast seasonal univoltine and multivoltine, respectively. 3. Temporal sequencing of periods of peak resource utilization were not demonstrated by the members of this guild. A null analysis indicated that periods of peak resource utilization were aggregated.

The Influence of Environmental Predictability/Disturbance Characteristics on the Structure of a Guild of Mountain Stream Insects

Resource overlap in the scraper/collector-gatherer guild along temporal, trophic and microspatial niche dimensions was determined at two sites in the Upper Colorado River. The amount of resource overlap within the scraper/collector-gatherer guild in a predictable, benign regulated section of the river was compared with the amount of overlap within the scraper/collector-gatherer guild of a less predictable, frequently disturbed free-flowing section. Thirty years of daily flow records were used to determine predictability and frequency of disturbance events. Trophic, temporal and microspatial overlap between the two guilds was not significantly different. There was no evidence to support the hypothesis that resource overlap should be reduced or more regularly spaced in guilds inhabiting predictable benign environments.

Importance of Light and Nutrients in Structuring an Algal Community in a Rocky Mountain Stream

Abstract The effects of light regime and nutrient enrichment on periphytic algae in a second-order Rocky Mountain stream were determined in a field experiment. We established a light gradient along a 400 m stretch of West St. Louis Creek using the following four light treatments: artificially shaded in the open reach, closed-canopy, artificially illuminated in the closed reach, and open-canopy. Nitrate and phosphorus were manipulated with nutrient-diffusing substrata. Despite order-of-magnitude differences in light levels, metrics of periphytic and algal abundance were very similar under the open and closed canopy treatments. Under the artificially illuminated treatment, however, all parameters were two to four times greater, indicating that light was limiting under both of the natural light regimes, (i.e., insufficient light under the closed canopy and photoinhibition in the open section). Addition of phosphorus caused significant increases in Chl-a, periphytic ash-free-dry-mass, and algal cell density, ...

Mayfly production in a Colorado mountain stream: an assessment of methods for synchronous and non-synchronous species

Year-round collections of mayflies (Ephemeroptera) from a Colorado mountain stream allowed critical examination of several methods of calculating production for species with different life cycles. Five of the six numerically dominant species exhibited slow seasonal, univoltine life cycles. Baetis tricaudatus was bivoltine. Two species demonstrated well synchronized development, three species were poorly synchronized and a sixth was intermediate. Mean density and biomass data from each sampling date were used to ascertain the goodness-of-fit of each species to the Allen curve. It is proposed that such information can provide quantitative criteria for identifying species with well synchronized development and thereby determine when it is appropriate to directly apply cohort methods while avoiding time intensive body size (e.g. head width) measurements necessary for size-frequency analyses. In addition, these data demonstrate that species specific production varies with gross changes in elevation.