A Stable Isotope Tracer Study of the Influences of Adjacent Land Use and Riparian Condition on Fates of Nitrate in Streams

The influence of land use on potential fates of nitrate (NO3-) in stream ecosystems, ranging from denitrification to storage in organic matter, has not been documented extensively. Here, we describe the Pacific Northwest component of Lotic Intersite Nitrogen eXperiment, phase II (LINX II) to examine how land-use setting influences fates of NO3- in streams. We used 24 h releases of a stable isotope tracer ((NO3)-N-15-N) in nine streams flowing through forest, agricultural, and urban land uses to quantify NO3- uptake processes. NO3- uptake lengths varied two orders of magnitude (24-4247 m), with uptake rates (6.5-158.1 mg NO3-N m(-2) day(-1)) and uptake velocities (0.1-2.3 mm min(-1)) falling within the ranges measured in other LINX II regions. Denitrification removed 0-7% of added tracer from our streams. In forest streams, 60.4 to 77.0% of the isotope tracer was exported downstream as NO3-, with 8.0 to 14.8% stored in wood biofilms, epilithon, fine benthic organic matter, and bryophytes. Agricultural and urban streams with streamside forest buffers displayed hydrologic export and organic matter storage of tracer similar to those measured in forest streams. In agricultural and urban streams with a partial or no riparian buffer, less than 1 to 75% of the tracer was exported downstream; much of the remainder was taken up and stored in autotrophic organic matter components with short N turnover times. Our findings suggest restoration and maintenance of riparian forests can help re-establish the natural range of NO3- uptake processes in human-altered streams.Keywords: Oregon, Organic matter storage, Streams, Spiraling, Land use, N-15, Isotope tracer, Denitrification, Nitroge

Thinking outside the channel : modeling nitrogen cycling in networked river ecosystems

Author Posting. © Ecological Society of America, 2011. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Frontiers in Ecology and the Environment 9 (2011): 229–238, doi:10.1890/080211.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.This research was supported by NSF (DEB-0111410). Additional support was provided by NSF for BJP and SMT (DEB-0614301), for WMW (OCE-9726921 and DEB-0614282), for WHM and JDP (DEB-0620919), for SKH (DEB-0423627), and by the Gordon and Betty Moore Foundation for AMH, GCP, ESB, and JAS, and by an EPA Star Fellowship for AMH

A Landscape Plan Based on Historical Fire Regimes for a Managed Forest Ecosystem: the Augusta Creek Study

The Augusta Creek project was initiated to establish and integrate landscape and watershed objectives into a landscape plan to guide management activities within a 7600-hectare (19,000-acre) planning area in western Oregon. Primary objectives included the maintenance of native species, ecosystem processes and structures, and long-term ecosystem productivity in a federally managed landscape where substantial acreage was allocated to timber harvest. Landscape and watershed management objectives and prescriptions were based on an interpreted range of natural variability of landscape conditions and disturbance processes. A dendrochronological study characterized fire patterns and regimes over the last 500 years. Changes in landscape conditions throughout the larger surrounding watershed due to human uses (e.g., roads in riparian areas, widespread clearcutting, a major dam, and portions of a designated wilderness and an unroaded area) also were factored into the landscape plan. Landscape prescriptions include an aquatic reserve system comprised of small watersheds distributed throughout the planning area and major valley-bottom corridor reserves that connect the small-watershed reserves. Where timber harvest was allocated, prescriptions derived from interpretations of fire regimes differ in rotation ages (100 to 300 years), green-tree retention levels (15- to 50-percent canopy cover), and spatial patterns of residual trees. General prescriptions for fire management also were based on interpretations of past fire regimes. All these prescriptions were linked to specific blocks of land to provide an efficient transition to site-level planning and project implementation. Landscape and watershed conditions were projected 200 years into the future and compared with conditions that would result from application of standards, guidelines, and assumptions in the Northwest Forest Plan prior to adjustments resulting from watershed analyses. The contrasting prescriptions for aquatic reserves and timber harvest (rotation lengths, green-tree retention levels, and spatial patterns) in these two approaches resulted in strikingly different potential future landscapes. These differences have significant implications for some ecosystem processes and habitats. We view this management approach as a potential post watershed analysis implementation of the Northwest Forest Plan and offer it as an example of how ecosystem management could be applied in a particular landscape by using the results of watershed analysis

Stream denitrification across biomes and its response to anthropogenic nitrate loading

Author Posting. © The Author(s), 2008. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 452 (2008): 202-205, doi:10.1038/nature06686.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.Funding for this research was provided by the National Science Foundation

and biological linkages within a stream geomorphic hierarchy: A modeling approach.’’

Abstract. Geology and precipitation interact to determine the geomorphology of a stream basin. We propose that stream geomorphology in turn interacts with sunlight, air temperature, precipitation, and geology to produce a distribution of environmental drivers (incident radiation, discharge, water temperature, nutrients) that is largely responsible for determining the distribution of organisms in streams. GEOMOD, a physically explicit stream ecosystem model, was designed to examine this proposal. The model has a geomorphically based hierarchical structure with basin, reach, and channel-unit levels of resolution. We used GEOMOD: 1) to simulate annual cycles of the biota in 3rd-and 5th-order stream sections at the basin level of resolution and 2) to predict organism distributions at the reach and channel-unit level of resolution. Stream physical structure and the 4 environmental drivers were the only factors that differed among the sites. Data from two 150-m sections of 3rd-order Mack Creek (one in old-growth and the other in clear-cut forest) and from a 1.5-km section of 5th-order Lookout Creek in the Cascade mountains of Oregon were used to parameterize the physical structure and initial standing crops and calibrate the drivers. Uniform parameters were determined by curve-fitting. GEOMOD simulated annual magnitudes and cycles for abiotic (e.g., channel dimensions, fine particulate organic matter) and biotic (e.g., algae, invertebrates, fish) variables in Mack and Lookout creeks. With explicit parameterization of reach and channel-unit sequences, GEOMOD also predicted the distribution of organisms among channel units and reaches. Fish distributions were accurately predicted at the reach scale, while algal-invertebrate interactions and scouring effects became clear only when examined at the channel-unit level. These results demonstrate that organism distributions and interactions in highly structured streams such as those in the Pacific Northwest region of the USA can be effectively simulated with a physically explicit model. Although more complicated to design and parameterize than a uniform physical representation, a physical explicit model can be tailored to represent a wide variety of stream types

Stream nitrate enrichment and increased light yet no algal response following forest harvest and experimental manipulation of headwater riparian zones.

Disturbances to forested watersheds often result in increases of nutrients and light to nearby streams. Such changes are generally expected to produce a shift to a more autotrophic aquatic ecosystem, with measurable increases in algae, and associated implications for food webs and fisheries. Although this paradigm is widely established, results from our 10-year study (2007-2016) in 12 headwater streams and four sites downstream in the Trask River Watershed (Oregon, USA), did not concur. In 2012, one watershed was thinned, three were clearcut harvested with variable buffers and three with uniform riparian buffers. After harvest, light to the stream surface significantly increased at the three watersheds with variable buffers while dissolved inorganic nitrogen (DIN) significantly increased in all of the clearcut harvested streams. Despite the increase in DIN and light, algal standing stocks and chlorophyll a concentrations did not significantly increase. The common assumption of increased autotrophic responses in stream food webs following increases of nitrogen and light was not supported here. We postulate the co-limitation of nutrients, driven by low phosphorus concentrations, which unlike DIN did not increase post-harvest, and the characteristics of the algal community, which were dominated by low light adapted diatoms rather than green algae, contributed to our findings of no responses for standing stocks of epilithic algae or concentrations of chlorophyll a. The inclusion of multiple statistical analyses provided more certainty around our findings. This study documents responses to current forest practices and provides cautionary information for management and restoration activities aiming to increase fish abundance and standing stocks by opening riparian canopies and adding nutrients

A STABLE ISOTOPE TRACER STUDY OF NITROGEN UPTAKE AND TRANSFORMATION IN AN OLD‐GROWTH FOREST STREAM

The understanding of nitrogen dynamics in streams of temperate forest biomes historically has been constrained by a combination of anthropogenic disturbances and technical limitations. We report here on a study in an undisturbed stream in Oregon, USA, using a stable isotopic tracer to quantify uptake, transformation, and retention of nitrogen. We added 15NH4Cl for six weeks to Mack Creek, a third‐order stream in a 500‐year‐old‐growth coniferous forest and monitored 15N in dissolved, aquatic, and terrestrial riparian food web components. Data collected before, during, and for four weeks after the tracer addition allowed us to derive uptake rates of inorganic N and to trace its fates. Short uptake lengths (35–55 m) and residence times (8–12 min) of ammonium indicated strong demand. Despite nitrate concentrations of 55–68 μg/L, nitrification rates were also high, with 40– 50% of the 15NH4+ converted to nitrate over the 220‐m study reach. Aquatic bryophytes and biofilm on large wood (“epixylon”) showed the highest biomass‐adjusted uptake rates. All aquatic consumers sampled, both vertebrate and invertebrate, showed incorporation of tracer 15N by the end of the experiment; small invertebrate grazers were more strongly labeled than their food sources. Increased 15N label in 15 of the 17 riparian plant species sampled suggested transfer of aquatic N to the terrestrial ecosystem. At the end of the release, 81% of the added tracer was accounted for, with 49% exported (primarily as 15NO3−) and 32% retained within the stream and riparian biota (primarily by bryophytes, epixylon, and fine benthic organic material). Our results suggest that, in streams within undisturbed primary forests, uptake and retention of nitrogen may be highly efficient and that there may be strong connections between terrestrial and aquatic ecosystems

ArgerichComprehensiveMultiyearCarbonSupportingInfo.pdf

Headwater streams comprise nearly 90% of the total length of perennial channels in global catchments. They mineralize organic carbon entering from terrestrial systems, evade terrestrial carbon dioxide (CO₂ ), and generate and remove carbon through in-stream primary production and respiration. Despite their importance, headwater streams are often neglected in global carbon budgets primarily because of a lack of available data. We measured these processes, in detail, over a 10 year period in a stream draining a 96 ha forested watershed in western Oregon, USA. This stream, which represents only 0.4% of the watershed area, exported 159 kg C ha⁻¹ yr⁻¹, similar to the global exports for large rivers. Stream export was dominated by downstream transport of dissolved inorganic carbon (63 kg C ha⁻¹ yr⁻¹) and by evasion of CO₂ to the atmosphere (42 kg C ha⁻¹ yr⁻¹), leaving the remainder of 51 kg C ha⁻¹ yr⁻¹ for downstream transport of organic carbon (17 kg C ha⁻¹ yr⁻¹ and 34 kg C ha⁻¹ yr⁻¹ in dissolved and particulate form, respectively)Keywords: evasion, carbon fluxes, biogeochemistry, carbon, strea