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  1. The concurrent reduction in acid deposition and increase in precipitation impact stream solute dynamics in complex ways that make predictions of future water quality difficult. To understand how changes in acid deposition and precipitation have influenced dissolved organic carbon (DOC) and nitrogen (N) loading to streams, we investigated trends from 1991 to 2018 in stream concentrations (DOC, ~3,800 measurements), dissolved organic nitrogen (DON, ~1,160 measurements), and dissolved inorganic N (DIN, ~2,130 measurements) in a forested watershed in Vermont, USA. Our analysis included concentration-discharge (C-Q) relationships and Seasonal Mann-Kendall tests on long-term, flow-adjusted concentrations. To understand whether hydrologic flushing and changes in acid deposition influenced long-term patterns by liberating DOC and dissolved N from watershed soils, we measured their concentrations in the leachate of 108 topsoil cores of 5 cm diameter that we flushed with solutions simulating high and low acid deposition during four different seasons. Our results indicate that DOC and DON often co-varied in both the long-term stream dataset and the soil core experiment. Additionally, leachate from winter soil cores produced especially high concentrations of all three solutes. This seasonal signal was consistent with C-Q relation showing that organic materials (e.g., DOC and DON), which accumulate during winter, are flushed into streams during spring snowmelt. Acid deposition had opposite effects on DOC and DON compared to DIN in the soil core experiment. Low acid deposition solutions, which mimic present day precipitation, produced the highest DOC and DON leachate concentrations. Conversely, high acid deposition solutions generally produced the highest DIN leachate concentrations. These results are consistent with the increasing trend in stream DOC concentrations and generally decreasing trend in stream DIN we observed in the long-term data. These results suggest that the impact of acid deposition on the liberation of soil carbon (C) and N differed for DOC and DON vs. DIN, and these impacts were reflected in long-term stream chemistry patterns. As watersheds continue to recover from acid deposition, stream C:N ratios will likely continue to increase, with important consequences for stream metabolism and biogeochemical processes. 
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  2. Abstract

    Tree-derived dissolved organic matter (DOM) comprises a significant carbon flux within forested watersheds. Few studies have assessed the optical properties of tree-derived DOM. To increase understanding of the factors controlling tree-derived DOM quality, we measured DOM optical properties, dissolved organic carbon (DOC) and calcium concentrations in throughfall and stemflow for 17 individual rain events during summer and fall in a temperate deciduous forest in Vermont, United States. DOC and calcium fluxes in throughfall and stemflow were enriched on average 4 to 70 times incident fluxes in rain. A multiway model was developed using absorbance and fluorescence spectroscopy to further characterize DOM optical properties. Throughfall contained a higher percentage of protein-like DOM fluorescence than stemflow while stemflow was characterized by a higher percentage of humic-like DOM fluorescence. DOM absorbance spectral slopes in yellow birch (Betula alleghaniensis) stemflow were significantly higher than in sugar maple (Acer saccharum) stemflow. DOM optical metrics were not influenced by rainfall volume, but percent protein-like fluorescence increased in throughfall during autumn when leaves senesced. Given the potential influence of tree-derived DOM fluxes on receiving soils and downstream ecosystems, future modeling of DOM transport and soil biogeochemistry should represent the influence of differing DOM quality in throughfall and stemflow across tree species and seasons.

     
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  4. Abstract

    A paired‐catchment study began in 2000 to assess the hydrologic effects of high‐elevation development on Mt. Mansfield, Vermont's highest summit (1340 m). West Branch Little River drains 12.08 km2and encompasses a large ski resort. Adjacent Ranch Brook drains 9.83 km2of minimally disturbed second‐growth forest. The two catchments have similar elevation, aspect, surficial and bedrock geology, and vegetation. The resort was well established before this study, but it underwent a major expansion during the period 2004–2008. The expansion included new ski lifts and trails, a large hotel, roads and second home development, a 435 000‐m3snowmaking storage pond and a nine‐hole golf course, increasing the extent of cleared/open land from 17% to 24%. Runoff from the developed West Branch Little River catchment was 21% greater than Ranch Brook over the duration of the study, but varied widely each year from 10% to 42%. This high variability occurs both on the interannual and individual storm scales, and is consistent with expectations from future climate projections. Hydrologic variability is on the rise, as shown by an increase in stream flashiness in both catchments over the 20 years of our study. Resort expansion, which provided for stormwater management, had no discernible effect on the overall runoff difference nor the flow distribution at the scale of the catchments, but sedimentation, water quality impacts and localized erosion cannot be ruled out. Forest clearing, impervious and hardened surfaces, and skier‐compacted and machine‐made snow may all cause enhanced runoff. However, the greater runoff at West Branch, which occurs primarily during snowmelt and summer, may arise partly from greater precipitation capture in the complex mountain topography. Development pressure on the mountain landscape continues to mount, but managers may also need to consider the confounding effects of a changing climate.

     
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  5. Abstract

    This study focuses on characterizing the contributions of key terrestrial pathways that deliver dissolved organic carbon (DOC) to streams during hydrological events and on elucidating factors governing variation in water and DOC fluxes from these pathways. We made high‐frequency measurements of discharge, specific conductance (SC), and fluorescent dissolved organic matter (FDOM) during 221 events recorded over 2 years within four Vermont (USA) watersheds that range in area from 0.4 to 139 km2. Using the SC measurements, together with statistical information on discharge, we separated the event hydrographs into contributions from three terrestrial pathways, which we refer to as riparian quickflow, subsurface quickflow, and slow‐flow groundwater. The pathway discharges were used as input to a mixing model that closely approximated sub‐hourly streamwater DOC concentrations as measured with the FDOM sensors. Subsurface quickflow, comprised of pre‐event water, was the leading contributor to streamwater DOC fluxes, while riparian quickflow, comprised of event water, was the second‐leading contributor to streamwater DOC fluxes, despite comprising the smallest proportion of streamflow yield among the three end‐member pathways. Fixed‐effects regression analysis revealed that the relationship between DOC fluxes from the end‐member pathways and event magnitude was consistent across the four watersheds. This analysis also showed that DOC fluxes from the quickflow pathways increased significantly with temperature and varied inversely, but weakly, with catchment antecedent wetness. We believe that our approach, which leverages in‐stream sensors that enable high‐frequency measurements over extended periods, may be applicable for evaluating controls on DOC export from other watersheds within and beyond our study region.

     
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  6. Abstract

    Aquatic fluxes of carbon and nutrients link terrestrial and aquatic ecosystems. Within forests, storm events drive both the delivery of carbon and nitrogen to the forest floor and the export of these solutes from the land via streams. To increase understanding of the relationships between hydrologic event character and the relative fluxes of carbon and nitrogen in throughfall, stemflow and streams, we measured dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations in each flow path for 23 events in a forested watershed in Vermont, USA. DOC and TDN concentrations increased with streamflow, indicating their export was limited by water transport of catchment stores. DOC and TDN concentrations in throughfall and stemflow decreased exponentially with increasing precipitation, suggesting that precipitation removed a portion of available sources from tree surfaces during the events. DOC and TDN fluxes were estimated for 76 events across a 2‐year period. For most events, throughfall and stemflow fluxes greatly exceeded stream fluxes, but the imbalance narrowed for larger storms (>30 mm). The largest 10 stream events exported 40% of all stream event DOC whereas those same 10 events contributed 14% of all throughfall export. Approximately 2–5 times more DOC and TDN was exported from trees during rain events than left the catchment via streams annually. The diverging influence of event size on tree versus stream fluxes has important implications for forested ecosystems as hydrological events increase in intensity and frequency due to climate change.

     
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  7. Abstract

    We examined how climate variability affects the mobilization of material from six watersheds. We analyzed one to seven years of high‐frequency sensor data from a temperate ecosystem and a tropical rainforest. We applied a windowed analysis to correlate concentration‐discharge (C‐Q) behavior with climate anomalies, providing insight into how hydrological and biogeochemical processes change in response to climate variability. Positive precipitation anomalies homogenized the C‐Q responses for dissolved organic matter, nitrate, specific conductance and turbidity, indicating that hydrological processes dominate the C‐Q signal and watersheds act as “conveyor belts” of material. In contrast, drier and warmer conditions led to C‐Q behavior associated with variation in solute concentration, suggesting that biogeochemical processes are a primary control on solute export and their response to flow. Results indicate that climate variability can move watersheds along a continuum from transporter‐to‐transformer of biologically active solutes and responses can potentially vary by biome.

     
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