Todd Wellnitz

Current velocity and invertebrate grazing regulate stream algae: results of an in situ electrical exclusion

Current velocity is a pervasive feature of lotic systems, yet this defining environmental variable is rarely examined as a factor for regulating stream herbivory. To investigate how current modifies herbivory in the upper Colorado River, U.S.A., loops of electrified fencing wire were used to reduce in situ grazer densities on 30 × 30 cm tile substrates. After 45 d, electrified tiles had significantly fewer grazers (P = 0.03) and >2X more algal biomass than controls (P = 0.0002). Reduced grazing on electrified tiles yielded periphytic assemblages having more diatoms and chlorophytes, as well as greater algal species richness. Current velocity effects alone did not significantly regulate algal abundance; however, the interaction between current velocity and grazer exclusion resulted in more algae in slow vs. fast current (P = 0.02). Grazer abundances were similar between fast and slow current velocities, suggesting that grazers in the Colorado River differ in their ability to regulate algae across the current velocity gradient. Our results indicate that stream current-mediated herbivory in streams may be more important than is generally recognized.

Current-mediated periphytic structure modifies grazer interactions and algal removal

By shaping the architecture and taxonomic composition of periphyton, stream current may create periphytic mats on which some grazers can feed and forage more effectively than others. Current-mediated periphytic structure also has the potential to foster positive interactions among grazers if one grazer’s foraging facilitates another’s access to algal food. To examine the extent to which these indirect effects of current influenced periphytic removal and grazer interactions, we conducted a mesocosm experiment with two common grazers, the caddisfly (Trichoptera) Glossosoma verdona and the mayfly (Ephemeroptera) Drunella grandis. Periphyton was allowed to colonize ungrazed tiles for 30xa0d and assume its natural growth form under three ranges of near-bed current, “slow” (1–5xa0cmxa0s−1), “medium” (15–20xa0cmxa0s−1), and “fast” (30–40xa0cmxa0s−1). Tiles were then exposed to the two grazer species at five densities. A streambed survey quantified Glossosoma and Drunella distributions in relation to near-bed current and periphytic structure (i.e., diatom films vs. filamentous mats) in the Colorado River. After 22xa0days of grazing, periphytic removal by Glossosoma was influenced by near-bed current and attendant periphytic structure. In slow current, where senescent Ulothrix filaments were abundant, increased Glossosoma density was correlated with an increase in periphyton biomass. Larvae became entangled and immobilized by the diffuse and senescent Ulothrix mat that characterized slow velocity, and Glossosoma mortality and weight loss was greatest in this treatment. By contrast, Drunella reduced periphyton across all density and current treatments. Drunella density correlated with increased Glossosoma survivorship and weight gain in slow current. The driving mechanism for this facilitation appeared to be removal of entangling overstory filaments by Drunella. The streambed survey showed that Glossosoma were negatively associated with filamentous mats, lending support to the hypothesis that clearing action by Drunella in the slow current/senescent Ulothrix treatment facilitated Glossosoma growth and survival. Our study helps underscore the importance of evaluating species interactions over ranges of abiotic conditions and consumer pressure to understand the patterns and processes shaping benthic communities.

Using the Science Writing Heuristic to Improve Undergraduate Writing in Biology

Our objective was to investigate the impact of the Science Writing Heuristic (SWH) on undergraduates’ ability to express logical conclusions and include appropriate evidence in formal writing assignments. Students in three laboratory sections were randomly allocated to the SWH treatment (nu2009=u200951 students) with another three sections serving as a control (nu2009=u200947 students). All sections received an identical formal writing assignment to report results of laboratory activities. Four blinded raters used a 6-point rating scheme to evaluate the quality of students’ writing performance. Raters’ independent scoring agreement was evaluated using Cronbachs α. Paper scores were compared using a t-test, then papers were combined into low-scoring (3.5 of 6 points) or high-scoring (>3.5 of 6 points) sets and SWH and control cohorts were compared using Pearsons chi-square test. Papers from the SWH cohort were significantly (Pu2009=u20090.02) more likely to receive a high score than those from the control cohort. Overall scores of SWH cohort papers tended to be higher (Pu2009=u20090.07) than those from the control cohort. Gains in student conceptual understanding elicited by the SWH approach improved student ability to express logical conclusions about their data and include appropriate evidence to support those conclusions in formal research reports. Extending the writing tasks of the SWH to formal writing assignments can improve the ability of undergraduates to argue effectively for their research findings.

Current velocity influences the facilitation and removal of algae by stream grazers

The modification of flows in lotic ecosystems can have dramatic effects on abiotic and biotic processes and change the structure of basal trophic levels. In high-gradient streams, most of the biota are benthic, and decreased flow may homogenize and reduce benthic current velocity, potentially changing stream ecosystem function. Grazing by macroinvertebrates is an important component of stream function because grazers regulate energy flow from primary producers to higher trophic levels. We conducted an experiment to examine how macroinvertebrate grazers facilitated or removed algal biomass across a gradient of benthic current velocity (0–40xa0cmxa0s−1). We chose three grazers (Drunella coloradensis, Cinygmula spp., and Epeorus deceptivus) from a montane stream and conducted our experiment using 24 artificial stream channels that had three treatments: no grazers (control), single-grazer, and combined-grazer treatments. In the absence of grazers, algal biomass increased with benthic current velocity. Grazer treatments differed from the control in that more algal biomass was removed at higher velocities, whereas algal accrual was largely facilitated at low velocities. The transition from facilitation to removal ranged from 4.5 to 5.9xa0cmxa0s−1 for individual grazer treatments and occurred at 11.7xa0cmxa0s−1 for the combined-grazer treatment. Our data suggest that velocity plays a significant role in the facilitation and removal of algae by macroinvertebrate grazers. Additionally, the patterns revealed here could have general implications for algal accrual in systems where flow is reduced.

Microhabitat influences on stream insect emergence

Understanding the factors controlling insect emergence from streams has applications to ecological theory regarding cross-boundary flux, along with practical value for monitoring stream function after restoration projects. We hypothesized that stream microhabitat would have effects on emergence that were independent of those mediated by the local stock of benthic macroinvertebrates. We set 50 emergence traps in a third-order stream in northern Minnesota, USA, during two study periods and used structural equation modeling to examine direct and indirect effects of benthic stock and microhabitat features on emergence. Emergence by biomass showed direct positive relationships to substrates of fines and detritus in the first sampling period, and to shallow depth and wood area in the second period. Emergence by abundance had direct positive relationships with benthic stock, CPOM, and fewer macrophytes in the first period, and with benthic stock and periphyton in the second period. Fine substrates may act to concentrate burrowing larvae, whereas CPOM and particularly wood may intercept drifting pre-emergent insects and provide exiting surfaces. Shallow depths may reduce the extent to which resident insects drift downstream (and leave the sample area) while emerging. Periphyton may be an indicator for patches with greater illumination, which itself attracts emergers. Our results suggest emergence is sensitive to environmental conditions at the microhabitat scale, and that stream restoration activities should consider habitat for emerging insects when designing projects.

Current velocity as a factor in determining macroinvertebrate assemblages on wood surfaces

The velocity of moving water near submerged wood surfaces may be an important factor for shaping benthic communities in sandy-bottomed streams. As wood can be suspended above the streambed and be kept free of inundating sediments, wood substrates may provide the primary hard surface for macroinvertebrate colonization. To examine this, we sampled macroinvertebrate assemblages from the surface of 20 pieces of submerged wood in a sandy-bottomed stream in the west-central Wisconsin. Near-surface velocity was characterized at the wood surface and the streambed macroinvertebrate assemblage was sampled near each wood piece for comparison. We found that the density of macroinvertebrates was five times greater on wood than the sandy streambed and the average richness on wood was twice as great. Macroinvertebrate abundance and richness on wood surfaces increased with velocity; however, abundance increased as a consequence of adding taxa rather than adding individuals of the dominant taxa. These data suggest that near-surface current velocity on wood may be an important determinant of benthic community composition when the availability of other hard surfaces is limited.

Biogeographic patterns of lichens and trees on islands of the Boundary Waters Canoe Area Wilderness

Abstract. Different biogeographic factors may influence the richness of lichen and tree assemblages found on islands within boreal lakes. To examine the relative importance of island area, insularity and habitat for shaping these richness patterns, we surveyed lichen and tree species on islands of the Boundary Waters Canoe Area Wilderness in northern Minnesota, USA. We hypothesized that dispersal would be a more important limiting factor for trees, whereas lichen richness would be limited primarily by habitat. Thirty islands were sampled during a one-week study in September-October 2009. For each island we measured island area, distance from lakeshore, and counted the number of habitats. A total of 51 lichen and 15 tree species were found. Lichens occurred on every island and ranged between 2 and 28 species per island. Trees were found on just 13 of the 30 islands and their richness ranged from 1-12 species. Regression analysis and structural equation modeling showed that habitat number and island size were the principle factors affecting the richness of both taxa. Insularity had a small effect on lichen richness and no effect on tree richness. When trees and lichens were examined separately, island area influenced tree richness more than did habitat, whereas lichen richness was most strongly influenced by tree richness.

Macroinvertebrate grazers, current velocity, and bedload transport rate influence periphytic accrual in a field-scale experimental stream

Periphyton plays an important role in stream ecology, and can be sensitive to macroinvertebrate grazers, near-bed current velocity, and bedload abrasion. We manipulated conditions to examine influences on periphytic accrual in the St. Anthony Falls Laboratory Outdoor StreamLab in Minneapolis, MN, USA. Macroinvertebrate grazers were excluded from 27 of 65 clay tiles using electric pulses. We examined periphytic biomass accrual as a function of grazer presence, sampling run, and near-bed current velocity using ANCOVA. We found significant temporal differences between sampling runs but no significant effect of grazer presence. Along with a strong association between bedload transport rates and mean periphytic biomass, our results suggest that grazers are relatively unimportant in stream systems with high levels of physical disturbance from floods and associated sand bedload. However, the interaction between grazer presence and velocity was marginally significant. Regression analyses showed no relation between velocity and periphyton in the absence of grazers but a negative relation when grazers were present, suggesting that mechanical dislodgement of periphyton by grazers may increase with velocity. We conclude that grazers can have subtle effects on periphyton, particularly in streams with high bedload transport rates.

Does ambient substrate composition influence consumer diversity effects on algal removal

Benthic substrates constitute an important habitat template for aquatic communities and may affect the contributions of benthic organisms to ecological processes. To test the effects of ambient substrate composition on the process of algae accrual and removal, we conducted an experiment to examine how substrate type influenced consumer richness effects. We hypothesized that algal removal from focal substrates (ceramic tiles) would be influenced by the surrounding ambient substrate through its effect on nutrient cycling and subsequent algal growth. We manipulated consumer richness in mesocosms at one or three species while holding consumer biomass constant. Aquatic consumers were an amphipod, a snail, and a water boatman, and ambient substrates were either sand or gravel. After 21xa0days, ambient substrate influenced epilithic algal accrual on tiles, affected physio-chemical parameters within mesocosms, and modified consumer behavior. Chlorophyll a was approximately 2× greater on control tiles surrounded by sand, and FPOM and turbidity were greater on sand than gravel when consumers were present. Substrate modified consumer behavior such that consumers congregated around focal substrates in sand, but dispersed around them in gravel. Consumers also had substrate-specific influences on epilithic chlorophyll, causing a decrease in sand and an increase in gravel. Algal assemblages on focal tiles were dominated by diatoms, and their composition responded to consumer richness and identity, but not substrate. Our data suggest that direct effects (e.g., consumptive removal of epilithon from focal tiles) were more pronounced in sand, whereas indirect effects (e.g., bioturbation and enhanced mixing) promoted algal accrual in gravel. These results show that algae production on exposed surfaces may change as underlying substrate composition changes, and that substrate type can alter consumer diversity effects on algal removal.

How do stream grazers partition their benthic habitat

AbstractEmphasizing species’ trait similarities over differences, functional feeding guilds underrepresent functional diversity within stream communities. Species within guilds commonly overlap in their distributions and must partition their shared resources to coexist. Guild partitioning strategies may be revealed by the dissimilar responses the species show to environmental conditions, and these responses may be predicted from species-specific traits. To test this, I examined a grazer guild in a mountain stream dominated by three mayflies, Baetis, Epeorus, and Cinygmula. My objective was to determine how these species responded to gradients of current velocity, stream depth, and rock size and the abundance of algal food. I sampled 35 streambed locations and measured near-bed current velocity and depth at nine points at each location before sampling macroinvertebrates with a Surber sampler. Nine of the sampled rocks were measured for size, and benthic algae were collected from four of them. Multiple regression and non-metric multi-dimensional scaling (NMDS) revealed that each mayfly had unique responses to current velocity, depth, rock size, and algal abundance. Baetis correlated with depth, current velocity, and the interaction of these variables. Epeorus correlated exclusively with current velocity, and Cinygmula correlated with depth in interaction with other variables. Algal abundance by itself was a poor predictor of mayfly abundance, and NMDS indicated that individual species responses were not good predictors of mayfly assemblage patterns. Nevertheless, the mayflies’ idiosyncratic responses to current velocity, depth, and rock size supported the notion that these variables are components of unique partitioning strategies.n