01995nas 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/263459098203417nas a2200193   4500008004100000245006200041210006200103260006200165490000800227520280000235653001503035653001903050653002103069653001203090100002203102700002603124700002503150856004803175       2021                        eng d00aStream corridor connectivity controls on nitrogen cycling0 aStream corridor connectivity controls on nitrogen cycling  aBoulder, CO, USAbUniversity of Colorado Boulderc05/20210 vPhD3 a<p>As water flows downstream, it is transported to and from environments that surround the visible stream. Along with surface water, these laterally and vertically connected environments comprise the stream corridor. Stream corridor connectivity influences many ecosystem services, including retention of excess nutrients. The subsurface area where stream water and groundwater mixes&mdash;the hyporheic zone&mdash;represents one of the most biogeochemically active parts of stream corridors.</p><p>The goal of my research is to advance understanding of how connectivity between different parts of a stream corridor controls the availability and retention of nitrogen (N), a nutrient that can limit primary productivity (low-N) and negatively impact water quality (excess N). First, I developed and applied a new machine learning method to objectively characterize the extent and variability of hyporheic exchange in terms of statistically unique functional zones using geophysical data. In applying this method to a benchmark dataset, I found that hyporheic extent does not scale uniformly with streamflow and that changes in the heterogeneity of connectivity differ over small (&lt;10 m) distances. Next, I leveraged the relative simplicity of ephemeral streams of the McMurdo Dry Valleys (MDVs), Antarctica, to isolate stream corridor processes that influence the fate of N. Through intensive field sampling campaigns, I found that the hyporheic zone can be a persistent source of N even in this low nutrient environment. Next, I combined historic sample data and remote sensing analysis to estimate how much N is stored in an MDV stream corridor. My results indicate that up to 103 times more N is stored in this system than is exported each year, with most of this storage in the shallow (&lt; 10 cm) hyporheic zone. Lastly, I examined 25 years of data for 10 streams to assess how stream corridor processes control concentration-discharge relationships. I found that in the absence of hillslope connectivity, stream corridor processes alone can maintain chemostasis &ndash; relatively small concentration changes with large fluctuations in streamflow &ndash; of both geogenic solutes and primary nutrients. My analysis also revealed that solutes subject to greater control by biological processes exhibit more variability within chemostatic relationships than weathering solutes that are only minimally influenced by biota.</p><p>Altogether, this research advances understanding of processes that are difficult to measure or are often overlooked in typical studies of temperate stream corridors. My findings provide insight into the surprising ways in which N is mobilized, transformed, and retained due to stream corridor connectivity in intermittent stream systems with few N inputs.</p>10aAntarctica10ahyporheic zone10anitrogen cycling10astreams1 aSingley, Joel, G.1 aHinckley, Eve-Lyn, S.1 aGooseff, Michael, N.  uhttps://www.proquest.com/docview/257259312703418nas 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=1450300775nas a2200217   4500008004100000245010100041210006900142260001500211300001200226490000700238653002100245653001200266100002100278700002700299700002400326700002200350700002400372700001700396700002200413856012200435       2008                        eng d00aHydrologic response to extreme warm and cold summers in the McMurdo Dry Valleys, East Antarctica0 aHydrologic response to extreme warm and cold summers in the McMu  c05/16/2008  a499-5090 v2010aClimate Response10astreams1 aDoran, Peter, T.1 aMcKay, Christopher, P.1 aFountain, Andrew, G1 aNylen, Thomas, H.1 aMcKnight, Diane, M.1 aJaros, Chris1 aBarrett, John, E.  uhttps://mcm.lternet.edu/content/hydrologic-response-extreme-warm-and-cold-summers-mcmurdo-dry-valleys-east-antarctica