02348nas a2200217   4500008004100000245023200041210006900273260001200342300001200354490000700366520154300373653001501916653002001931653000801951653002001959653001401979653002101993100002102014700002402035856007102059       2025                        eng d00aChemical signatures of microbial life in an ecological end-member: Shifting hydroclimate and sediment fluxes influence DOM biogeochemistry in Lake Fryxell, a permanently ice-covered lake in the McMurdo Dry Valleys of Antarctica0 aChemical signatures of microbial life in an ecological endmember  c04/2025  a24786780 v573 a<p>The ice-covered lakes in the McMurdo Dry Valleys (MDV) of Antarctica provide end-member ecosystems for understanding the production of dissolved organic matter (DOM) in aquatic ecosystems in the absence of vegetation on the landscape and under resource and nutrient constraints. Given these constraints, DOM in MDV lakes is derived solely from microbial phototrophs and heterotrophic bacteria, contrasting with the dominant terrestrial sources in temperate regions. Previous research developed fluorometric approaches for characterizing DOM, including in MDV lakes. In this study we leveraged these approaches along with contemporary molecular-based techniques to elucidate changes in DOM composition across the depth profile for Lake Fryxell in the MDV. The results showed that the presence of organic molecules containing sulfur increased at depth where anoxic conditions prevailed. To evaluate the influences of climate-induced rising lake levels and multiple flood events in the MDV, we compared recent and historical samples. The results indicated a remarkable consistency in source-related fluorescence metrics over time, whereas a twofold decrease in sulfur content of the fulvic acid fraction was observed in samples from above and below the oxycline. Biogeochemical processes associated with the influx of iron oxide&ndash;rich sediments during flood events may have contributed to this change, and similar processes may stimulate biogeochemical cycling and remineralization in temperate lakes during seasonal transitions.</p>10aAntarctica10abiogeochemistry10aDOM10alake metabolism10alimnology10anutrient cycling1 aRue, Garrett, P.1 aMcKnight, Diane, M.  uhttps://www.tandfonline.com/doi/full/10.1080/15230430.2025.247867802474nas a2200229   4500008004100000245007500041210006900116260001200185300001800197490000800215520178800223653001502011653001602026653002802042653000802070653002202078653000902100100002502109700002502134700002402159856006102183       2022                        eng d00aDissolved organic carbon chemostasis in Antarctic polar desert streams0 aDissolved organic carbon chemostasis in Antarctic polar desert s  c07/2022  ae2021JG0066490 v1273 a<p>Dissolved organic carbon (DOC) is a key variable impacting stream biogeochemical processes. The relationship between DOC concentration (C) and stream discharge (<i>q</i>) can elucidate spatial and temporal DOC source dynamics in watersheds. In the ephemeral glacial meltwater streams of the McMurdo Dry Valleys (MDV), Antarctica, the C-<i>q</i> relationship has been applied to dissolved inorganic nitrogen and weathering solutes including silica, which all exhibit chemostatic C-<i>q</i> behavior; but DOC-<i>q</i> dynamics have not been studied. DOC concentrations here are low compared to temperate streams, in the range of 0.1-2 mg C l<sup>-1</sup>, and their chemical signal clearly indicates derivation from microbial biomass (benthic mats and hyporheic biofilm). To investigate whether the DOC generation rate from these autochthonous organic matter pools was sufficient to maintain chemostasis for DOC, despite these streams&#39; large diel and interannual fluctuations in discharge, we fit the long-term DOC-<i>q</i> data to a power law and an advection-reaction model. Model outputs and coefficients of variation characterize the DOC-<i>q</i> relationship as chemostatic for several MDV streams. We propose a conceptual model in which hyporheic carbon storage, hyporheic exchange rates, and net DOC generation rates are key interacting components that enable chemostatic DOC-<i>q</i> behavior in MDV streams. This model clarifies the role of autochthonous carbon stores in maintaining DOC chemostasis and may be useful for examining these relationships in temperate systems, which typically have larger sources of bioavailable autochthonous organic carbon than MDV streams but where this autochthonous signal could be masked by a stronger allochthonous contribution.</p>10aAntarctica10achemostasis10aconcentration-discharge10aDOC10aephemeral streams10aLTER1 aTorrens, Christa, L.1 aGooseff, Michael, N.1 aMcKnight, Diane, M.  uhttps://onlinelibrary.wiley.com/doi/10.1029/2021JG00664901995nas a2200205   4500008004100000245012000041210006900161260004800230490000900278520132200287653001501609653001101624653001801635653001201653653001501665653001301680100002401693700002401717856004801741       2021                        eng d00aControls on microbial mat coverage and diatom species turnover in Antarctic desert streams: A transplant experiment0 aControls on microbial mat coverage and diatom species turnover i  aBoulder, CObUniversity of Colorado Boulder0 vM.S.3 a<p>This thesis evaluates how polar desert streams regulate benthic microbial mat coverage, chlorophyll-a biomass, and diatom species composition. Microbial mats growing on rocks (eplithon) and on sandy substrate (epipelon) were reciprocally transplanted among four glaciers meltwater streams and monitored through time. The selected study streams were Green Creek, Bowles Creek, Delta Stream, Von Guerard Stream in the Lake Fryxell Basin in the McMurdo Dry Valleys, Antarctica. These streams vary in length, streamflow intermittency, and diatom community composition of microbial mats. Results demonstrate that streams strongly control mat biomass (coverage and chlorophyll-a) differently for epilithon and epipelon. However, diatom species composition did not vary between these growth habitats but instead varied by stream, suggesting adaptive niche separation related to environmental conditions. Diatom species composition of transplants in Green Creek became dissimilar from their initial stream communities suggesting downstream dispersal and within stream connectivity regulates community assembly. This experiment confirms that environmental characteristics and intra-stream dispersal processes exert strong control over maintaining microbial mat coverage, biomass accrual, and diatom species composition.</p>10aAntarctica10adiatom10amicrobial mat10astreams10atransplant10aturnover1 aDarling, Joshua, P.1 aMcKnight, Diane, M.  uhttps://www.proquest.com/docview/263459098202603nas a2200217   4500008004100000022001400041245012400055210006900179260001200248520186200260653001502122653002702137653001902164653002102183653001502204100002202219700002502241700002402266700002602290856006902316       2021                        eng d  a2169-895300aThe role of hyporheic connectivity in determining nitrogen availability: Insights from an intermittent Antarctic stream0 arole of hyporheic connectivity in determining nitrogen availabil  c04/20223 a<p>Due to widespread manipulation of nitrogen (N), much research has focused on processes controlling the fate of anthropogenic N in streams. Yet, in a variety of oligotrophic systems, N fixed by periphyton is a significant driver of ecosystem metabolism. Due to difficulties partitioning allochthonous and autochthonous sources, there is limited information regarding how the latter is processed. Autochthonous N may be particularly important in alpine, arid, or polar environments. We test the hypothesis that the availability of remineralized autochthonous N is controlled by connectivity between the hyporheic zone and main channel due to the contrasting biogeochemical functions of benthic autotrophs (including N‐fixing <em>Nostoc</em>) and hyporheic heterotrophs in an intermittent Antarctic stream. There, we collected surface water and hyporheic water concurrently at 4‐6 hour intervals over a 32.5‐hr period during one flow season and opportunistically throughout a second. Hyporheic water had 7 to 30 times greater nitrate‐N concentrations relative to surface water across all flow conditions. In contrast, ammonium concentrations were generally lower, although similar among locations. Additionally, nitrate in hyporheic water was positively correlated with silica, an indicator of hyporheic residence time. A laboratory assay confirmed prior inferences that hyporheic microbial communities possess the functional potential to perform nitrification. Together, these findings suggest that remineralized autochthonous N accumulates in the hyporheic zone even as streamflow varies and likely subsidizes stream N availability&mdash;which supports prior inferences from N stable isotope data at this site. These results highlight the importance of hyporheic connectivity in controlling autochthonous N cycling and availability in streams.</p>10aAntarctica10aautochthonous nitrogen10ahyporheic zone10anitrogen cycling10astreamflow1 aSingley, Joel, G.1 aGooseff, Michael, N.1 aMcKnight, Diane, M.1 aHinckley, Eve-Lyn, S.  uhttps://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021JG00630903499nas a2200229   4500008004100000245016600041210006900207260001200276300001100288490000700299520269000306653002302996653001503019653004103034653001603075653002403091653002003115100002203135700002203157700002403179856006603203       2021                        eng d00aSupporting simultaneous air revitalization and thermal control in a crewed habitat with temperate <i>Chlorella vulgaris</i> and eurythermic Antarctic Chlorophyta0 aSupporting simultaneous air revitalization and thermal control i  c08/2021  a7097460 v123 a<p>Including a multifunctional, bioregenerative algal photobioreactor for simultaneous air revitalization and thermal control may aid in carbon loop closure for long-duration surface habitats. However, using water-based algal media as a cabin heat sink may expose the contained culture to a dynamic, low temperature environment. Including psychrotolerant microalgae, native to these temperature regimes, in the photobioreactor may contribute to system stability. This paper assesses the impact of a cycled temperature environment, reflective of spacecraft thermal loops, to the oxygen provision capability of temperate <i>Chlorella vulgaris</i> and eurythermic Antarctic Chlorophyta. The tested 28-min temperature cycles reflected the internal thermal control loops of the International Space Station (<i>C. vulgaris</i>, 9&ndash;27&deg;C; Chlorophyta-Ant, 4&ndash;14&deg;C) and included a constant temperature control (10&deg;C). Both sample types of the cycled temperature condition concluded with increased oxygen production rates (<i>C. vulgaris</i>; initial: 0.013 mgO<sub>2</sub> L<sup>-1</sup>, final: 3.15 mgO<sub>2</sub> L<sup>&ndash;1</sup> and Chlorophyta-Ant; initial: 0.653 mgO<sub>2</sub> L<sup>&ndash;1</sup>, final: 1.03 mgO<sub>2</sub> L<sup>&ndash;1</sup>) and culture growth, suggesting environmental acclimation. Antarctic sample conditions exhibited increases or sustainment of oxygen production rates normalized by biomass dry weight, while both <i>C. vulgaris</i> sample conditions decreased oxygen production per biomass. However, even with the temperature-induced reduction, cycled temperature <i>C. vulgaris</i> had a significantly higher normalized oxygen production rate than Antarctic Chlorophyta. Chlorophyll fluorometry measurements showed that the cycled temperature conditions did not overly stress both sample types (F<sub>V</sub>/F<sub>M</sub>: 0.6&ndash;0.75), but the Antarctic Chlorophyta sample had significantly higher fluorometry readings than its <i>C. vulgaris</i> counterpart (<i>F</i> = 6.26, <i>P</i> &lt; 0.05). The steady state <i>C. vulgaris</i> condition had significantly lower fluorometry readings than all other conditions (F<sub>V</sub>/F<sub>M</sub>: 0.34), suggesting a stressed culture. This study compares the results to similar experiments conducted in steady state or diurnally cycled temperature conditions. Recommendations for surface system implementation are based off the presented results. The preliminary findings imply that both <i>C. vulgaris</i> and Antarctic Chlorophyta can withstand the dynamic temperature environment reflective of a thermal control loop and these data can be used for future design models.</p>10aair revitalization10aAntarctica10abioregenerative life support systems10aChlorophyta10aMcMurdo Dry Valleys10athermal control1 aMatula, Emily, E.1 aNabity, James, A.1 aMcKnight, Diane, M.  uhttps://www.frontiersin.org/article/10.3389/fmicb.2021.70974603261nas a2200241   4500008004100000245013300041210006900174260001200243490000600255520247400261653001502735653002202750653001402772653001102786653001902797100002002816700002202836700002202858700002402880700002402904700002102928856007002949       2020                        eng d00aEvaluating alternative metacommunity hypotheses for diatoms in the McMurdo Dry Valleys using simulations and remote sensing data0 aEvaluating alternative metacommunity hypotheses for diatoms in t  c09/20200 v83 a<p>Diatoms are diverse and widespread freshwater Eukaryotes that make excellent microbial subjects for addressing questions in metacommunity ecology. In the McMurdo Dry Valleys of Antarctica, the simple trophic structure of glacier-fed streams provides an ideal outdoor laboratory where well-described diatom assemblages are found within two cyanobacterial mat types, which occupy different habitats and vary in coverage within and among streams. Specifically, black mats of <em>Nostoc</em> spp. occur in marginal wetted habitats, and orange mats (<em>Oscillatoria</em> spp. and <em>Phormidium</em> spp.) occur in areas of consistent stream flow. Despite their importance as bioindicators for changing environmental conditions, the role of dispersal in structuring dry valley diatom metacommunities remains unclear. Here, we use MCSim, a spatially explicit metacommunity simulation package for R, to test alternative hypotheses about the roles of dispersal and species sorting in maintaining the biodiversity of diatom assemblages residing in black and orange mats. The spatial distribution and patchiness of cyanobacterial mat habitats was characterized by remote imagery of the Lake Fryxell sub-catchment in Taylor Valley. The available species pool for diatom metacommunity simulation scenarios was informed by the Antarctic Freshwater Diatoms Database, maintained by the McMurdo Dry Valleys Long Term Ecological Research program. We used simulation outcomes to test the plausibility of alternative community assembly hypotheses to explain empirically observed patterns of freshwater diatom biodiversity in the long-term record. The most plausible simulation scenarios suggest species sorting by environmental filters, alone, was not sufficient to maintain biodiversity in the Fryxell Basin diatom metacommunity. The most plausible scenarios included either (1) neutral models with different immigration rates for diatoms in orange and black mats or (2) species sorting by a relatively weak environmental filter, such that dispersal dynamics also influenced diatom community assembly, but there was not such a strong disparity&nbsp;in immigration rates between mat types. The results point to the importance of dispersal for understanding current and future biodiversity patterns for diatoms in this ecosystem, and more generally, provide further evidence that metacommunity theory is a useful framework for testing hypotheses about microbial community assembly.</p>10aAntarctica10aBacillariophyceae10adispersal10aNostoc10astream ecology1 aSokol, Eric, R.1 aBarrett, John, E.1 aKohler, Tyler, J.1 aMcKnight, Diane, M.1 aSalvatore, Mark, R.1 aStanish, Lee, F.  uhttps://www.frontiersin.org/article/10.3389/fevo.2020.521668/full02540nas a2200289   4500008004100000245012600041210006900167260001200236490000800248520163000256653002101886653001501907653001701922653001501939653002401954653001901978100002101997700002102018700001902039700002402058700001902082700001702101700002402118700001902142700001702161856007202178       2020                        eng d00aGeochemistry of aeolian material from the McMurdo Dry Valleys, Antarctica: Insights into Southern Hemisphere dust sources0 aGeochemistry of aeolian material from the McMurdo Dry Valleys An  c10/20200 v5473 a<p>In the Southern Hemisphere, the major sources of dust and other aeolian materials are from Patagonia, South Africa, Australia, and New Zealand. Dust from Patagonia and New Zealand has been identified in ice cores throughout Antarctica, suggesting that during arid and windy periods, such as glacial periods, dust can be entrained and transported onto the continent. However, little information exists on modern Antarctic dust sources, transport, and its role in the Southern Hemisphere dust cycle. We present the first geochemical characterization of aeolian materials collected at five heights (between 5 cm and 100 cm) above the surface in four valleys within the McMurdo Dry Valleys, the largest ice-free area in Antarctica. Our mineralogy data indicate that these materials are primarily derived from local rocks of the McMurdo Volcanics, Ferrar Dolerite, Beacon Sandstone and Granite Harbor Intrusives, with varying contributions of each rock type dependent on the valley location. While major oxide, trace element and rare earth element data show that low elevation and coastal locations (with respect to the Ross Sea) are dominated by local sources, high elevation and inland locations have accumulated both local materials and dust from other distant Southern Hemisphere sources. This far-traveled material may not be accumulating today, but represents a paleo source that is resuspended from the soils. By geochemically &ldquo;fingerprinting&rdquo; aeolian materials from the MDV, we can better inform future studies on the transport of materials within Antarctica and between Southern Hemisphere land masses.</p>10aaeolian material10aAntarctica10amajor oxides10amineralogy10arare earth elements10atrace elements1 aDiaz, Melisa, A.1 aWelch, Susan, A.1 aSheets, J., M.1 aWelch, Kathleen, A.1 aKhan, Alia, L.1 aAdams, Byron1 aMcKnight, Diane, M.1 aCary, Craig, S1 aLyons, Berry  uhttps://www.sciencedirect.com/science/article/pii/S0012821X2030404002639nas a2200301   4500008004100000245008400041210006900125260001200194520169800206653001501904653002101919653002401940653002701964653001301991100002102004700002602025700001702051700001602068700002502084700002502109700001702134700002202151700002302173700002002196700002402216700002402240856007302264       2019                        eng d00aThe hydroecology of an ephemeral wetland in the McMurdo Dry Valleys, Antarctica0 ahydroecology of an ephemeral wetland in the McMurdo Dry Valleys   c11/20193 a<p>The McMurdo Dry Valleys (MDV) is a polar desert on the coast of East Antarctica where ephemeral wetlands become hydrologically active during warm and sunny summers when sub‐surface flows are generated from melting snowfields. To understand the structure and function of polar wetland ecosystems, we investigated the hydroecology of one such wetland, the Wormherder Creek wetland, during the warm and sunny summer of 2008 &ndash; 2009, when the wetland was hydrologically reactivated. Conservative tracer (LiCl) was injected for a 2‐hour period into a stream above the wetland to determine flow path orientations and hydrologic residence times. Tracer results indicated that surface water is rapidly exchanged with wetland groundwater and wetland residence times may exceed two austral summers. Major ion concentrations were uniform in samples from surface water and shallow groundwater throughout the wetland. Microbial mats in the wetland had high autotrophic index values (the ratios of chlorophyll a [Chl‐a]/ash‐free dry mass [AFDM]), ranging from 9‐38 μg Chl‐a/mg AFDM, indicative of actively photosynthesizing mat communities. The diatom communities in the mats were relatively uniform compared to those in mats from regularly flowing MDV streams, with four endemic and one widespread diatom taxa of the genus <em>Luticola</em> accounting for an average of 86% of the community. These results indicate that the hydrologic characteristics of the wetland contribute to uniform geochemical conditions. In turn, uniform geochemical conditions may explain the high autotrophic index values of the microbial mats and relatively low spatial variation of the diatom community.</p>10aAntarctica10adesert hydrology10adiatom biodiversity10ahyporheic interactions10awetlands1 aWlostowski, Adam1 aSchulte, Nicholas, O.1 aAdams, Byron1 aBall, Becky1 aEsposito, Rhea, M.M.1 aGooseff, Michael, N.1 aLyons, Berry1 aNielsen, Uffe, N.1 aVirginia, Ross, A.1 aWall, Diana, H.1 aWelch, Kathleen, A.1 aMcKnight, Diane, M.  uhttps://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JG00515302748nas a2200241   4500008004100000245014000041210006900181260004600250300000800296490001000304520196700314653001502281653002102296653002602317653001502343653001902358653001202377653001402389653000902403100001902412700002402431856005102455       2018                        eng d00aBiogeochemistry, contaminant transport, and atmospheric exchange in glacial cryoconite meltwater of the McMurdo Dry Valleys, Antarctica0 aBiogeochemistry contaminant transport and atmospheric exchange i  aBoulder, CObUniversity of Coloradoc2018  a2450 vPh.D.3 a<p>Polar regions serve as a global sink for many forms of semi-volatile pollution emitted from low- or midlatitudes of the populated world. This study examined the longrange atmospheric transport, fate, and phase partitioning of semi-volatile organic contaminants from air masses into meltwater and aeolian sediment on six glaciers in the McMurdo Dry Valleys of Antarctica. A novel low-cost, field-portable instrument was developed for the in-situ solid-phase extraction of trace contaminants in extreme environmental conditions without access to electricity or traditional laboratory facilities. Beyond polar research, this equipment is applicable for rapid field extraction and stabilization of samples assessing air and water quality after natural disasters. This is the first published study to identify the presence of anthropogenic perfluorinated compounds in the Transantarctic Mountain region and indicates a longer range of poleward contaminant transport than prior estimates in the Southern Hemisphere. Additional research examined the biochemistry and climatic variability of open and sealed cryoconite holes on glacial surfaces throughout the initial melt, equilibrium, and refreezing periods in 2013&ndash;2015. High solute concentrations relative to glacial ice indicate that the pools can remain isolated from hydrologic connectivity for more than a decade. Microbial carbon cycling in pools enclosed by ice led to atmospheric disequilibrium and extreme pH. Analysis of unique air, liquid, and ice stratification in cryoconite holes revealed vertical patterns representing a highly accurate, multi-year record of past weather conditions sensitive enough to identify individual dates. This research identifies fluctuations in atmospheric contaminant transport, specific timeframes for deposition events, and may be used in back-trajectory models to help identify the source and variability of semi-volatile emissions in the Southern hemisphere.</p>10aAntarctica10aapplied sciences10acontaminant transport10acryoconite10aearth sciences10aglacier10ahydrology10amelt1 aMass, Alex, Q.1 aMcKnight, Diane, M.  uhttps://search.proquest.com/docview/204831467803401nas a2200241   4500008004100000245013400041210006900175260004000244300000800284490001000292520259800302653001002900653001502910653002402925653001902949653001602968653001902984653002403003653001903027100002203046700002403068856006703092       2015                        eng d00aPhysical and chemical controls on the abundance and composition of stream microbial mats from the McMurdo Dry Valleys, Antarctica0 aPhysical and chemical controls on the abundance and composition   aBoulder, CObUniversity of Colorado  a2720 vPh.D.3 a<p>&nbsp;</p><div title="Page 4"><div><div><p>The McMurdo Dry Valleys of Antarctica are a cold, dry desert, yet perennial microbial mats are abundant in the ephemeral glacial meltwater streams that flow during austral summers. Three types of mats are present (orange, black, and green), and are primarily comprised of filamentous cyanobacteria,&nbsp;Nostoc, and chlorophytes, respectively. Mat types furthermore occupy distinct habitats within streams, utilizing the benthos, hyporheic zone, and water column, which expose them to different environmental conditions. Due to a lack of lateral inflows, allochthonous organic inputs, and negligible grazing activity, these streams are ideal for the controlled ecological study of microbial mats. Here, I investigated how mats will respond to physical disturbance, alterations in the hydrologic regime, and nutrient liberation from permafrost melt in the future. Specifically, I: 1) quantified and characterized the regrowth of mat biomass, community structure, and elemental stoichiometry after a scouring disturbance, 2) investigated how geomorphology and taxonomic identity influences the response of mat biomass to hydrologic regime in transects monitored over two decades, and 3) evaluated relationships between water chemistry and the elemental and isotopic composition of mat types over longitudinal and valley-wide gradients in Taylor Valley. I found that mats recovered ~20-50% of their biomass over the course of an austral summer following scour. Algal communities were significantly different in composition between disturbed and control treatments, but all samples naturally varied in species and elemental stoichiometry over the study period. When the long- term record of mat biomass was compared with hydrologic variables, stream channel mats (orange and green) had the greatest correlations, while marginal mats (black) showed weaker relationships with flow regime. Relationships also differed as a function of stream geomorphology, indicating the importance of substrata and gradient in conjunction with discharge. Lastly, mats showed unique elemental and isotopic compositions. Green and orange mats within the stream channel most reflected water column nutrient concentrations, while black mats showed significant nitrogen fixation. These results highlight the importance of taxonomic identity and habitat to modeling primary production here and elsewhere, and provide insight to how stream microbial mat communities are formed, maintained, and ultimately persist in an isolated polar desert.</p></div></div></div><p>&nbsp;</p>10aalgae10aAntarctica10abiological sciences10aclimate change10aDisturbance10aearth sciences10aMcMurdo Dry Valleys10amicrobial mats1 aKohler, Tyler, J.1 aMcKnight, Diane, M.  uhttps://search.proquest.com/docview/1690497718?accountid=1450303418nas a2200217   4500008004100000245009200041210006900133260004000202490001000242520270900252653001502961653002402976653002203000653001203022653001903034653002403053653001203077100002103089700002403110856006603134       2011                        eng d00aEcological controls on stream diatom communities in the McMurdo Dry Valleys, Antarctica0 aEcological controls on stream diatom communities in the McMurdo   aBoulder, CObUniversity of Colorado0 vPh.D.3 a<p>&nbsp;</p><div title="Page 4"><div><div><p>Diatoms are vital primary producers in aquatic ecosystems and useful indicators of environmental change. In climatically sensitive polar areas, diatoms have been used as beacons of climate change, allowing us to monitor physical, chemical, and biological changes. This research aims to improve our understanding of diatom ecology in the pristine and dynamic McMurdo Dry Valleys of Antarctica, where diatoms reside in stream microbial mats. These results aid in understanding how changes in hydrologic regime will affect stream microbial communities, biodiversity, and ecosystem function in a changing environment.</p><p>Relationships between hydrologic regime and diatom community composition were investigated using long-term data. Diatom communities were structured by stream physical features, and streams with more similar hydrologic characteristics had more similar communities. Variation in diatom community composition was best explained by hydrologic regime. Small diatoms increased in relative abundances with increasing streamflow, suggesting a role of diatom size in structuring communities. Overall, diatom communities were resistant to flood and drought-like conditions, suggesting an adaptation to frequent disturbances.</p><p>The importance of hydrologic regime on drift activity was investigated during three 24- hour experiments. Diel variations in drift could be attributed to diurnal flow peaks. Biomass and diatom cell densities followed a clockwise pattern with stream discharge and support the dominant role of hydraulic processes. The quality of source material differed between seasons and throughout the day. Drifting diatom communities were dominated by&nbsp;<em>Fistulifera pelliculosa</em>, which is rarely found in stream mats and suggests a different source. Modeling results suggest that the less firmly anchored marginal mats contribute more to the drift than channel mats in low- flow seasons, while the channel mats become more important during high-flow seasons.</p><p>The relationships between diatom and bacterial assemblages in microbial mats were assessed based on phylogenetic and functional relatedness in five Dry Valley streams. Significant relationships between diatom and bacterial communities were found, and co-occurrence analysis identified numerous correlations between individual diatom and bacterial taxa. A consistency in metabolic lifestyles of correlated taxa suggests that the relationships are ecologically relevant. Diatom and bacterial diversity showed opposite patterns, which indicate differences in environmental drivers of diversity for bacteria and eukaryotes.</p></div></div></div><p>&nbsp;</p>10aAntarctica10abiological sciences10acommunity ecology10adiatoms10aearth sciences10aMcMurdo Dry Valleys10astreams1 aStanish, Lee, F.1 aMcKnight, Diane, M.  uhttps://search.proquest.com/docview/915694114?accountid=1450302203nas a2200217   4500008004100000245008900041210006900130260001200199300001100211490000800222520155200230653001501782653002601797653002301823653002101846653001801867100001301885700002401898700001701922856004601939       2010                        eng d00aEffect of unsteady flow on nitrate loss in an oligotrophic, glacial meltwater stream0 aEffect of unsteady flow on nitrate loss in an oligotrophic glaci  c03/2010  aG010010 v1153 a<p>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 (NO<sub>3</sub><sup>-</sup>) enrichment tracer injection in Huey Creek to quantify NO<sub>3</sub><sup>-</sup> 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 NO<sub>3</sub><sup>-</sup>, nitrite (NO<sub>2</sub><sup>-</sup>), ammonium (NH<sub>4</sub><sup>+</sup>), and dissolved organic carbon (DOC) varied significantly during the injection, with a net loss of NO<sub>3</sub><sup>-</sup>, NO<sub>2</sub><sup>-</sup>, 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 NO<sub>3</sub><sup>-</sup> 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.</p>10aAntarctica10ahot spots/hot moments10ahyporheic exchange10anitrogen cycling10aunsteady flow1 aKoch, J.1 aMcKnight, Diane, M.1 aBaeseman, J.  uhttp://doi.wiley.com/10.1029/2009JG00103003833nas a2200217   4500008004100000245017200041210006900213260004600282300000800328490001000336520305700346653001503403653001903418653002103437653002503458653001903483653002303502100001603525700002403541856005003565       2009                        eng d00aControls on stream and hyporheic temperatures, Taylor Valley, Antarctica and large-scale climate influences on interannual flow variation in the Onyx River, Antarctica0 aControls on stream and hyporheic temperatures Taylor Valley Anta  aBoulder, CObUniversity of Coloradoc2009  a3170 vPh.D.3 a<p>The McMurdo Dry Valleys comprise the largest ice-free polar desert on the continent of Antarctica. My Ph.D. research investigated summertime glacial meltwater streams flowing through this region. This work is presented in Chapters 2 through 6 of my thesis. Chapters 2-5 present the work I have done related to hyporheic processes while Chapter 6 focuses on the hydroclimatological investigations I have carried out.</p><p>More specifically, Chapter 2 addresses the question: what are the dominant processes controlling dry valley stream temperatures? In particular, this investigation quantified the role of hyporheic exchange. The study found that in the Dry Valleys, exchange acted to decrease stream temperatures, accounting for 6&ndash;21% of cooling.</p><p>Chapter 3 discusses a follow up tracer study to investigate whether the comparatively large daily changes in dry valley stream temperatures (6-9&deg;C) affect hyporheic processes, for instance through viscosity effects. Results showed that the hyporheic zone volume and exchange coefficient were lower during the warmer, afternoon stream/streambed temperature regime than during the cooler, morning one. A temperature-induced feedback mechanism that increases subsurface flow path preferentiality is proposed as a possible explanation for the reduction in hyporheic volume under warmer conditions. The tracer results also suggested a &ldquo;Swiss Cheese&rdquo; type conceptual model of the hyporheic zone in which flow takes place along paths weaving their way through isolated areas.</p><p>Chapter 4 presents work done to elucidate individual hyporheic flow path lengths and residence times. A streambed injection revealed some long (over 100 m) paths that were also fast, having subsurface travel times on par with the surface water. Hyporheic pipeflow is proposed as an explanation.</p><p>In Chapter 5 research is presented showing that nitrate and phosphate concentrations at specific locations in the hyporheic zone increase with the decreasing connectivity of that location to the stream.</p><p>Finally, Chapter 6 describes the large-scale climate conditions that prevailed during December and January during the highest and lowest flow summers of the Onyx River record, the longest flow record for Antarctica. Climate variables and regions in the Southern Hemisphere that had a statistically significant linear correlation to Onyx River flows were also identified. The highest flow summer on record, 2001-2, was found to have some unusual climate features when compared to the other high flow summers. It stands out as having an anomalous wind pattern that would have increased katabatic winds in the valleys, raising air temperatures and possibly depositing sediment on the glaciers, decreasing their albedo. It is also characterized by anomalously high incoming shortwave radiation. We postulate that those high levels may have been due in part to the unusually low concentrations of radiation absorbing stratospheric ozone prevalent over the valleys that particular summer.</p>10aAntarctica10aearth sciences10ahydroclimatology10ahyporheic flow paths10ahyporheic zone10astream temperature1 aCozzetto, K1 aMcKnight, Diane, M.  uhttps://search.proquest.com/docview/304866366