An index to characterize the spatial distribution of land use within watersheds and implications for river network nutrient removal and export

The spatial distribution of land use and associated nutrient inputs may influence the efficacy of in-stream nutrient removal; however, the effect of source location on N removal and watershed N export has not been quantified. We present the skewness index, a metric to quantify the spatial distribution of land use within watersheds. Using this index and a river network nitrogen removal model, we quantified the effect of varying the location of developed land use within two watersheds on nutrient removal and export. The quantity and location of developed land use as well as runoff affected nitrogen removal and export. Because river network nitrogen removal is bypassed when sources are skewed toward the watershed mouth, varying the location of land use alone can double aquatic nitrogen removal. Nutrient sources skewed toward the distant headwaters maximized in-stream removal which in turn can reduce watershed export

Examining the ecosystem service of nutrient removal in a coastal watershed

Globally, managers are trying to prevent or halt the eutrophication of valuable estuaries and bays by reducing nutrient inputs, but justifying the cost of conservation or processing facility upgrades often proves challenging. We focus on a coastal watershed in Maine and New Hampshire struggling with the financial burdens of nitrogen pollution mandates due to the eutrophication of the Great Bay estuary. After creating two future watershed land cover scenarios comparing plausible extremes, we ran them through two models, the Natural Capital Project’s InVEST (Integrated Valuation of Ecosystem Services and Tradeoffs) and a detailed hydrologic and biogeochemical river network model FrAMES (Framework for Aquatic Modeling of the Earth System). Through this work, we both evaluated and valued the ecosystem service of nitrogen retention. We find that both models provide numerical arguments for conservation efforts, and decision makers would benefit from using either an ecosystem services model or a biogeochemical model when dealing with complex issues like nutrient overenrichment. According to both our modeling results, modest watershed conservation efforts as defined by our expert stakeholders, ie: protecting wetlands and forests, could reduce the amount of total nitrogen entering the Great Bay estuary in the range of 3–28 metric tons per year

Development of scenarios for land cover, population density, impervious cover, and conservation in New Hampshire, 2010–2100

Future changes in ecosystem services will depend heavily on changes in land cover and land use, which, in turn, are shaped by human activities. Given the challenges of predicting long-term changes in human behaviors and activities, scenarios provide a framework for simulating the long-term consequences of land-cover change on ecosystem function. As input for process-based models of terrestrial and aquatic ecosystem function, we developed scenarios for land cover, population density, and impervious cover for the state of New Hampshire for 2020–2100. Key drivers of change were identified through information gathered from six sources: historical trends, existing plans relating to New Hampshire’s land-cover future, surveys, existing population scenarios, key informant interviews with diverse stakeholders, and input from subject-matter experts. Scenarios were developed in parallel with information gathering, with details added iteratively as new questions emerged. The final scenarios span a continuum from spatially dispersed development with a low value placed on ecosystem services (Backyard Amenities) to concentrated development with a high value placed on ecosystem services (the Community Amenities family). The Community family includes two population scenarios (Large Community and Small Community), to be combined with two scenarios for land cover (Protection of Wildlands and Promotion of Local Food), producing combinations that bring the total number of scenarios to six. Between Backyard Amenities and Community Amenities is a scenario based on linear extrapolations of current trends (Linear Trends). Custom models were used to simulate decadal change in land cover, population density, and impervious cover. We present raster maps and proportion of impervious cover for HUC10 watersheds under each scenario and discuss the trade-offs of our translation and modeling approach within the context of contemporary scenario projects

Dissolved organic carbon uptake in streams: A review and assessment of reach‐scale measurements

Quantifying the role that freshwater ecosystems play in the global carbon cycle requires accurate measurement and scaling of dissolved organic carbon (DOC) removal in river networks. We reviewed reach‐scale measurements of DOC uptake from experimental additions of simple organic compounds or leachates to inform development of aquatic DOC models that operate at the river network, regional, or continental scale. Median DOC uptake velocity (vf) across all measurements was 2.28 mm min−1. Measurements using simple compound additions resulted in faster vf (2.94 mm min−1) than additions of leachates (1.11 mm min−1). We also reviewed published data of DOC bioavailability for ambient stream water and leaf leachate DOC from laboratory experiments. We used these data to calculate and apply a correction factor to leaf leachate uptake velocity to estimate ambient stream water DOC uptake rates at the reach scale. Using this approach, we estimated a median ambient stream DOC vf of 0.26 mm min−1. Applying these DOC vf values (0.26, 1.11, 2.28, and 2.94 mm min−1) in a river network inverse model in seven watersheds revealed that our estimated ambient DOC vf value is plausible at the network scale and 27 to 45% of DOC input was removed. Applying the median measured simple compound or leachate vf in whole river networks would require unjustifiably high terrestrial DOC inputs to match observed DOC concentrations at the basin mouth. To improve the understanding and importance of DOC uptake in fluvial systems, we recommend using a multiscale approach coupling laboratory assays, with reach‐scale measurements, and modeling

A coupled terrestrial and aquatic biogeophysical model of the Upper Merrimack River watershed, New Hampshire, to inform ecosystem services evaluation and management under climate and land-cover change

Accurate quantification of ecosystem services (ES) at regional scales is increasingly important for making informed decisions in the face of environmental change. We linked terrestrial and aquatic ecosystem process models to simulate the spatial and temporal distribution of hydrological and water quality characteristics related to ecosystem services. The linked model integrates two existing models (a forest ecosystem model and a river network model) to establish consistent responses to changing drivers across climate, terrestrial, and aquatic domains. The linked model is spatially distributed, accounts for terrestrial–aquatic and upstream–downstream linkages, and operates on a daily time-step, all characteristics needed to understand regional responses. The model was applied to the diverse landscapes of the Upper Merrimack River watershed, New Hampshire, USA. Potential changes in future environmental functions were evaluated using statistically downscaled global climate model simulations (both a high and low emission scenario) coupled with scenarios of changing land cover (centralized vs. dispersed land development) for the time period of 1980–2099. Projections of climate, land cover, and water quality were translated into a suite of environmental indicators that represent conditions relevant to important ecosystem services and were designed to be readily understood by the public. Model projections show that climate will have a greater influence on future aquatic ecosystem services (flooding, drinking water, fish habitat, and nitrogen export) than plausible changes in land cover. Minimal changes in aquatic environmental indicators are predicted through 2050, after which the high emissions scenarios show intensifying impacts. The spatially distributed modeling approach indicates that heavily populated portions of the watershed will show the strongest responses. Management of land cover could attenuate some of the changes associated with climate change and should be considered in future planning for the region

Deliberative multiattribute valuation of ecosystem services across a range of regional land-use, socioeconomic, and climate scenarios for the upper Merrimack River watershed, New Hampshire, USA

We evaluate the relative desirability of alternative futures for the upper Merrimack River watershed in New Hampshire, USA based on the value of ecosystem services at the end of the 21st century as gauged by its present-day inhabitants. This evaluation is accomplished by integrating land-use and socioeconomic scenarios, downscaled climate projections, biogeophysical simulation models, and the results of a citizen-stakeholder deliberative multicriteria evaluation. We find that although there are some trade-offs between alternative plausible futures, for the most part, it can be expected that future inhabitants of the watershed will be most satisfied if land-use planning in the intervening years prioritizes water supply and flood protection as well as maintenance of existing farmland and forest cover. With respect to climate change, it is expected that future watershed inhabitants will be more negatively affected by the projected loss of snow cover than the anticipated increase in hot summer days. More important than the specific results for the upper Merrimack River watershed, this integrative assessment demonstrates the complex yet ultimately informative potential to link stakeholder engagement with scenario generation, ecosystem models, and multiattribute evaluation for informing regional-scale planning and decision making

Aquatic Effects of a Terrestrial Invader: A 35-year Natural Experiment Shows Russian Olive Alters Basal Resources and Community Metabolism of a Desert Spring Stream

Invasive species can have ecosystem effects that span aquatic-terrestrial boundaries. We investigated the effects of an invasive riparian tree, Russian olive (Eleagnus angustifolia), on the ecosystem function of a desert spring stream, Deep Creek, Idaho. Reaches of Deep Creek were studied in the early 1970s as a representative desert stream for the International Biological Program (IBP). The results of that study revealed that Deep Creek was an autotrophic (with primary production within the system meeting the energy demands for that community) stream which was exceptional because until then all streams were thought to be heterotrophic (relying on external terrestrial energy inputs to meet the energy demand in the system). One of the IBP study reaches has since been invaded by Russian olive, setting the stage for a unique before-after-invasion-control comparison. We repeated measures of community metabolism (which assesses primary production (P) and respiration (R)), allochthonous (leaf litter) inputs and standing stocks of aquatic primary producers in invaded and control reaches. The ratio of gross primary production to respiration (P/R) at the control site and was similar to estimates from 1971 and 1972 in the summer and fall. At the invaded site P/R only decreased slightly during the summer but decreased from 1.37 to 0.47 in the fall, a change that appears to be driven by both decreased photosynthesis and by increased respiration. This change in trophic status may be associated with the more than 10-fold increase in allochthonous input we observed at the invaded site during the fall. However, standing stock of aquatic primary producers showed no evidence of having been affected by the presence of Russian olive. We are constructing an annual carbon budget and quantitative food web to further assess the effects of this invasion on organisms and energy flows in this system. The progress of this and other invasive riparian tree species is rapid and widespread in the great basin therefore the responses observed at Deep Creek due to its altered riparian community will be applicable to many streams experiencing similar changes in this region

A Non-native riparian tree (Elaeagnus angustifolia) changes nutrient dynamics in streams

Russian olive (Elaeagnus angustifolia) is a non-nativeri parian tree that has become common and continues to rapidly spread throughout the western United States. Due to its dinitrogen (N2)-fixingability and proximity to streams, Russian olive has the potential to subsidize stream ecosystems with nitrogen (N), which may in turn alter nutrient processing in these systems. We tested these potential effects by comparing background N concentrations; nutrient limitation of biofilms; and uptake of ammonium (NH4-N), nitrate (NO3-N),and phosphate (PO4-P) in paired upstream-reference and downstream-invaded reaches in streams in southeastern Idaho and central Wyoming. We found that stream reaches invaded by Russian olive had higher organic N concentrations and exhibited reduced N limitation of biofilms compared to reference reaches. However, at low inorganic N back-ground concentrations, reaches invaded by Russian olive exhibited higher demand for both NH4-N and NO3-N compared to their paired reference reaches, suggesting these streams have the potential to retain the N subsidy from Russian olive N2 fixation and diminish its downstream export and effects. Our findings demonstrate the potential for a non-native riparian plant to significantly alter biogeochemical cycling in streams. Finally, we used our results to develop a conceptual model that describes predicted effects of Russian olive and other non-native riparian N2 fixers on in-stream N dynamics

Characterizing storm-event nitrate fluxes in a fifth order suburbanizing watershed using in situ sensors

Land use influences the distribution of nonpoint nitrogen (N) sources in urbanizing watersheds and storm events interact with these heterogeneous sources to expedite N transport to aquatic systems. In situ sensors provide high frequency and continuous measurements that may reflect storm-event N variability more accurately compared to grab samples. We deployed sensors from April to December 2011 in a suburbanizing watershed (479 km2) to characterize storm-event nitrate-N (NO3–N) and conductivity variability. NO3–N concentrations exhibited complex patterns both within and across storms and shifted from overall dilution (source limitation) before summer baseflows to subsequent periods of flushing (transport limitation). In contrast, conductivity generally diluted with increasing runoff. Despite diluted NO3–N concentrations, NO3–N fluxes consistently increased with flow. Sensor flux estimates for the entire deployment period were similar to estimates derived from weekly and monthly grab samples. However, significant differences in flux occurred at monthly time scales, which may have important implications for understanding impacts to temporally sensitive receiving waters. Evidence of both supply (nutrient-poor) and transport (nutrient-rich) limitation patterns during storms is consistent with watersheds undergoing land use transitions. Tracking shifts in these patterns could indicate N accumulation in developing watersheds and help identify mitigation opportunities prior to N impairment