Nancy C. Tuchman

DENSITY-DEPENDENT GROWTH, ECOLOGICAL STRATEGIES, AND EFFECTS OF NUTRIENTS AND SHADING ON BENTHIC DIATOM SUCCESSION IN STREAMS1

The importance of immigration, growth, and competition for nutrients and light in benthic diatom succession was studied in experimental channels in a low‐nutrient stream. Diatom accumulation was greater in channels enriched with nitrate and phosphate (NP) than in control channels, reaching about 5 × 106 and 2 × 106 cells‐cm−2, respectively, after 30 d. Shading during late stages of community development reduced algal standing crop.

Changes in the vertical microdistribution of diatoms within a developing periphyton mat

We followed periphyton succession for 120 d in an artificial stream system to: 1) document an increase in cell densities and associated decrease in irradiance to the base of developing periphyton mats; 2) determine the vertical location of motile and nonmotile diatom species within the mat associated with these changes; and 3) determine the viability of diatom cells at the base of the mat compared with those of the same species at different vertical locations within the mat. We developed a novel method for producing thin cross-sections of intact periphyton mats to document the vertical distribution of algal species in horizontal 0.1-mm strata from the base of the mat to the canopy. Results demonstrated that the adnate nonmotile diatom Achnanthidium minutissimum exploited the substratum during early succession when the mat was relatively thin and without a canopy, and the population retained live cells at the base of the mat throughout the 120-d experiment, demonstrating a high tolerance to low light levels. Gomphonema angustatum also was positioned at the base of the mat and was common during early stages of succession, but viable cell densities declined sharply by day 37 when increasing cell densities reduced irradiance at the mat base by 90% (to ca 8 μmol quanta m-2· s-1). The only highly motile species studied, Nitzschia palea, was distributed throughout the vertical profile of the periphyton mat and its live cell densities did not change throughout the study, suggesting that it moved into resource-rich microhabitats and avoided stress. The vertical distribution of a tychoplanktonic chain-forming diatom species appeared to be related to the physical architecture of the developing mat and not to resource limitations within the mat. Our data suggest that diatom species differ in their tolerance to resource-limiting conditions in developing periphyton mats, and these differences affect autecological characteristics.

Differential heterotrophic utilization of organic compounds by diatoms and bacteria under light and dark conditions

The heterotrophic utilization of organic substrates by diatoms is likely an important survival strategy when light levels are too low for photosynthesis. The objectives of this study were: (1) to determine if heterotrophic utilization of a large array of organic compounds by eight common freshwater benthic diatom taxa was light-dependent, and (2) to determine if organic substrate utilization patterns differed between darkgrown diatoms and bacteria as a possible means of reducing competition by niche separation. Eight lightand dark-grown diatom taxa and five bacterial species were incubated in 96-well Biolog® Microtiter plates with each well containing 1 of 95 different organic substrates. Oxidation rates of each organic substrate were measured through time. There was a substantial increase in the number of organic substrates oxidized by diatoms grown in the dark compared to their light-grown counterparts, indicating that the transport systems for these molecules may be light activated. Therefore, diatoms likely only utilize these metabolically expensive uptake mechanisms when they are necessary for survival, or when substrates are plentiful. A principal components analysis indicated discernible differences in the types of organic-C substrates utilized by dark-grown diatoms and bacteria. Although bacteria were able to oxidize a more diverse array of organic substrates including carboxylic acids and large polymers, diatoms appeared to more readily utilize the complex carbohydrates. By oxidizing different organic substrates than bacteria, heterotrophically metabolizing diatoms may be reducing direct competition and enhancing coexistence with bacteria.

Elevated Atmospheric CO2 Alters Soil Microbial Communities Associated with Trembling Aspen (Populus tremuloides) Roots

Global atmospheric CO2 levels are expected to double within the next 50 years. To assess the effects of increased atmospheric CO2 on soil ecosystems, cloned trembling aspen (Populustremuloides) seedlings were grown individually in 1 m3 open bottom root boxes under either elevated (720 ppm, ELEV) or ambient CO2 (360 ppm, AMB). After 5 years, soil cores (40 cm depth) were collected from the root boxes and divided into 0–20 cm and 20–40 cm fractions. ELEV treatment resulted in significant decreases in both soil nitrate and total soil nitrogen in both the 0–20 cm and 20–40 cm soil fractions, with a 47% decrease in soil nitrate and a 50% decrease in total soil nitrogen occurring in the 0–20 cm fraction. ELEV treatment did not result in a significant change in the amount of soil microbial biomass. However, analysis of indicator phospholipid fatty acids (PLFA) indicated that ELEV treatment did result in significant increases in PLFA indicators for fungi and Gram-negative bacteria in the 0–20 cm fraction. Terminal restriction fragment length polymorphism (T-RFLP) analysis was used to analyze the composition of the soil bacterial communities (using primers targeting the 16SrRNA gene) and the soil fungal communities (using primers targeting the intergenic transcribed spacer region). T-RFLP analysis revealed shifts in both bacterial and fungal community structure, as well as increases in both bacterial and fungal species richness with ELEV treatment. These results indicated that increased atmospheric CO2 had significant effects on both soil nutrient availability and the community composition of soil microbes associated with aspen roots.

PATTERNS OF ENVIRONMENTAL CHANGE ASSOCIATED WITH TYPHA X GLAUCA INVASION IN A GREAT LAKES COASTAL WETLAND

Typha x glauca (hybrid cattail) is an aggressive invader of wetlands in the upper Midwest, USA. There is widespread concern about declines in plant diversity followingTypha invasion. However, relatively little is known about howTypha alters habitat characteristics, i.e., its potential to act as an “ecosystem engineer”. Over five years, we measured physical, chemical, and plant community changes associated withTypha invasion in a Lake Huron wetland in northern lower Michigan. We compared uninvaded areas with patches varying in invasion intensity. Our study was observational, but we used statistical inference to try to separate effects ofTypha and confounding variables, particularly water depth. We used space-for-time substitution to investigate whetherTypha-associated changes increased over time, as predicted ifTypha invasion was in part a cause (not only a consequence) of abiotic changes. Relative to uninvaded areas,Typha-invaded areas differed in plant-community composition and had lower species richness, higher litter mass, and higher soil organic matter and nutrient concentrations (all P < 0.001). Overall,Typha invasion appeared to displace native species and enrich wetland soils. These changes could benefitTypha at the expense of native species, potentially generating plant-soil feedbacks that pose special challenges for wetland management and restoration.

Effects of Selective Grazing by Snails on Benthic Algal Succession

Effects of ambient levels of snail grazing on rates and direction of benthic algal succession were investigated in experimental chambers located near two lakes. Benthic algal environments were manipulated by introducing snails (15 individuals/m2) to flow-through experimental systems (for 26-28 d) where clay tiles were colonized with algae from littoral lake water from Douglas Lake, Michigan, and Kentucky Lake, Kentucky. We found that algal succession was not always evident in ungrazed conditions; but when successional changes were evident, many different growth forms were observed among early and late succession species. Early succession species were typically non-motile unicellular or colonial growth forms, whereas late succession species were motile unicellular or filamentous diatoms and cyanobacteria. Grazing by the snail Elimia livescens arrested succession of non-diatom algae by selectively removing the overstory of filamentous cyanobacteria. However, grazing by E. livescens enhanced succession among diatoms by selectively grazing the early succession diatoms and leaving small motile late succession species. Whether grazing enhanced or arrested succession, or changed the direction of succession, was dependent upon growth forms of early and late succession algal species, which varied among habitats and corresponding species assemblages.

Resource stress alters hydrological disturbance effects in a stream periphyton community

Scouring reduces stream periphyton biomass during floods, and levels of light and nutrient resource supply can influence growth and community physiology between floods. We hypothesised that for a given taxonomic structure, resource stressed communities would have lower resistance and resilience to scour disturbance than resource replete communities. We investigated this in experimental streams with four combinations of light and nutrients. Communities in all treatments were dominated by similar diatoms. Resource supply significantly influenced the resistance to scouring of some components of the communities. In particular, high nutrient stress resulted in low resistance of chlorophyll a biomass (up to 94% loss), whereas moderate light stress and low nutrient stress resulted in the most resistant communities (43% loss). Resilience of chlorophyll a was negatively correlated, and resilience of taxonomic composition was positively correlated, with resource stress. In the treatments with high nutrient supply, chlorophyll a returned to pre-disturbance levels within 6 d, but regeneration of the taxonomic structure took >9 d. For the treatments with low nutrient supply, chlorophyll a took >18 d to recover, but taxonomic composition took only a few days to recover reflecting the dominance of immigration processes (and lack of in situ growth). In all treatments the scour disturbance caused a reduction in growth rates of the regenerating communities relative to the undisturbed control communities. Overall, the disturbance had a longer-term influence on community persistence under moderate to high resource stress than under low resource stress.

Colonial aggregates: effects of spatial position on zebra mussel responses to vertical gradients in interstitial water quality

Vertical gradients in interstitial water quality may develop within densely organized assemblages of sessile aquatic organisms. These gradients may compromise the survival of individuals. We examined whether a vertical gradient of interstitial water chemistry (NO3-N, NH4-N, and dissolved oxygen [DO]) would develop within dense zebra mussel (Dreissena polymorpha) colonies in a laboratory flume (flow rate ∼1 cm/s). Over a 4-h duration, we found that NO3-N concentrations increased, DO decreased, and NH4-N concentrations remained the same from the surface to the base of 6-cm thick zebra mussel colonies. These results were supported by trends found in natural Lake Michigan zebra mussel colonies at 4 to 6 m depths, where NO3-N concentrations at the base of colonies measured 162% of NO3-N concentrations in open water above the colonies. We also examined how vertical water-quality gradients influenced zebra mussel movement and mortality by tracking the vertical position and survivorship of individual zebra mussels in colonies. Substantial movement out of the base of the colony occurred after 7 and 30 d of incubation. After 30 d, 69% of smaller-sized mussels (≤6 mm) moved upward within the colony from the base, in contrast to 0% movement of larger mussels (>20 mm), whose motility may have been impeded. After 30 d, mortality of all size classes significantly increased, with >50% mortality occurring in the bottom layer. Our studies suggest that dense colonies produce vertical interstitial water-quality gradients at low flow, and that movement out of the base of the colony by smaller mussels may be an important mechanism for survival in dense colonies.

Water Level Decline Promotes Typha X glauca Establishment and Vegetation Change in Great Lakes Coastal Wetlands

Climate change is predicted to reduce Laurentian Great Lakes water levels, altering coastal wetland ecosystems and potentially stimulating invasive macrophytes, like Typha X glauca. Recent prolonged low water levels, which climaxed in 2007, created conditions comparable to those predicted by climate change science. In 2008, we examined ecosystem and plant community properties in 14 intact northern Great Lakes coastal wetlands and compared community data with data from a 1987–1989 high-water period, before T. X glauca invasion. In 2008, T. X glauca occurred in 50% of wetlands and 16% of plots; was associated with reduced Floristic Quality and increased soil organic matter, soil nutrients, and leaf litter (all p < 0.05); and plant community composition had shifted and was more homogeneous than in 1988 (both p < 0.05). Additionally, T. X glauca was more dominant when growing behind barrier beach ridges, which form in high-water conditions and persist in low-water, than in lake-exposed marshes (p < 0.05), revealing a physiographic mechanism for increased dominance. Beach ridges protect T. X glauca from wave and seiche energy, and as water levels decline, these energy-insulating microtopographic features will likely stimulate further invasion and dominance by T. X glauca, even in high quality wetlands.

Elevated-CO2-induced changes in the chemistry of quaking aspen (Populus tremuloides Michaux) leaf litter: subsequent mass loss and microbial response in a stream ecosystem

Changes in chemistry of quaking aspen (Populus tremuloides Michaux) leaf litter were examined under ambient (AMB = 360 ppm) and elevated (ELE = 720 ppm) levels of atmospheric CO2. Senesced ELE leaves were significantly higher in phenolic compounds, lignin, and C:N than AMB leaves. A 30-d in situ experiment in a northern Michigan stream analyzed changes in leaf mass, the concentration of phenolic compounds as a result of chemical leaching, and the growth responses of fungi and bacteria. ELE leaves lost less mass than AMB leaves after a 30-d incubation. Although ELE leaves were initially higher in total phenolic compounds and condensed tannins, differences between the treatments were no longer observed after 48 h of chemical leaching. Bacterial biomass and community respiration were higher on the AMB leaves for the first 12 d of incubation, whereas fungal biomass and community respiration were higher in the AMB treatment by the end of the 30-d experiment. Fungal biomass was negatively correlated with C:N and positively correlated with bacterial biomass on the ELE leaves, but not on the AMB leaves. These results indicate that a doubling in concentration of atmospheric CO2 could lead to leaf litter that is more recalcitrant toward microbial breakdown, which may decrease the availability of C and N for higher trophic levels.