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Abstract
AbstractWatershed resilience is the ability of a watershed to maintain its characteristic system state while concurrently resisting, adapting to, and reorganizing after hydrological (for example, drought, flooding) or biogeochemical (for example, excessive nutrient) disturbances. Vulnerable waters include non-floodplain wetlands and headwater streams, abundant watershed components representing the most distal extent of the freshwater aquatic network. Vulnerable waters are hydrologically dynamic and biogeochemically reactive aquatic systems, storing, processing, and releasing water and entrained (that is, dissolved and particulate) materials along expanding and contracting aquatic networks. The hydrological and biogeochemical functions emerging from these processes affect the magnitude, frequency, timing, duration, storage, and rate of change of material and energy fluxes among watershed components and to downstream waters, thereby maintaining watershed states and imparting watershed resilience. We present here a conceptual framework for understanding how vulnerable waters confer watershed resilience. We demonstrate how individual and cumulative vulnerable-water modifications (for example, reduced extent, altered connectivity) affect watershed-scale hydrological and biogeochemical disturbance response and recovery, which decreases watershed resilience and can trigger transitions across thresholds to alternative watershed states (for example, states conducive to increased flood frequency or nutrient concentrations). We subsequently describe how resilient watersheds require spatial heterogeneity and temporal variability in hydrological and biogeochemical interactions between terrestrial systems and down-gradient waters, which necessitates attention to the conservation and restoration of vulnerable waters and their downstream connectivity gradients. To conclude, we provide actionable principles for resilient watersheds and articulate research needs to further watershed resilience science and vulnerable-water management.
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Sabo RD, Elmore AJ, Nelson DM, Clark CM, Fisher T, Eshleman KN. Positive correlation between wood δ 15N and stream nitrate concentrations in two temperate deciduous forests. ENVIRONMENTAL RESEARCH COMMUNICATIONS 2020; 2:1-17. [PMID: 36313933 PMCID: PMC9610404 DOI: 10.1088/2515-7620/ab77f8] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
A limitation to understanding drivers of long-term trends in terrestrial nitrogen (N) availability in forests and its subsequent influence on stream nitrate export is a general lack of integrated analyses using long-term data on terrestrial and aquatic N cycling at comparable spatial scales. Here we analyze relationships between stream nitrate concentrations and wood δ 15N records (n = 96 trees) across five neighboring headwater catchments in the Blue Ridge physiographic province and within a single catchment in the Appalachian Plateau physiographic province in the eastern United States. Climatic, acidic deposition, and forest disturbance datasets were developed to elucidate the influence of these factors on terrestrial N availability through time. We hypothesized that spatial and temporal variation of terrestrial N availability, for which tree-ring δ 15N records serve as a proxy, affects the variation of stream nitrate concentration across space and time. Across space at the Blue Ridge study sites, stream nitrate concentration increased linearly with increasing catchment mean wood δ 15N. Over time, stream nitrate concentrations decreased with decreasing wood δ 15N in five of the six catchments. Wood δ 15N showed a significant negative relationship with disturbance and acidic deposition. Disturbance likely exacerbated N limitation by inducing nitrate leaching and ultimately enhancing vegetative uptake. As observed elsewhere, lower rates of acidic deposition and subsequent deacidification of soils may increase terrestrial N availability. Despite the ephemeral modifications of terrestrial N availability by these two drivers and climate, long-term declines in terrestrial N availability were robust and have likely driven much of the declines in stream nitrate concentration throughout the central Appalachians.
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Affiliation(s)
- Robert D Sabo
- Oak Ridge Institute for Science and Education, United States Environmental Protection Agency, Office of Research and Development, National Center for Environmental Assessment, US EPA (8623-P),1200 Pennsylvania Ave NW; Washington, DC 20460, United States of America
| | - Andrew J Elmore
- University of Maryland Center for Environmental Science, Appalachian Laboratory, 301 Braddock Road, Frostburg, MD 21532, United States of America
| | - David M Nelson
- University of Maryland Center for Environmental Science, Appalachian Laboratory, 301 Braddock Road, Frostburg, MD 21532, United States of America
| | - Christopher M Clark
- United States Environmental Protection Agency, Office of Research and Development, National Center for Environmental Assessment, US EPA (8623-P),1200 Pennsylvania Ave NW; Washington, DC 20460, United States of America
| | - Thomas Fisher
- University of Maryland Center for Environmental Science, Appalachian Laboratory, 301 Braddock Road, Frostburg, MD 21532, United States of America
| | - Keith N Eshleman
- University of Maryland Center for Environmental Science, Appalachian Laboratory, 301 Braddock Road, Frostburg, MD 21532, United States of America
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Bellmore RA, Compton JE, Brooks JR, Fox EW, Hill RA, Sobota DJ, Thornbrugh DJ, Weber MH. Nitrogen inputs drive nitrogen concentrations in U.S. streams and rivers during summer low flow conditions. THE SCIENCE OF THE TOTAL ENVIRONMENT 2018; 639:1349-1359. [PMID: 29929300 PMCID: PMC6361169 DOI: 10.1016/j.scitotenv.2018.05.008] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/30/2018] [Revised: 03/28/2018] [Accepted: 05/01/2018] [Indexed: 05/26/2023]
Abstract
Ecological and human health impairments related to excess nitrogen (N) in streams and rivers remain widespread in the United States (U.S.) despite recent efforts to reduce N pollution. Many studies have quantified the relationship between N loads to streams in terms of N mass and N inputs to watersheds; however, N concentrations, rather than loads, are more closely related to impacts on human health and aquatic life. Additionally, concentrations, rather than loads, trigger regulatory responses. In this study, we examined how N concentrations are related to N inputs to watersheds (atmospheric deposition, synthetic fertilizer, manure applied to agricultural land, cultivated biological N fixation, and point sources), land cover characteristics, and stream network characteristics, including stream size and the extent of lakes and reservoirs. N concentration data were collected across the conterminous U.S. during the U.S. Environmental Protection Agency's 2008-09 National Rivers and Streams Assessment (n = 1966). Median watershed N inputs were 15.7 kg N ha-1 yr-1. Atmospheric deposition accounted for over half the N inputs in 49% of watersheds, but watersheds with the highest N input rates were dominated by agriculture-related sources. Total N input to watersheds explained 42% and 38% of the variability in total N and dissolved inorganic N concentrations, respectively. Land cover characteristics were also important predictors, with wetland cover muting the effect of agricultural N inputs on N concentrations and riparian disturbance exacerbating it. In contrast, stream variables showed little correlation with N concentrations. This suggests that terrestrial factors that can be managed, such as agricultural N use practices and wetland or riparian areas, control the spatial variability in stream N concentrations across the conterminous U.S.
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Affiliation(s)
- R A Bellmore
- National Research Council, in residence at the Environmental Protection Agency, Western Ecology Division, 200 SW 35th St., Corvallis, OR 97333, United States.
| | - J E Compton
- U.S. Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Western Ecology Division, 200 SW 35th St., Corvallis, OR 97333, United States.
| | - J R Brooks
- U.S. Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Western Ecology Division, 200 SW 35th St., Corvallis, OR 97333, United States.
| | - E W Fox
- U.S. Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Western Ecology Division, 200 SW 35th St., Corvallis, OR 97333, United States.
| | - R A Hill
- Oak Ridge Institute for Science and Education Post-doctoral Participant c/o U.S. EPA, Western Ecology Division, 200 SW 35th St., Corvallis, OR 97333, United States.
| | - D J Sobota
- Water Quality Program, Oregon Department of Environmental Quality, 700 NE Multnomah Street, Suite 600, Portland, OR 97232, United States.
| | - D J Thornbrugh
- Oak Ridge Institute for Science and Education Post-doctoral Participant c/o U.S. EPA, Western Ecology Division, 200 SW 35th St., Corvallis, OR 97333, United States.
| | - M H Weber
- U.S. Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Western Ecology Division, 200 SW 35th St., Corvallis, OR 97333, United States.
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