Showing papers by "Diane M. McKnight published in 2010"

Effect of instrument‐specific response on the analysis of fulvic acid fluorescence spectra

Abstract: Fluorescence spectroscopy has been extensively employed to characterize the source, age, and reactivity of aquatic dissolved organic matter (DOM). However, there is no consensus on the protocols for collecting and correcting DOM fluorescence spectra for the instrument-specific response associated with each component on a fluorometer involved in the excitation of DOM and the resulting detection of DOM emission. The central objective of this study was to evaluate the removal of instrument-specific response from DOM fluorescence spectra collected on three different fluorometers using manufacturer-provided emission and excitation correction files. We evaluated criteria and protocols for comparison of removal of instrument response, using quinine sulfate, a well-characterized fluorophore, as well as the International Humic Substance Society's microbially and terrestrially derived reference end-member fulvic acids: Pony Lake and Suwannee River fulvic acids, respectively. Our results demonstrate that sample spectra collected on different fluorometers differed significantly before correction. Although the effectiveness of manufacturer-provided correction factors in removal of instrument response from sample spectra varied by instrument, spectral overlap of the same sample on multiple instruments improved after correction. Our results suggest that conclusions based on analysis of trends within a dataset of DOM fluorescence spectra should be expected to be independent of the fluorometer employed.

Dissolved organic matter sources and consequences for iron and arsenic mobilization in Bangladesh aquifers.

Abstract: Iron (Fe) and dissolved organic matter (DOM) cycling have been implicated in arsenic mobilization via microbially mediated Fe oxide reduction. To evaluate the sources and multiple roles of DOM in Bangladesh aquifers, we conducted spectroscopic analyses on various types of surface water and groundwater samples from a site representative of aquifer chemistry and hydrology. Surface water contained humic substances with oxidized quinone-like moieties and high concentrations of labile microbially derived DOM. In contrast, in shallow groundwater where dissolved iron and arsenic concentrations were high, the quinone-like moieties of humic substances were more reduced, with less abundant labile DOM than that of surface water. Instead, DOM at these depths was characterized by terrestrial (plant/soil) signatures. A sediment microcosm experiment demonstrated that Fe(II) and terrestrially derived DOM were released from sediment over time. The results provide new evidence to support a dual role of natural DOM in Bangladesh aquifers (1) as a labile substrate for Fe- and humic-reducing bacteria and (2) as an electron shuttle via humic substances to enhance microbial iron reduction. Fluorescence index, amino acid-like fluorescence, and redox index may serve as useful indicators of the type of DOM likely to be involved in Fe solubilization and potentially As mobilization reactions.

Comparison of seasonal changes in fluorescent dissolved organic matter among aquatic lake and stream sites in the Green Lakes Valley

Abstract: [1] The spectral characteristics of whole water dissolved organic matter (DOM) and fulvic acid were studied in samples collected from an alpine lake, a subalpine lake, and a subalpine stream during snowmelt and the summer growing season. Excitation-emission matrices of whole water DOM and fulvic acid were analyzed by parallel factor analysis (PARAFAC). Allochthonous inputs of terrestrially derived fulvic acid DOM were dominant during snowmelt at the alpine lake, and during both snowmelt and summer at the subalpine sites. At the alpine lake, autochthonous inputs of DOM dominated during the summer phytoplankton bloom, and the spectral characteristics of the whole water DOM diverged from those of the fulvic acid. For example, the quinone-like fluorophores in whole water DOM at the alpine lake were more oxidized and microbially derived than the fulvic acid fraction during the summer. At the subalpine sites, the seasonal changes in the source and redox state of the quinone-like fluorophores of the whole water DOM tracked those of the fulvic acid pool. However, at both lake sites there was a greater contribution of amino acid-like fluorophores in the whole water DOM than the fulvic acid fraction. This trend was not observed at the subalpine stream site. Principal components analysis (PCA) of the PARAFAC components suggests that during snowmelt, the chemical quality of the DOM at the alpine lake was similar to that of the subalpine stream; whereas the alpine site was more similar to the subalpine lake during the summer. Spectral characterization and PCA of the PARAFAC components suggest that nonhumic quinone-like and amino acid-like fluorophores were produced in the alpine lake during the summer phytoplankton bloom. Our results show that different types of water bodies produce different seasonal patterns in whole water DOM and fulvic acid quantity and quality.

Spatial variations in the geochemistry of glacial meltwater streams in the Taylor Valley, Antarctica

Abstract: Streams in the McMurdo Dry Valleys, Antarctica, flow during the summer melt season (4-12 weeks) when air temperatures are close to the freezing point of water. Because of the low precipitation rates, streams originate from glacial meltwater and flow to closed-basin lakes on the valley floor. Water samples have been collected from the streams in the Dry Valleys since the start of the McMurdo Dry Valleys Long-Term Ecological Research project in 1993 and these have been analysed for ions and nutrient chemistry. Controls such as landscape position, morphology of the channels, and biotic and abiotic processes are thought to influence the stream chemistry. Sea-salt derived ions tend to be higher in streams that are closer to the ocean and those streams that drain the Taylor Glacier in western Taylor Valley. Chemical weathering is an important process influencing stream chemistry throughout the Dry Valleys. Nutrient availability is dependent on landscape age and varies with distance from the coast. The streams in Taylor Valley span a wide range in composition and total dissolved solids and are surprisingly similar to aw ide range of much larger temperate and tropical river systems.

Communicating with the public: opportunities and rewards for individual ecologists

Abstract: Many ecologists are interested in communicating science to the public and addressing societal concerns about environmental issues. Individual ecologists need to consider whether, when, and how this should be done. We propose that public outreach activities can be beneficial for ecologists at all stages of their career. There are diverse opportunities for such involvement, and these can vary enormously in terms of time and expertise required. Trends within the science of ecology, especially research focused on social-ecological systems, are likely to promote increased interactions with stakeholders and policy makers. To be effective in these interactions, ecologists should consider new approaches to communication and be aware of the potential roles scientists can play in public policy debates. Professional ecologists need to engage with non-scientific audiences; a review of such activities should be included in considerations for promotion, recognition, and awards, while also acknowledging variations in the inclinations and abilities of individual scientists. There are, however, few current standards for how much time ecologists should commit to public outreach, how time allocation might change over a career, or how to evaluate the quality of such activities. We ask ecologists to consider ways to evaluate the quality of interactions with the public and how to reward these efforts appropriately.

New light on a dark subject: comment

Abstract: In their article, “New light on a dark subject: On the use of fluorescence data to deduce redox states of natural organic matter,” Macalady and Walton-Day (2009) subjected natural organic matter (NOM) samples to oxidation, reduction, and photochemical transformation. Fluorescence spectra were obtained on samples, which were diluted “to bring maximum uvvisible absorbance values below 1.0.” The spectra were fit to the Cory and McKnight (2005) parallel factor analysis (PARAFAC) model, and consistent variation in the redox state of quinone-like moieties was not detected. Based on these results they concluded that fitting fluorescence spectra to the Cory and McKnight (2005) PARAFAC model to obtain information about the redox state of quinone-like moieties in NOM is problematic. Recognizing that collection and correction of fluorescence spectra requires consideration of many factors, we investigated the potential for inner-filter effects to obscure the ability of fluorescence spectroscopy to quantify the redox state of quinone-like moieties. We collected fluorescence spectra on Pony Lake and Suwannee River fulvic acid standards that were diluted to cover a range of absorbance wavelengths, and fit these spectra to the Cory and McKnight (2005) PARAFAC model. Our results suggest that, in order for the commonly used inner-filter correction to effectively remove inner-filter effects, samples should be diluted such that the absorbance at 254 nm is less than 0.3 prior to the collection of fluorescence spectra. This finding indicates that inner-filter effects may have obscured changes in the redox signature of fluorescence spectra of the highly absorbing samples studied by Macalady and Walton-Day (2009).

Effects of short-term drying and irrigation on CO2 and CH4 production and emission from mesocosms of a northern bog and an alpine fen.

Abstract: Atmospheric CO2 and CH4 exchange in peatlands is controlled by water table levels and soil moisture, but impacts of short periods of dryness and rainfall are poorly known. We conducted drying-rewetting experiments with mesocosms from an ombrotrophic northern bog and an alpine, minerotrophic fen. Efflux of CO2 and CH4 was measured using static chambers and turnover and diffusion rates were calculated from depth profiles of gas concentrations. Due to a much lower macroporosity in the fen compared to the bog peat, water table fluctuated more strongly when irrigation was stopped and resumed, about 11 cm in the fen and 5 cm in the bog peat. Small changes in air filled porosity caused CO2 and CH4 concentrations in the fen peat to be insensitive to changes in water table position. CO2 emission was by a factor of 5 higher in the fen than in the bog mesocosms and changed little with water table position in both peats. This was probably caused by the importance of the uppermost, permanently unsaturated zone for auto- and heterotrophic CO2 production, and a decoupling of air filled porosity from water table position. CH4 emission was 12.6 mmol m−2 day−1 in the fen peat, where it was lowered by water table fluctuations. CH4 production was limited to the saturated zone in the bog peat but proceeded in the capillary fringe of the fen peat. Water table drawdown partly led to inhibition of methanogenesis in the newly unsaturated zone, but CH4 production appeared to continue after irrigation without time-lag. The identified effects of irrigation on soil moisture and respiration highlight the importance of peat physical properties for respiratory dynamics; but the atmospheric carbon exchange was fairly insensitive to the small-scale fluctuations induced.

Effects of short-term drying and irrigation on electron flow in mesocosms of a northern bog and an alpine fen.

Abstract: Methane emissions and element mobility in wetlands are controlled by soil moisture and redox conditions. We manipulated soil moisture by weekly drying and irrigation of mesocosms of peat from a bog and iron and sulfur rich fen. Water table changed more strongly in the decomposed fen peat ( approximately 11 cm) than in the fibric bog peat ( approximately 5 cm), where impacts on redox processes were larger due to larger change in air filled porosity. Methanogenesis was partly decoupled from acetogenesis and acetate accumulated up to 5.6 mmol L(-1) in the fen peat after sulfate was depleted. Irrigation and drying led to rapid redox-cycles with sulfate, hydrogen sulfide, nitrate, and methane being produced and consumed on the scale of days, contributing substantially to the total electron flow and suggesting short-term resilience of the microbial community to intermittent aeration. Anaerobic CO2 production was partly balanced by methanogenesis (0-34%), acetate fermentation (0-86%), and sulfate reduction (1-30%) in the bog peat. In the fen peat unknown electron acceptors and aerenchymatic oxygen influx apparently drove respiration. The results suggest that regular rainfall and subsequent drying may lead to local oxidation-reduction cycles that substantially influence electron flow in electron acceptor poor wetlands.

Life in Antarctic Deserts and other Cold Dry Environments: Astrobiological Analogs

Abstract: 1. Introduction Peter T. Doran, W. Berry Lyons and Diane M. McKnight 2. Geologic analogies between the surface of Mars and the McMurdo Dry Valleys: microclimate-related geomorphic features and evidence for climate change David R. Marchant and James W. Head 3. The legacy of aqueous environments on soils of the McMurdo Dry Valleys: contexts for future exploration of Martian soils Jeb E. Barrett, Michael A. Poage, Michael A. Gooseff and Cristina Takacs-Vesbach 4. The Antarctic cryptoendolithic microbial ecosystem Henry J. Sun, James A. Nienow and Christopher P. McKay 5. Antarctic McMurdo Dry Valley stream ecosystems as analog to fluvial systems on Mars Michael A. Gooseff, Diane M. McKnight, Michael H. Carr and Jenny Baeseman 6. Saline lakes and ponds in the McMurdo Dry Valleys: ecological analogs to Martian paleolake environments Jill Mikucki, W. Berry Lyons, I. Hawes, Brian D. Lanoil and Peter T. Doran 7. The biogeochemistry and hydrology of Dry Valley glaciers: is there life on Martian ice now? Martyn Tranter, Elizabeth Bagshaw, Andrew Fountain and Christine Foreman 8. Factors promoting microbial diversity in the McMurdo Dry Valleys, Antarctica Cristina Takacs-Vesbach, Lydia Zeglin, Jeb E. Barrett, Michael A. Gooseff and John C. Priscu 9. Other analogs to Mars: high altitude, subsurface, desert, and polar environments Nathalie A. Cabrol, Dale T. Andersen, Carol R. Stoker, Pascal Lee, Christopher P. McKay and David S. Wettergreen.

Overcoming “ecophobia”: fostering environmental empathy through narrative in children's science literature

Abstract: Efforts to communicate with children about ecological themes often emphasize environmental threats. For some children, however, this approach can backfire, as they try to avoid continued exposure to problems they cannot solve. Another innovative approach is to promote the development of environmental empathy and environmental literacy through the use of narrative. Children's books that use narrative to convey key concepts about a given ecosystem could potentially reach a broad audience by making information about the books available on the internet. Examples of this approach are a special New York City edition of a book in the Magic School Bus series (Scholastic Press) and the Schoolyard Book series produced by the Long Term Ecological Research Network program.

Effect of unsteady flow on nitrate loss in an oligotrophic, glacial meltwater stream

Abstract: [1] The McMurdo Dry Valleys of Antarctica are among the coldest, driest ecosystems on Earth. During the austral summer, glacial meltwater supports cyanobacterial mat communities in some streams, but they are not ubiquitous. We conducted a nitrate (NO3−) enrichment tracer injection in Huey Creek to quantify NO3− loss in a Dry Valley stream where algal mats would not obscure hyporheic microbial processes. Unsteady streamflow led to diel variability in the tracer concentration and in surface/subsurface water and solute exchange. Subsequently, concentrations of NO3−, nitrite (NO2−), ammonium (NH4+), and dissolved organic carbon (DOC) varied significantly during the injection, with a net loss of NO3−, NO2−, and DOC, and production of nitrous oxide. These mass changes within a reach were often coincident with high streamflows. Reactivity also coincided with the highest DOC concentrations, suggesting that DOC is the primary limitation to heterotrophic microbial activity in the stream. Together, streamflow and DOC availability create the hot spots and hot moments that dominate NO3− reactivity and removal in this polar desert ecosystem. The combination of spatially and temporally variable hyporheic dynamics and solute availability underscore the limitations of common nutrient uptake metrics and transient storage models when unsteady flow conditions exist.