03072nas a2200253   4500008004100000022001400041245011200055210006900167260001200236490000700248520227400255653001902529653001702548653002102565653002002586653001902606653001502625100002202640700002402662700002502686700002402711700002602735856005702761       2023                        eng d  a2161-954900aDifferentiating physical and biological storage of nitrogen along an intermittent Antarctic stream corridor0 aDifferentiating physical and biological storage of nitrogen alon  c09/20230 v423 a<p>In many temperate streams, biological uptake of N acts to attenuate the transport of excess N from allochthonous anthropogenic imports. Relatively few studies have determined how this N uptake relates to the magnitude of physical vs. biological N storage in the stream corridor, especially for intermittent systems where allochthonous N imports are often low and N transport may only occur during brief periods of streamflow. Glacial meltwater streams in the McMurdo Dry Valleys of Antarctica provide an excellent setting to quantify autochthonous N cycling and storage processes supported by abundant algal mats and well-connected hyporheic zones. We combined historic point-scale sediment and periphyton sample datasets with remote sensing-based modeling of periphyton coverage to estimate how much N was stored in periphyton biomass and the hyporheic zone of a 5-km long McMurdo Dry Valley stream corridor (&gt;100,000 m<sup>2</sup>). We contextualized these N storage calculations by estimating the magnitude of annual N imports to and exports from the stream corridor based on &gt;2 decades of streamflow and surface water data, source glacier ice cores and meltwater data, and past studies of local aeolian deposition and biological N fixation rates. We found that in this highly oligotrophic system, stream corridor-scale N storage was ~1000x that of total annual N import or export fluxes. More than 90% of this temporarily stored N was autochthonous organic matter in the shallow (&lt;10 cm) hyporheic zone, which acts as a reservoir that sustains N availability in the water column. Despite its location in a polar desert devoid of higher-order vegetation, area-normalized N storage (~40 g N/m<sup>2</sup>) was greater than that reported for streams at lower latitudes (~1&ndash;22 g N/m<sup>2</sup>). We also demonstrated that NH<sub>4</sub><sup>+</sup> sorption to stream sediment may be an important physicochemical N storage mechanism that responds to short-term fluctuations in streamflow and governs the mobility of inorganic N. Altogether, this research illustrates the importance of quantifying N storage within stream corridors when evaluating the significance of internal cycling and physical retention processes that modulate N availability.</p>10ahyporheic zone10aMcMurdo LTER10anitrogen cycling10anutrient budget10aorganic matter10aperiphyton1 aSingley, Joel, G.1 aSalvatore, Mark, R.1 aGooseff, Michael, N.1 aMcKnight, Diane, M.1 aHinckley, Eve-Lyn, S.  uhttps://www.journals.uchicago.edu/doi/10.1086/72567602809nas a2200337   4500008004100000245012000041210006900161260001200230300001800242490000800260520168500268653002201953653002001975653004901995653001902044653001202063653004302075653001902118653002402137653002102161653003102182100002102213700002402234700002202258700002402280700002402304700002502328700002402353700002502377856006902402       2021                        eng d00aDiatoms in hyporheic sediments trace organic matter retention and processing in the McMurdo Dry Valleys, Antarctica0 aDiatoms in hyporheic sediments trace organic matter retention an  c02/2021  ae2020JG0060970 v1263 a<p>In low‐nutrient streams in cold and arid ecosystems, the spiraling of autochthonous particulate organic matter (POM) may provide important nutrient subsidies downstream. Because of its lability and the spatial heterogeneity of processing in hyporheic sediments, the downstream transport and fate of autochthonous POM can be difficult to trace. In Antarctic McMurdo Dry Valley (MDV) streams, any POM retained in the hyporheic zone is expected to be derived from surface microbial mats that contain diatoms with long‐lasting silica frustules. We tested whether diatom frustules can be used to trace the retention of autochthonous POM in the hyporheic zone and whether certain geomorphic locations promote this process. The accumulation of diatom frustules in hyporheic sediments, measured as biogenic silica, was correlated with loss‐on‐ignition organic matter and sorbed ammonium, suggesting that diatoms can be used to identify locations where POM has been retained and processed over long timescales, regardless of whether the POM remains intact. In addition, by modeling the upstream sources of hyporheic diatom assemblages, we found that POM was predominantly derived from N‐fixing microbial mats of the genus Nostoc. In terms of spatial variability, we conclude that the hyporheic sediments adjacent to the stream channel that are regularly inundated by daily flood pulses are where the most POM has been retained over long timescales. Autochthonous POM is retained in hyporheic zones of low‐nutrient streams beyond the MDVs, and we suggest that biogenic silica and diatom composition can be used to identify locations where this transfer is most prevalent.</p>10abenthic processes10abiogenic silica10abiogeochemical cycles processes and modeling10acarbon cycling10adiatoms10agroundwater/surface water interactions10ahyporheic zone10aMcMurdo Dry Valleys10anitrogen cycling10aparticulate organic matter1 aHeindel, Ruth, C1 aDarling, Joshua, P.1 aSingley, Joel, G.1 aBergstrom, Anna, J.1 aMcKnight, Diane, M.1 aLukkari, Braeden, M.1 aWelch, Kathleen, A.1 aGooseff, Michael, N.  uhttps://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JG00609702603nas 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/10.1029/2021JG00630903417nas 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/2572593127