Showing papers by "Diane M. McKnight published in 2017"

Microbial formation of labile organic carbon in Antarctic glacial environments

Abstract: Bioavailable glacial carbon has been thought to be largely ancient or anthropogenic. Analyses of carbon dynamics in an Antarctic supraglacial stream reveal that non-photosynthetic production relies on organic carbon from photosynthetic microbes.

Decadal ecosystem response to an anomalous melt season in a polar desert in Antarctica

Abstract: Amplified climate change in polar regions is significantly altering regional ecosystems, yet there are few long-term records documenting these responses. The McMurdo Dry Valleys (MDV) cold desert ecosystem is the largest ice-free area of Antarctica, comprising soils, glaciers, meltwater streams and permanently ice-covered lakes. Multi-decadal records indicate that the MDV exhibited a distinct ecosystem response to an uncharacteristic austral summer and ensuing climatic shift. A decadal summer cooling phase ended in 2002 with intense glacial melt (‘flood year’)—a step-change in water availability triggering distinct changes in the ecosystem. Before 2002, the ecosystem exhibited synchronous behaviour: declining stream flow, decreasing lake levels, thickening lake ice cover, decreasing primary production in lakes and streams, and diminishing soil secondary production. Since 2002, summer air temperatures and solar flux have been relatively consistent, leading to lake level rise, lake ice thinning and elevated stream flow. Biological responses varied; one stream cyanobacterial mat type immediately increased production, but another stream mat type, soil invertebrates and lake primary productivity responded asynchronously a few years after 2002. This ecosystem response to a climatic anomaly demonstrates differential biological community responses to substantial perturbations, and the mediation of biological responses to climate change by changes in physical ecosystem properties. The McMurdo Dry Valleys is the largest ice-free ecosystem in Antarctica. Here, the varied community responses to an anomalous melt season are documented.

Dissolved black carbon in the global cryosphere: Concentrations and chemical signatures

Abstract: Black carbon (BC) is derived from the incomplete combustion of biomass and fossil fuels and can enhance glacial recession when deposited on snow and ice surfaces Here we explore the influence of environmental conditions and the proximity to anthropogenic sources on the concentration and composition of dissolved black carbon (DBC), as measured by benzenepolycaroxylic acid (BPCA) markers, across snow, lakes, and streams from the global cryosphere Data are presented from Antarctica, the Arctic, and high alpine regions of the Himalayas, Rockies, Andes, and Alps DBC concentrations spanned from 062 µg/L to 170 µg/L The median and (25, 975) quantiles in the pristine samples were 18 µg/L (062, 12), and non-pristine samples were 21 µg/L (16, 170) DBC is susceptible to photodegradation when exposed to solar radiation This process leads to a less condensed BPCA signature In general, DBC across the dataset was comprised of less-polycondensed DBC However, DBC from the Greenland Ice Sheet (GRIS) had a highly-condensed BPCA molecular signature This could be due to recent deposition of BC from Canadian wildfires Variation in DBC appears to be driven by a combination of photochemical processing and the source combustion conditions under which the DBC was formed Overall, DBC was found to persist across the global cryosphere in both pristine and non-pristine snow and surface waters The high concentration of DBC measured in supra-glacial melt on the GRIS suggests DBC can be mobilized across ice surfaces This is significant because these processes may jointly exacerbate surface albedo reduction in the cryosphere

Impacts of coal dust from an active mine on the spectral reflectance of Arctic surface snow in Svalbard, Norway

Abstract: Light absorbing particles (LAPs) in snow such as dust and black carbon (BC) influence the radiative forcing at the Earth's surface, which has major implications for global climate models. LAPs also significantly influence the melting of glaciers, sea ice, and seasonal snow. Here, we present an in situ study of surface snow near an active coalmine in the Norwegian Arctic. We couple measurements of spectral hemispherical directional reflectance factor (HDRF) with measurements of LAPs characterized two ways, as refractory black carbon (rBC) using a Single Particle Soot Photometer (SP2) and the total light absorption of LAPs measured with the Light Absorption Heating Method (LAHM). The Snow Ice and Aerosol Radiation model was constrained by LAP measurements. Results were compared to observed spectral albedo measurements. Modeled and observed albedos were similar at the cleaner and more remote sites. However, the modeled spectral albedos do not fully account for the low spectral albedo measured next to the mine. LAP measurements also showed a large variation in particle sizes (tenths-to-tens of microns) related to transport distance of the particles from the mine. Here, we find that LAPs from coal dust reduce the spectral HDRF by up to 84% next to the mine, and 55% 0.5 km downwind of the mine. The coupling of extreme LAP observations, (1 ng g-1 to 4863 ng g-1), with HDRF measurements from 350 – 2500 nm has facilitated the development of spectral band pairs, which could be used in the future to remotely assess LAPs in Arctic snow.

Freshwater diatom biogeography and the genus Luticola: an extreme case of endemism in Antarctica

Abstract: Historical views have characterized Antarctica as a frozen desert with low diversity, although recent studies suggest that this may not be true for microscopic organisms. For microbes, assessing endemism in the Antarctic region has been particularly important, especially against a backdrop of debate regarding their presumed cosmopolitan nature. To contribute to this conversation, we highlight the observed endemism of the freshwater diatom genus Luticola in Antarctica by synthesizing the results of a modern high-resolution taxonomy from the Continental, Maritime, and sub-Antarctic regions. We report that Luticola has one of the highest endemic rates of any diatom genus in Antarctica, in terms of total number of species (taxon endemism) and percentage of the entire genus (phylogenetic endemism). Of the over 200 species of Luticola globally, nearly 20% (43) occur in the Antarctic, with 42 of these being endemic. Within regions, Maritime Antarctica has the largest number of Luticola species and endemics (28 and 23, respectively), followed by Continental Antarctica (14, 9) and sub-Antarctic islands (8, 6). Thus, 38 of the 42 endemics are found in a single region only. While the timing of Luticola diversification has not been established, fossil evidence suggests recent invasions and/or diversification over a relatively short geologic timescale. Understanding the origin and evolution of endemic diatom species in Antarctica will help us better understand microbial biogeography, as well as assess and interpret impacts of large-scale environmental change taking place at southern latitudes.

Concentration-discharge relationships during an extreme event: contrasting behavior of solutes and changes to chemical quality of dissolved organic material in the Boulder Creek Watershed during the September 2013 flood.

Abstract: During the week of September 9-15, 2013, about 44 cm of rain fell across Boulder County, Colorado, USA, representing a very rare precipitation event The resultant stream flows corresponded to an extreme event not seen since the historical flood of 1894 For the Boulder Creek Critical Zone Observatory (BcCZO), this event provided an opportunity to study the effect of extreme rainfall on solute concentration-discharge relationships and biogeochemical processes We measured weathering-derived lithologic solutes (Ca, Mg, Na, K, and Si) and dissolved organic carbon (DOC) concentrations at two sites on Boulder Creek during the recession of peak flow We also isolated four distinct fractions of dissolved organic matter (DOM) for chemical characterization At the upper and lower sites, all solutes had their highest concentration at peak flow At the upper site, which represented a mostly forested catchment, the concentrations of lithologic solutes decreased slightly during flood recession In contrast, DOC and K concentrations decreased by a factor of three At the lower site within the urban corridor, concentration of lithologic solutes decreased substantially for a few days before rebounding, whereas the DOC and K concentrations continued to decrease Additionally, we found spatiotemporal trends in the chemical quality of DOM that were consistent with a limited reservoir of soluble organic matter in surficial soils becoming depleted and deeper layers of the Critical Zone contributing DOM during the flood recession Overall, these results suggest that despite the extreme flood event, concentration-discharge relationships were similar to typical snowmelt periods in this Rocky Mountain region

Dissolved black carbon in the global cryosphere: concentrations and chemical signatures

Abstract: Black carbon (BC) is derived from the incomplete combustion of biomass and fossil fuels and can enhance glacial recession when deposited on snow and ice surfaces. Here we explore the influence of environmental conditions and the proximity to anthropogenic sources on the concentration and composition of dissolved black carbon (DBC), as measured by benzenepolycaroxylic acid (BPCA) markers, across snow, lakes, and streams from the global cryosphere. Data are presented from Antarctica, the Arctic, and high alpine regions of the Himalayas, Rockies, Andes, and Alps. DBC concentrations spanned from 0.62 µg/L to 170 µg/L. The median and (2.5, 97.5) quantiles in the pristine samples were 1.8 µg/L (0.62, 12), and non-pristine samples were 21 µg/L (1.6, 170). DBC is susceptible to photodegradation when exposed to solar radiation. This process leads to a less condensed BPCA signature. In general, DBC across the dataset was comprised of less-polycondensed DBC. However, DBC from the Greenland Ice Sheet (GRIS) had a highly-condensed BPCA molecular signature. This could be due to recent deposition of BC from Canadian wildfires. Variation in DBC appears to be driven by a combination of photochemical processing and the source combustion conditions under which the DBC was formed. Overall, DBC was found to persist across the global cryosphere in both pristine and non-pristine snow and surface waters. The high concentration of DBC measured in supra-glacial melt on the GRIS suggests DBC can be mobilized across ice surfaces. This is significant because these processes may jointly exacerbate surface albedo reduction in the cryosphere.

Debates—Hypothesis testing in hydrology: A view from the field: The value of hydrologic hypotheses in designing field studies and interpreting the results to advance hydrology

Abstract: Advances in hydrology are greatly needed and approaches that employ hypotheses to guide research have the potential to contribute to future advances. In this context, hypotheses can serve a range of purposes. Overarching hypotheses can provide a common integrating framework for collaborative research and can be revised as research progresses over time. Hypotheses that attempt to explain unexpected field observations or experimental results can provide a guide for designing further field studies. Focused testable hypotheses can facilitate effective presentation of proposed research, and clarify alternative hypotheses. Finally, the value of employing a hypothesis-based approach depends upon the research environment, which can act as an “environmental filter” in determining successful research outcomes.

Thermal autecology describes the occurrence patterns of four benthic diatoms in McMurdo Dry Valley streams

Abstract: Benthic microbial mats in the glacial-fed meltwater streams are hotspots of productivity in the McMurdo Dry Valleys (MDV), Antarctica. Benthic diatoms are common in these mats and the >45 primarily endemic taxa represent the most diverse group of eukaryotes in the MDV. In this harsh polar desert, streams are thermally dynamic with daily water temperatures varying 6–9 °C and daily maximum temperatures as high as 15 °C. Stream temperature may play a role in determining growth rates and survival strategies. To understand taxon-specific adaptations to their environment, we measured the growth rates of unialgal cultures of four diatom taxa (Psammothidium papilio, Hantzschia abundans, Hantzschia amphioxys, and Hantzschia amphioxys f. muelleri) under three temperature conditions (7.6, 10, and 15 °C) that were representative of maximum daily stream temperatures. We found that P. papilio exhibited a constant growth rate across the full temperature range; this species is most common in streams that begin to flow early in the summer and with less variable thermal regimes. Growth rates for H. abundans were greatest at 15 °C, but showed a non-linear relationship with temperature. H. amphioxys f. muelleri grew faster than the other taxa studied and thrived at 10 °C. Hantzschia amphioxys grew only at the two lower temperatures. These results aligned with the observed relationships between each taxon’s relative abundance and stream temperatures in the long-term record maintained by the MDV Long-Term Ecological Research program. Overall, our observations suggest that differences in thermal optima may be one factor contributing to and maintaining the diversity of benthic diatom flora in the MDV.

Biogeophysical properties of an expansive Antarctic supraglacial stream

Abstract: Supraglacial streams are important hydrologic features in glaciated environments as they are conduits for the transport of aeolian debris, meltwater, solutes and microbial communities. We characterized the basic geomorphology, hydrology and biogeochemistry of the Cotton Glacier supraglacial stream located in the McMurdo Dry Valleys of Antarctica. The distinctive geomorphology of the stream is driven by accumulated aeolian sediment from the Transantarctic Mountains, while solar radiation and summer temperatures govern melt in the system. The hydrologic functioning of the Cotton Glacier stream is largely controlled by the formation of ice dams that lead to vastly different annual flow regimes and extreme flushing events. Stream water is chemically dilute and lacks a detectable humic signature. However, the fluorescent signature of dissolved organic matter (DOM) in the stream does demonstrate an extremely transitory red-shifted signal found only in near-stream sediment leachates and during the initial flushing of the system at the onset of flow. This suggests that episodic physical flushing drives pulses of DOM with variable quality in this stream. This is the first description of a large Antarctic supraglacial stream and our results provide evidence that the hydrology and geomorphology of supraglacial streams drive resident microbial community composition and biogeochemical cycling.

Linkages among Clostridia, recalcitrant organic matter, and methylmercury production in oligotrophic sediments

Abstract: Recent advances have allowed for greater investigation into microbial regulation of mercury toxicity in the environment. In wetlands in particular, dissolved organic matter (DOM) may influence methylmercury (MeHg) production both through chemical interactions and through substrate effects on microbiomes. We conducted microcosm experiments in two disparate wetland environments (unvegetated and vegetated sediments) to examine the impacts of plant leachate and inorganic mercury loadings on microbiomes, DOM cycling, and MeHg production. We show that while leachate influenced the microbiome in both environment types, sediment with high organic carbon content was more resistant to change than oligotrophic sediment. Oligotrophic unvegetated sediments receiving leachate produced more MeHg than unamended microcosms, coincident with an increase in putative chemoorganotrophic methylators belonging to Clostridia . Further, metagenomic shifts toward fermentation, and secondarily iron metabolisms, in these microcosms as well as degradation of complex DOM also support a possible association between rarely acknowledged microorganisms and MeHg. Our research provides a basis for future investigation into the role of fermenting organisms in mercury toxicity and generates a new hypothesis that DOM can stimulate mercury methylation either 1) via direct methylation by fermenting bacteria or 2) via enhancing carbon bioavailability for sulfate- and iron-reducing bacteria through breakdown of complex DOM.