Gregory Stanley V.
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ArticleThinking 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, LydiaAgricultural 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.
PreprintStream 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.