McDowell William H.

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McDowell
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William H.
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  • Article
    Thinking outside the channel : modeling nitrogen cycling in networked river ecosystems
    (Ecological Society of America, 2010-09-08) Helton, Ashley M. ; Poole, Geoffrey C. ; Meyer, Judy L. ; Wollheim, Wilfred M. ; Peterson, Bruce J. ; Mulholland, Patrick J. ; Bernhardt, Emily S. ; Stanford, Jack A. ; Arango, Clay P. ; Ashkenas, Linda R. ; Cooper, Lee W. ; Dodds, Walter K. ; Gregory, Stanley V. ; Hall, Robert O. ; Hamilton, Stephen K. ; Johnson, Sherri L. ; McDowell, William H. ; Potter, Jody D. ; Tank, Jennifer L. ; Thomas, Suzanne M. ; Valett, H. Maurice ; Webster, Jackson R. ; Zeglin, Lydia
    Agricultural and urban development alters nitrogen and other biogeochemical cycles in rivers worldwide. Because such biogeochemical processes cannot be measured empirically across whole river networks, simulation models are critical tools for understanding river-network biogeochemistry. However, limitations inherent in current models restrict our ability to simulate biogeochemical dynamics among diverse river networks. We illustrate these limitations using a river-network model to scale up in situ measures of nitrogen cycling in eight catchments spanning various geophysical and land-use conditions. Our model results provide evidence that catchment characteristics typically excluded from models may control river-network biogeochemistry. Based on our findings, we identify important components of a revised strategy for simulating biogeochemical dynamics in river networks, including approaches to modeling terrestrial–aquatic linkages, hydrologic exchanges between the channel, floodplain/riparian complex, and subsurface waters, and interactions between coupled biogeochemical cycles.
  • Preprint
    Stream denitrification across biomes and its response to anthropogenic nitrate loading
    ( 2007-06-06) Mulholland, Patrick J. ; Helton, Ashley M. ; Poole, Geoffrey C. ; Hall, Robert O. ; Hamilton, Stephen K. ; Peterson, Bruce J. ; Tank, Jennifer L. ; Ashkenas, Linda R. ; Cooper, Lee W. ; Dahm, Clifford N. ; Dodds, Walter K. ; Findlay, Stuart E. G. ; Gregory, Stanley V. ; Grimm, Nancy B. ; Johnson, Sherri L. ; McDowell, William H. ; Meyer, Judy L. ; Valett, H. Maurice ; Webster, Jackson R. ; Arango, Clay P. ; Beaulieu, Jake J. ; Bernot, Melody J. ; Burgin, Amy J. ; Crenshaw, Chelsea L. ; Johnson, Laura T. ; Niederlehner, B. R. ; O'Brien, Jonathan M. ; Potter, Jody D. ; Sheibley, Richard W. ; Sobota, Daniel J. ; Thomas, Suzanne M.
    Worldwide, anthropogenic addition of bioavailable nitrogen (N) to the biosphere is increasing and terrestrial ecosystems are becoming increasingly N saturated, causing more bioavailable N to enter groundwater and surface waters. Large-scale N budgets show that an average of about 20-25% of the N added to the biosphere is exported from rivers to the ocean or inland basins, indicating substantial sinks for N must exist in the landscape. Streams and rivers may be important sinks for bioavailable N owing to their hydrologic connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favor microbial denitrification. Here, using data from 15N tracer experiments replicated across 72 streams and 8 regions representing several biomes, we show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of instream nitrate that is removed from transport. Total uptake of nitrate was related to ecosystem photosynthesis and denitrification was related to ecosystem respiration. Additionally, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.
  • Article
    Quantifying the production of dissolved organic nitrogen in headwater streams using 15N tracer additions
    (Association for the Sciences of Limnology and Oceanography, 2013-07) Johnson, Laura T. ; Tank, Jennifer L. ; Hall, Robert O. ; Mulholland, Patrick J. ; Hamilton, Stephen K. ; Valett, H. Maurice ; Webster, Jackson R. ; Bernot, Melody J. ; McDowell, William H. ; Peterson, Bruce J. ; Thomas, Suzanne M.
    Most nitrogen (N) assimilation in lake and marine ecosystems is often subsequently released via autochthonous dissolved organic nitrogen (DON) production, but autochthonous DON production has yet to be quantified in flowing waters. We measured in-stream DON production following 24 h 15N-nitrate () tracer additions in 36 headwater streams, a subset of sites from the second Lotic Intersite Nitrogen eXperiment. Streams were located in five North American ecoregions and drained basins dominated by native vegetation, agriculture, or urban land use. Using a two-compartment model, we could quantify DON production in 15 streams as a function of DO15N derived from 15N tracer in biomass compartments. The streams with detectable DON production had higher % modified land use (agriculture + urban) in their basins than did streams with undetectable DON production. Median DON production represented 8% of total uptake when we used N biomass estimates based on N assimilated over 1 d (measured directly from the 15N additions). Median DON production was 17% of total uptake when we used N assimilated over 42 d (extrapolated from previous 15N tracer studies). Variation in DON production was positively correlated with ecosystem respiration, indicating that stream heterotrophy may influence DON production. In-stream DON production was similar in magnitude to stream denitrification and nitrification, indicating that the production of autochthonous DON can represent a substantial transformation of stream N. Our results confirm that headwater streams can quickly convert inorganic N into organic forms, although the ultimate fate of DON remains unclear.
  • Preprint
    Consequence of altered nitrogen cycles in the coupled human and ecological system under changing climate: the need for long-term and site-based research.
    ( 2014-07) Shibata, Hideaki ; Branquinho, Cristina ; McDowell, William H. ; Mitchell, Myron J. ; Monteith, Don T. ; Tang, Jianwu ; Arvola, Lauri ; Cruz, Cristina ; Cusack, Daniela F. ; Halada, Lubos ; Kopacek, Jiri ; Maguas, Cristina ; Sajidu, Samson ; Schubert, Hendrik ; Tokuchi, Naoko ; Zahora, Jaroslav
    Anthropogenically derived nitrogen (N) has a central role in global environmental changes, including climate change, biodiversity loss, air pollution, greenhouse gas emission, water pollution, as well as food production and human health. Current understanding of the biogeochemical processes that govern the N cycle in coupled human–ecological systems around the globe is drawn largely from the long-term ecological monitoring and experimental studies. Here, we review spatial and temporal patterns and trends in reactive N emissions, and the interactions between N and other important elements that dictate their delivery from terrestrial to aquatic ecosystems, and the impacts of N on biodiversity and human society. Integrated international and long-term collaborative studies covering research gaps will reduce uncertainties and promote further understanding of the nitrogen cycle in various ecosystems.
  • Article
    Denitrification and total nitrate uptake in streams of a tropical landscape
    (Ecological Society of America, 2010-12) Potter, Jody D. ; McDowell, William H. ; Merriam, J. L. ; Peterson, Bruce J. ; Thomas, Suzanne M.
    Rapid increases in nitrogen (N) loading are occurring in many tropical watersheds, but the fate of N in tropical streams is not well documented. Rates of nitrate uptake and denitrification were measured in nine tropical low-order streams with contrasting land use as part of the Lotic Intersite Nitrogen eXperiment II (LINX II) in Puerto Rico using short term (24-hour) additions of K15NO3 and NaBr. Background nitrate concentrations ranged from 105 to 997 μg N/L, and stream nitrate uptake lengths were long, varying from 315 to 8480 m (median of 1200 m). Other indices of nitrate uptake (mass transfer coefficient, Vf [cm/s], and whole-stream nitrate uptake rate, U [μg N·m−2·s−1]) were low in comparison to other regions and were related to chemical, biological, and physical parameters. Denitrification rates were highly variable (0–133 μg N·m−2·min−1; median = 15 μg N·m−2·min−1), were dominated by the end product N2 (rather than N2O), and were best predicted by whole-stream respiration rates and stream NO3 concentration. Denitrification accounted for 1–97% of nitrate uptake with five of nine streams having 35% or more of nitrate uptake via denitrification, showing that denitrification is a substantial sink for nitrate in tropical streams. Whole-stream nitrate uptake and denitrification in our study streams closely followed first-order uptake kinetics, indicating that NO3 uptake is limited by delivery of substrate (NO3) to the organisms involved in uptake or denitrification. In the context of whole-catchment nitrogen budgets, our finding that in-stream denitrification results in lower proportional production of N2O than terrestrial denitrification suggests that small streams can be viewed as the preferred site of denitrification in a watershed in order to minimize greenhouse gas N2O emissions. Conservation of small streams is thus critical in tropical ecosystem management.