Kevin H. Wyatt

Identifying Key Features of Effective Active Learning: The Effects of Writing and Peer Discussion

This study compared the effectiveness of three different methods of implementing active-learning exercises in an introductory biology course. The results suggest that individual writing should be implemented as part of active learning whenever possible and that instructors may need training and practice to become effective with active learning.

Regulation of algal structure and function by nutrients and grazing in a boreal wetland

Abstract We evaluated the potential for grazers to regulate benthic algal biomass and taxonomic composition in an Alaskan marsh after enrichment with nutrients that are expected to increase in the region with ongoing climate change. We nested caged and uncaged substrates together inside mesocosm enclosures with natural abundances of snails or no snails and with or without nutrient enrichment (NO3 + PO4 + Si). Algal biomass was greater in all nutrient-enriched enclosures than in controls. Algal biomass was greater in enclosures where grazers were present but excluded by a cage than in enclosures where grazers were allowed to graze or where grazers were absent. In the presence of nutrients, grazed communities were dominated by small coccoid green algae and cyanobacteria, which were overgrown by filamentous green algae when grazers were excluded. In the absence of nutrients, grazers had little effect on algal biomass or taxonomic composition. However, grazers recycled a small but potentially important amount of nutrients in their waste, suggesting that consumer-driven nutrient recycling may have played a role in maintaining algal biomass when grazers were present. Our data show that grazers regulate algal responses to nutrients by suppressing algal accumulation but increasing productivity through nutrient recycling in a northern boreal wetland.

Effects of nutrient limitation on the release and use of dissolved organic carbon from benthic algae in Lake Michigan

Abstract: Compared to phytoplankton, relatively little information exists about the importance of benthic algae as a source of dissolved organic C (DOC) in lakes. We enriched outdoor tanks with N and P in a full factorial design (unenriched, +N, +P, N+P) to evaluate effects of nutrients on the release, composition, and decomposition of DOC from Cladophora glomerata (L.) Kütz. in Lake Michigan. After 10 d of enrichment, biomass-specific rates of gross primary productivity (GPP) were significantly greater in the N+P treatment than in the +N or +P treatments. The fraction of fixed C released as DOC was ∼37% of GPP in the N+P treatment, which was similar to material collected from the lake (initial samples). Biomass-specific rates of DOC release did not vary significantly with treatment, whereas GPP declined in the nutrient-limited treatments (+N, +P, and unenriched). Consequently, DOC release represented a higher percentage of GPP (>100%) under severe nutrient limitation. Specific ultraviolet (UV) absorbance, an indicator of aromatic compounds, indicated that DOC from material grown in the N+P treatment (DOCN+P) was more labile than DOC from material grown in the +N treatment (DOC+N). Both DOCN+P and DOC+N supported an exponential increase in bacterial density during a 19-d incubation, but DOCN+P was used more rapidly and supported higher bacterial density than DOC+N. Cladophora has become a major contributor to primary production in the shallow nearshore areas of the lower Laurentian Great Lakes, and our data suggest that ≥⅓ of this productivity is available for the benthic microbial loop as DOC. The amount of DOC released remains relatively constant as nutrient limitation reduces productivity, but the nutrient status of the alga affects the quality of DOC as a substrate for heterotrophic bacteria.

Algae alleviate carbon limitation of heterotrophic bacteria in a boreal peatland

Summary In most high-latitude wetlands, carbon accumulation as peat represents a balance between plant net primary productivity and heterotrophic decomposition. We hypothesized that this assessment of ecosystem metabolism is incomplete as it does not include information on energy inputs from microalgae, which form complex biofilms with heterotrophic micro-organisms on the peat surface. To investigate the potential for algae (and associated exudates) to support heterotrophic metabolism under ambient and elevated nutrient levels, we conducted a factorial enrichment of nutrients (nitrogen and phosphorus) and carbon (glucose) in mesocosms with and without the presence of algae (using light-transparent and dark treatments, respectively) in an Alaskan fen. We measured respiration rates and changes in bacterial biomass to characterize the response of heterotrophic bacteria to our experimental treatments. During incubation assays, algae released up to 38% of their net productivity as exudates and there was a positive relationship between algal production and concentrations of dissolved organic carbon inside mesocosms. Elevated algal productivity in the presence of nutrient enrichment stimulated heterotrophic bacterial respiration and biomass. These responses did not occur with nutrient enrichment in the dark (without algae). The response of bacteria to algae was similar in magnitude to bacterial responses to glucose enrichment. Synthesis. We conclude that bacteria in this boreal fen were primarily limited by labile carbon, and this constraint was alleviated in the presence of elevated algal production. Consequently, algae may facilitate hotspots of microbial activity in northern peatlands, especially in conditions of greater nutrient availability associated with more variable hydrology expected for this region with ongoing climate change.

Hot and toxic: Temperature regulates microcystin release from cyanobacteria

The mechanisms regulating toxin release by cyanobacteria are poorly understood despite the threat cyanotoxins pose to water quality and human health globally. To determine the potential for temperature to regulate microcystin release by toxin-producing cyanobacteria, we evaluated seasonal patterns of water temperature, cyanobacteria biomass, and extracellular microcystin concentration in a eutrophic freshwater lake dominated by Planktothrix agardhii. We replicated seasonal variation in water temperature in a concurrent laboratory incubation experiment designed to evaluate cause-effect relationships between temperature and toxin release. Lake temperature ranged from 3 to 27°C and cyanobacteria biomass increased with warming up to 18°C, but declined rapidly thereafter with further increases in temperature. Extracellular microcystin concentration was tightly coupled with temperature and was most elevated between 20 and 25°C, which was concurrent with the decline in cyanobacteria biomass. A similar trend was observed in laboratory incubations where productivity-specific microcystin release was most elevated between 20 and 25°C and then declined sharply at 30°C. We applied generalized linear mixed modeling to evaluate the strength of water temperature as a predictor of cyanobacteria abundance and microcystin release, and determined that warming≥20°C would result in a 36% increase in microcystin release when Chlorophyll a was ≤50μgl-1. These results show a temperature threshold for toxin release in P. agardhii, which demonstrates a potential to use water temperature to forecast bloom severity in eutrophic lakes where blooms can persist year-round with varying degrees of toxicity.

Spatial and temporal variability of algal community dynamics and productivity in floodplain wetlands along the Tanana River, Alaska

Abstract The boreal landscape is a mosaic of wetlands with distinct ecosystem properties. Algae are important for wetland functioning, but relatively little is known about the structure of algal communities among boreal wetlands. We documented spatial and temporal variability of algal community dynamics and productivity during a growing season in 6 wetlands (1 rich and 1 poor fen, 1 tussock, and 3 riverine marshes) in interior Alaska. Algal biomass and productivity were greater in the poor fen and a riverine marsh than in all other wetlands. Water depth and nutrients were significant predictors of benthic algal biomass and productivity among wetlands and were greatest immediately after the spring thaw and decreased during the growing season. Water depth and nutrients (N and P) explained the most variability in algal community structure. Algal community structure differed among wetlands, and temporal variation in environmental conditions was a significant predictor of the relative abundance of algal genera in individual wetlands. N2-fixing cyanobacteria increased in abundance with a seasonal decline in water depth and nutrient concentrations. Our characterization of algal community dynamics and productivity in relation to environmental characteristics will help to forecast future wetland function in a changing boreal landscape.

Algal community response to experimental and interannual variation in hydrology in an Alaskan boreal fen

Abstract.  Floristic studies indicate the abundance of microalgae in northern boreal peatlands, but we know relatively little about their ecology or how they will respond to changes in environmental conditions expected in this region as climate changes. We examined changes in algal community structure at sites exposed to a long-term water-table manipulation, including drought (lowered water-table treatment), flooding (raised water-table treatment), and control (no manipulation) treatments in an Alaskan fen. In previous years, continuous algal colonization typically would have occurred only in the raised water-table treatment, but a spring flood inundated experimental plots and provided a unique opportunity to examine algal community response to rewetting after long-term drought. This event allowed us to investigate how much ecosystem memory of the antecedent water-table manipulations regulated the ability of taxa to recolonize sites after prolonged drought compared to sites that had been continuously flooded. Despite no differences in water-table position among treatments at the time of sampling, surface-water nutrient concentrations were higher in the lowered water-table treatment relative to the other treatments after the spring thaw. This difference corresponded with greater algal abundance and biovolume in the lowered water-table treatment relative to the control and raised water-table treatments. Higher abundance and biovolume was driven mainly by filamentous green algae (Chlorophyta), especially Oedogonium, Spirogyra, and Microspora. Diatoms were most abundant in the raised water-table treatment, whereas chrysophytes were most abundant in the control treatment. Across all water-table treatments, N-fixing cyanobacteria increased as nutrients and water-table position declined over time. The differences in algal community structure among water-table treatments suggest that alternating drought and flooding events expected with climate change may significantly alter algal-driven functions in boreal wetlands.

The role of light availability and herbivory on algal responses to nutrient enrichment in a riparian wetland, Alaska

We investigated how the relative availability of solar radiation in the presence or absence of grazing alters the ability of benthic algae to respond to nutrient enrichment in an Alaskan marsh. We used a factorial mesocosm experiment that included nutrient enrichment (enriched or control), grazing (grazed or ungrazed), and light (unshaded or shaded) to simulate shading by macrophytes early and late in the growing season, respectively. We found stronger effects of grazers and nutrients compared to light on benthic algal biomass and taxonomic composition. Algal biomass increased in nutrient‐enriched treatments and was reduced by grazing. Shading did not have an effect on algal biomass or taxonomic composition, but the concentration of chl a per algal biovolume increased with shading, demonstrating the ability of algae to compensate for changes in light availability. Algal taxonomic composition was more affected by grazer presence than nutrients or light. Grazer‐resistant taxa (basal filaments of Stigeoclonium) were replaced by diatoms (Nitzschia) and filamentous green algae (Ulothrix) when herbivores were removed. The interacting and opposing influences of nutrients and grazing indicate that the algal community is under dual control from the bottom‐up (nutrient limitation) and from the top‐down (consumption by herbivores), although grazers had a stronger influence on algal biomass and taxonomic composition than nutrient enrichment. Our results suggest that low light availability will not inhibit the algal response to elevated nutrient concentrations expected with ongoing climate change, but grazers rapidly consume algae following enrichment, masking the effects of elevated nutrients on algal production.

Are long-term fish assemblage changes in a large US river related to the Asian Carp invasion? Test of the hostile take-over and opportunistic dispersal hypotheses

Gizzard shad is a dominant planktivore/detritivore in the Wabash River, and populations crashed in the early 1990s. Previous work (1974–2008) identified a substantial shift in body-size structure and functional trait composition in the Wabash River fish assemblage between 1989 and 1996. Invasive Asian Carp appeared in the Ohio River basin including the Wabash River in the 1990s. Our goal was to test for temporal changes in assemblage composition and trophic structure relative to the invasion of Asian carp from the early 1990s. We hypothesized that establishment of Asian Carp was a contributor to the assemblage composition shift, and that their presence altered the trophic pathways and food sources of native fishes including Gizzard Shad (hostile takeover hypothesis). Alternatively, Asian Carp may have found success through capitalizing on an empty niche, likely left vacant by the decline in Gizzard Shad, or abundance changes in other trophic groups (opportunistic hypothesis). We utilized archival fish and mussel collections to test for trophic changes in the ecosystem using δ13C and δ15N stable isotope analyses. We examined stomach contents of the dominant planktivore/detritivore consumer, Gizzard Shad, from archival and recent collections to test for changes in the phytoplankton community. Stable isotopes indicated a community reliance on other, more deplete, carbon sources than indicated by the algae, and a slight increase between the δ13C time periods. Although all functional feeding groups of fishes indicated some reduction in δ15N, the differences were only significant for omnivores, mussels, and planktivore/detritivores. Although Asian Carp may have contributed to the collapse of Gizzard Shad populations, other stressors were likely more important drivers of their decline. This is the first, albeit indirect, evidence of opportunistic “invasion” as opposed to the historically presumed hostile takeover model.