02541nas a2200241 4500008004100000245010600041210006900147260001200216520180200228653001202030653000602042653001802048653001902066653001302085653001902098653000602117653002202123100002202145700002202167700002102189700002402210856006502234 2023 eng d00aNitrogen fixation facilitates stream microbial mat biomass across the McMurdo Dry Valleys, Antarctica0 aNitrogen fixation facilitates stream microbial mat biomass acros c07/20233 a

Nitrogen (N) fixation is a fundamental mechanism by which N enters streams. Yet, because of modern N saturation, it is difficult to study the importance of N-fixation to stream nutrient budgets. Here, we utilized relatively simple and pristine McMurdo Dry Valley streams to investigate the role of N-fixing Nostoc abundance, streamwater dissolved inorganic N (DIN) concentration, and distance from the source glacier in regulating the elemental and isotopic composition of three microbial mat types (black, orange, and green) at the landscape scale. We found Nostoc-based black mats were the most enriched in δ15N, and δ15N signatures of mats increased where Nostoc was abundant, but did not surpass the atmospheric standard (δ15N ≈ 0‰). Furthermore, green and orange mat δ15N signatures became more depleted with increasing DIN, indicating that mats utilize glacial meltwater-sourced N when available. The distance from the source glacier explained limited variability in mat δ15N across sites, indicating the influence of individual stream characteristics on N spiraling. To further explore longitudinal N spiraling processes generating observed δ15Ν patterns, we developed a simple steady-state mathematical model. Analysis of plausible scenarios with this model confirmed that streams both have the capacity to remove allochthonous DIN over the plausible range of inputs, and that internal N sources are required to account for δ15N signatures and observed DIN concentrations at stream outlets. Collectively, these data and modeling results demonstrate that N-fixation exerts substantial influence within and across these streams, and is presumably dependent upon interconnected organic matter reserves, mineralization rates, and geomorphology.

10abiofilm10aC10acyanobacteria10ahyporheic zone10aMCM LTER10amineralization10aN10aP biogeochemistry1 aKohler, Tyler, J.1 aSingley, Joel, G.1 aWlostowski, Adam1 aMcKnight, Diane, M. uhttps://link.springer.com/article/10.1007/s10533-023-01069-002639nas 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

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 – 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 Luticola 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.

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/10.1029/2019JG00515302240nas a2200145 4500008004100000245010800041210006900149260001200218490000800230520171900238100002101957700002501978700001702003856007402020 2018 eng d00aSoil Moisture Controls the Thermal Habitat of Active Layer Soils in the McMurdo Dry Valleys, Antarctica0 aSoil Moisture Controls the Thermal Habitat of Active Layer Soils c01/20180 v1233 a

Antarctic soil ecosystems are strongly controlled by abiotic habitat variables. Regional climate change in the McMurdo Dry Valleys is expected to cause warming over the next century, leading to an increase in frequency of freeze-thaw cycling in the soil habitat. Previous studies show that physiological stress associated with freeze-thaw cycling adversely affects invertebrate populations by decreasing abundance and positively selecting for larger body sizes. However, it remains unclear whether or not climate warming will indeed enhance the frequency of annual freeze-thaw cycling and associated physiological stresses. This research quantifies the frequency, rate, and spatial heterogeneity of active layer freezing to better understand how regional climate change may affect active layer soil thermodynamics, and, in turn, affect soil macroinvertebrate communities. Shallow active layer temperature, specific conductance, and soil moisture were observed along natural wetness gradients. Field observations show that the frequency and rate of freeze events are nonlinearly related to freezable soil moisture (θf). Over a 2 year period, soils at θf < 0.080 m3/m3 experienced between 15 and 35 freeze events and froze rapidly compared to soils with θf > 0.080 m3/m3, which experienced between 2 and 6 freeze events and froze more gradually. A numerical soil thermodynamic model is able to simulate observed freezing rates across a range of θf, reinforcing a well-known causal relationship between soil moisture and active layer freezing dynamics. Findings show that slight increases in soil moisture can potentially offset the effect of climate warming on exacerbating soil freeze-thaw cycling.

1 aWlostowski, Adam1 aGooseff, Michael, N.1 aAdams, Byron uhttps://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JG00401801835nas a2200169 4500008004100000245013700041210006900178260001200247300001800259490000700277520122000284100002101504700002501525700002401550700001701574856007401591 2018 eng d00aTransit times and rapid chemical equilibrium explain chemostasis in glacial meltwater streams in the McMurdo Dry Valleys, Antarctica0 aTransit times and rapid chemical equilibrium explain chemostasis c12/2018 a13322 - 133310 v453 a

Fluid  transit  time  is  understood  to  be  an  important  control  on  the  shape  of concentration-discharge (C-q) relationships, yet empirical evidence supporting this linkage is limited. We investigated C-q relationships for weathering-derived solutes across seven Antarctic glacial meltwate streams. We hypothesized that (H1) solute fluxes in McMurdo Dry Valley streams are reaction limited so that C-q relationships are characterized by dilution and that (H2) transit time explains between-stream variability in the degree of C-q dilution. Results show that C-q relationships are chemostatic because solute equilibrium times are shorter than stream corridor fluid transit times. Between-stream variability in the efficiency of solute production is positively correlated with transit time, suggesting that transit time is an important control on the solute export regime. These results provide empirical evidence for the controls on weathering-derived C-q relationships and have important implications for Antarctic ecosystems and solute export regimes of watersheds worldwide.

1 aWlostowski, Adam1 aGooseff, Michael, N.1 aMcKnight, Diane, M.1 aLyons, Berry uhttps://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL08036902723nas a2200229 4500008004100000245015600041210006900197260001200266300001600278490000700294520192000301100002202221700002102243700002402264700002002288700002502308700001702333700002402350700002902374700002502403856006502428 2017 eng d00aCharacterizing hyporheic exchange processes using high-frequency electrical conductivity-discharge relationships on subhourly to interannual timescales0 aCharacterizing hyporheic exchange processes using highfrequency  c05/2017 a4124 - 41410 v533 a

Concentration-discharge (C-Q) relationships are often used to quantify source water contributions and biogeochemical processes occurring within catchments, especially during discrete hydrological events. Yet, the interpretation of C-Q hysteresis is often confounded by complexity of the critical zone, such as numerous source waters and hydrochemical nonstationarity. Consequently, researchers must often ignore important runoff pathways and geochemical sources/sinks, especially the hyporheic zone because it lacks a distinct hydrochemical signature. Such simplifications limit efforts to identify processes responsible for the transience of C-Q hysteresis over time. To address these limitations, we leverage the hydrologic simplicity and long-term, high-frequency Q and electrical conductivity (EC) data from streams in the McMurdo Dry Valleys, Antarctica. In this two end-member system, EC can serve as a proxy for the concentration of solutes derived from the hyporheic zone. We utilize a novel approach to decompose loops into subhysteretic EC-Q dynamics to identify individual mechanisms governing hysteresis across a wide range of timescales. We find that hydrologic and hydraulic processes govern EC response to diel and seasonal Q variability and that the effects of hyporheic mixing processes on C-Q transience differ in short and long streams. We also observe that variable hyporheic turnover rates govern EC-Q patterns at daily to interannual timescales. Last, subhysteretic analysis reveals a period of interannual freshening of glacial meltwater streams related to the effects of unsteady flow on hyporheic exchange. The subhysteretic analysis framework we introduce may be applied more broadly to constrain the processes controlling C-Q transience and advance understanding of catchment evolution. 

1 aSingley, Joel, G.1 aWlostowski, Adam1 aBergstrom, Anna, J.1 aSokol, Eric, R.1 aTorrens, Christa, L.1 aJaros, Chris1 aWilson, Colleen, E.1 aHendrickson, Patrick, J.1 aGooseff, Michael, N. uhttp://onlinelibrary.wiley.com/doi/10.1002/2016WR019739/full02751nas a2200181 4500008004100000022001300041245010100054210006900155260001200224300001200236490000800248520216000256100002502416700002102441700002402462700001702486856006602503 2017 eng d a0169555X00aHydrologic connectivity and implications for ecosystem processes - Lessons from naked watersheds0 aHydrologic connectivity and implications for ecosystem processes c01/2017 a63 - 710 v2773 a

Hydrologic connectivity has received great attention recently as our conceptual models of watersheds and water quality have evolved in the past several decades. However, the structural complexity of most temperate watersheds (i.e. connections among shallow soils, deep aquifers, the atmosphere and streams) and the dynamic seasonal changes that occur within them (i.e., plant senescence which impacts evapotranspiration) create significant challenges to characterizing or quantifying hydrologic connectivity. The McMurdo Dry Valleys, a polar desert in Antarctica, provide a unique opportunity to study hydrologic connectivity because there is no vegetative cover (and therefore no transpiration), and no deep aquifers connected to surface soils or streams. Glacier melt provides stream flow to well-established channels and closed-basin, ice-covered lakes on the valley floor. Streams are also connected to shallow hyporheic zones along their lengths, which are bounded at ~75 cm depth by ice-cemented permafrost. These hydrologic features and connections provide water for and underpin biological communities. Hence, exchange of water among them provides a vector for exchange of energy and dissolved solutes. Connectivity is dynamic on timescales of a day to a flow season (6–12 weeks), as streamflow varies over these timescales. The timescales over which these connections occur is also dynamic. Exchanges between streams and hyporheic zones, for example, have been estimated to be as short as hours to as long as several weeks. These exchanges have significant implications for the biogeochemistry of these systems and the biotic communities in each feature. Here we evaluate the lessons we can learn about hydrologic connectivity in the MDV watersheds that are simplified in the context of processes occurring and water reservoirs included in the landscape, yet are sensitive to climate controls and contain substantial physical heterogeneity. We specifically explore several metrics that are simple and/or commonly employed in hydrologic analyses and interpret them in the context of connectivity between and among hydrologic features.

1 aGooseff, Michael, N.1 aWlostowski, Adam1 aMcKnight, Diane, M.1 aJaros, Chris uhttp://linkinghub.elsevier.com/retrieve/pii/S0169555X1630248302884nas a2200133 4500008004100000245012700041210006900168260004800237490001000285520235800295100002102653700002502674856005102699 2017 eng d00aHydrologic connectivity in the McMurdo Dry Valleys, Antarctica: Water-mediated mass and energy fluxes in streams and soils0 aHydrologic connectivity in the McMurdo Dry Valleys Antarctica Wa aBoulder, CObUniversity of Colorado Boulder0 vPh.D.3 a

Chapter 1 synthesizes 20 years of stream gauge observations (discharge, water temperature, and specific conductance) to assess patterns of hydrologic connectivity between glaciers, streams and lakes. Results reveal hydrologic patterns across daily, annual and inter-annual timescales, which together characterize the hydrologic regime of MDV streams. Also, stream gauge data display a relationship between stream length and hydrologic regime. Longer streams are more intermittent, warmer, and saltier than shorter streams. This work provides physical context for understanding biological differences among MDV streams, while providing a methodological template for quantifying hydrologic connectivity.

Chapter 2 investigates the nature of concentration-discharge relationships for weathering-derived solutes in MDV streams. The relative simplicity of MDV “watersheds” permits the use of concentration-discharge relationships to infer hydrologic and chemical mixing dynamics occurring along the river corridor. Long-term stream geochemical data show that weathering derived solutes exhibit chemostatic C-Q relationships. Chemostasis implies that rates of solute production and/or mobilization scale proportionately with stream discharge. A numerical weathering and solute transport model suggests that chemostasis is maintained by a positive relationship between weathering rate and discharge along the stream corridor.

Finally, Chapters 3 and 4 investigate water-mediated energy fluxes within the soil habitat. Nematode communities in MDV are highly sensitive to the thermodynamic regime of active layer soils. Soil moisture and air temperature data were collected across natural wetness gradients adjacent to fluvial features to assess the control of soil moisture on the soil thermal regime. Observations show that wetter soils freeze less frequently and more gradually than drier soils. Also, a numerical soil heat transfer model suggests that increases in soil moisture and air temperature result in warmer average habitat temperature, an extension of the duration of time the soil habitat spends above freezing, and a reduction in the rate and frequency of freezing. The results of this chapter provide a physical context for understanding current and future patterns of ecosystem structure and function in MDV soils.

1 aWlostowski, Adam1 aGooseff, Michael, N. uhttps://scholar.colorado.edu/cven_gradetds/77/02242nas a2200181 4500008004100000245012300041210006900164260001200233300001400245490000700259520163300266100002101899700002501920700002401945700001701969700001701986856005702003 2016 eng d00aPatterns of hydrologic connectivity in the McMurdo dry valleys, Antarctica: a synthesis of 20 years of hydrologic data0 aPatterns of hydrologic connectivity in the McMurdo dry valleys A c04/2016 a2958-29750 v303 a

Streams in the McMurdo Dry Valleys (MDVs) of Antarctica moderate an important hydrologic and biogeochemical connection between upland alpine glaciers, valley-bottom soils, and lowland closed-basin lakes. Moreover, MDV streams are simple but dynamic systems ideal for studying interacting hydrologic and ecological dynamics. This work synthesizes 20 years of hydrologic data, collected as part of the MDVs Long-Term Ecological Research project, to assess spatial and temporal dynamics of hydrologic connectivity between glaciers, streams, and lakes. Long-term records of stream discharge (Q), specific electrical conductance (EC), and water temperature (T) from 18 streams were analysed in order to quantify the magnitude, duration, and frequency of hydrologic connections over daily, annual, and inter-annual timescales. At a daily timescale, we observe predictable diurnal variations in Q, EC, and T. At an annual timescale, we observe longer streams to be more intermittent, warmer, and have higher median EC values, compared to shorter streams. Longer streams also behave chemostatically with respect to EC, whereas shorter streams are more strongly characterized by dilution. Inter-annually, we observe significant variability in annual runoff volumes, likely because of climatic variability over the 20 record years considered. Hydrologic connections at all timescales are vital to stream ecosystem structure and function. This synthesis of hydrologic connectivity in the MDVs provides a useful end-member template for assessing hydrologic connectivity in more structurally complex temperate watersheds. 

1 aWlostowski, Adam1 aGooseff, Michael, N.1 aMcKnight, Diane, M.1 aJaros, Chris1 aLyons, Berry uhttp://onlinelibrary.wiley.com/doi/10.1002/hyp.1081800864nas a2200217 4500008004100000245020400041210006900245260001200314300001600326490000700342100002400349700001600373700002400389700002500413700001700438700001300455700001700468700002100485700002100506856011900527 2015 eng d00aPotential for real-time understanding of coupled hydrologic and biogeochemical processes in stream ecosystems: Future integration of telemetered data with process models for glacial meltwater streams0 aPotential for realtime understanding of coupled hydrologic and b c08/2015 a6725 - 67380 v511 aMcKnight, Diane, M.1 aCozzetto, K1 aCullis, James, D.S.1 aGooseff, Michael, N.1 aJaros, Chris1 aKoch, J.1 aLyons, Berry1 aNeupauer, R., M.1 aWlostowski, Adam uhttp://doi.wiley.com/10.1002/2015WR017618http://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2F2015WR017618