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- were subjected to external peer-review and accessible through library services;
- were published after January 1, 2016;
- were authored or co-authored by individuals employed with one of the seven principal CRC member institutions or with other major Chesapeake Bay Program partners; and
- have direct relevance to the science and/or management of the Chesapeake Bay or its watershed.
Further details about the the submission process can be found here. Further details about the purpose and "value added" of the RCCP collection (relative to other on-line databases) can be found here.
Effects of inset floodplains and hyporheic exchange induced by in-stream structures on nitrate removal in a headwater stream
Stream restoration efforts in the United States are increasingly aimed towards water quality improvement, yet little process-based guidance exists to compare pollutant removals from different restoration techniques for variable site conditions. Excess nitrate (NO3−) is a frequent pollutant of concern due to eutrophication in downstream waterbodies such as the Chesapeake Bay. We used MIKE SHE to simulate hydraulics and NO3− removal in a 90 m restored reach of Stroubles Creek, a second-order stream in Blacksburg, Virginia. Site specific geomorphic, hydrologic, and hydraulic data were used to calibrate the model. We evaluated in-stream structures that induce hyporheic zone denitrification during baseflow and inset floodplains that remove NO3− during storm flows. We varied hydraulic conditions (winter baseflow, summer baseflow, storm flow), biogeochemical parameters (literature hyporheic zone denitrification rates and newly available inset floodplain removal rates) and boundary conditions (upstream NO3− concentration), sediment conditions (hydraulic conductivity), and stream restoration design parameters (inset floodplain length). Our results indicate that NO3− removal rates within the 90 m reach were minimal. Structure-induced hyporheic zone denitrification did not exceed 3.1% of mass flowing in from the upstream channel, was achieved only during favorable background groundwater hydraulic conditions (i.e. summer baseflow), and was transport-limited such that non-trivial removal rates were achieved only when the streambed hydraulic conductivity (K) was at least 10−4 m/s. Inset floodplain nitrogen removal was limited by floodplain residence time and NO3− removal rate, and did not exceed 1% of inflowing mass. Summing these removals for both restoration practices over the course of the year based on the frequency of storm and summer baseflow conditions yielded ∼2.1% annual removal. Achieving 30% NO3− removal required increasing the length of stream reach restored to 0.9 km–819 km (depending on hydraulic conductivity) and 3.8–46 km (depending on inset floodplain length and nitrogen removal rate) for in-stream structures during baseflow and inset floodplains during storm flow, respectively. In one of the first comparisons of process-based modeling to the Chesapeake Bay Program stream restoration guidance, we found that the guidance overestimated hyporheic NO3− removal for our modeled reach, but correctly estimated inset floodplain removal. Overall, our results indicate that in-stream structures and inset floodplains can improve water quality, but overall required level of effort may be high to achieve desired results.
Get PDFAssessing and Enhancing Environmental Sustainability – A Conceptual Review
While sustainability is an essential concept to ensure the future of humanity and the integrity of the resources and ecosystems on which we depend, identifying a comprehensive yet realistic way to assess and enhance sustainability may be one of the most difficult challenges of our time. We review the primary environmental sustainability assessment approaches, categorizing them as either being design-based or those that employ computational frameworks and/or indicators. We also briefly review approaches used for assessing economic and social sustainability because sustainability necessitates integrating environmental, economic, and social elements. We identify the collective limitations of the existing assessment approaches, showing that there is not a consistent definition of sustainability, that the approaches are generally not comprehensive and are subject to unintended consequences, that there is little to no connection between bottom-up and top-down approaches, and that the field of sustainability is largely fragmented, with a range of academic disciplines and professional organizations pursuing similar goals, but without much formal coordination. We conclude by emphasizing the need for a comprehensive definition of sustainability (that integrates environmental, economic, and social aspects) with a unified system-of-systems approach that is causal, modular, tiered, and scalable, as well as new educational and organizational structures to improve systems-level interdisciplinary integration.
Get PDFWhole-Catchment Manipulations of Internal and External Loading Reveal the Sensitivity of a Century-Old Reservoir to Hypoxia
Climate change is predicted to have widespread impacts on freshwater lake and reservoir nutrient budgets by altering both hypolimnetic hypoxia and runoff, which will in turn alter the magnitude of internal and external nutrient loads. To examine the effects of these potential climate scenarios on nitrogen (N) and phosphorus (P) budgets, we conducted a whole-catchment manipulation of hypolimnetic oxygen conditions and external loads to Falling Creek Reservoir (FCR), an old, eutrophic reservoir in a reforested catchment with a history of agricultural land use. Throughout 2 years of monitoring, internal N and P loading during hypoxic conditions dominated the hypolimnetic mass of nutrients in FCR, regardless of changes in external loading. FCR commonly functioned as a net sink of N and P, except during hypoxic conditions, when the reservoir was a net source of ammonium ( NH+4 ) to downstream. We observed extremely high nitrate–nitrite ( NO−3−NO−2 ), soluble reactive P (SRP), total nitrogen (TN), and total phosphorus (TP) retention rates, indicating that the reservoir served as a sink for greater than 70% of NO−3−NO−2 inputs and greater than 30% of SRP, TN, and TP inputs, on average. Our study is notable in the length of time since reforestation (>80 years) that a reservoir is still exhibiting high N and P internal loading during hypoxia, potentially as a result of the considerable store of accumulated nutrients in its sediment from historical agricultural runoff. Our whole-catchment manipulations highlight the importance of understanding how multiple aspects of global change, waterbody and catchment characteristics, and land use history will interact to alter nutrient budgets in the future.
Get PDFLinking the Abundance of Estuarine Fish and Crustaceans in Nearshore Waters to Shoreline Hardening and Land Cover
Human alteration of land cover (e.g., urban and agricultural land use) and shoreline hardening (e.g., bulkheading and rip rap revetment) are intensifying due to increasing human populations and sea level rise. Fishes and crustaceans that are ecologically and economically valuable to coastal systems may be affected by these changes, but direct links between these stressors and faunal populations have been elusive at large spatial scales. We examined nearshore abundance patterns of 15 common taxa across gradients of urban and agricultural land cover as well as wetland and hardened shoreline in tributary subestuaries of the Chesapeake Bay and Delaware Coastal Bays. We used a comprehensive landscape-scale study design that included 587 sites in 39 subestuaries. Our analyses indicate shoreline hardening has predominantly negative effects on estuarine fauna in water directly adjacent to the hardened shoreline and at the larger system-scale as cumulative hardened shoreline increased in the subestuary. In contrast, abundances of 12 of 15 species increased with the proportion of shoreline comprised of wetlands. Abundances of several species were also significantly related to watershed cropland cover, submerged aquatic vegetation, and total nitrogen, suggesting land-use-mediated effects on prey and refuge habitat. Specifically, abundances of four bottom-oriented species were negatively related to cropland cover, which is correlated with elevated nitrogen and reduced submerged and wetland vegetation in the receiving subestuary. These empirical relationships raise important considerations for conservation and management strategies in coastal environments.
Get PDFUsing Bayesian hierarchical models to better understand nitrate sources and sinks in agricultural watersheds
Export coefficient models (ECMs) are often used to predict nutrient sources and sinks in watersheds because ECMs can flexibly incorporate processes and have minimal data requirements. However, ECMs do not quantify uncertainties in model structure, parameters, or predictions; nor do they account for spatial and temporal variability in land characteristics, weather, and management practices. We applied Bayesian hierarchical methods to address these problems in ECMs used to predict nitrate concentration in streams. We compared four model formulations, a basic ECM and three models with additional terms to represent competing hypotheses about the sources of error in ECMs and about spatial and temporal variability of coefficients: an ADditive Error Model (ADEM), a SpatioTemporal Parameter Model (STPM), and a Dynamic Parameter Model (DPM). The DPM incorporates a first-order random walk to represent spatial correlation among parameters and a dynamic linear model to accommodate temporal correlation. We tested the modeling approach in a proof of concept using watershed characteristics and nitrate export measurements from watersheds in the Coastal Plain physiographic province of the Chesapeake Bay drainage. Among the four models, the DPM was the best--it had the lowest mean error, explained the most variability (R2 = 0.99), had the narrowest prediction intervals, and provided the most effective tradeoff between fit complexity (its deviance information criterion, DIC, was 45.6 units lower than any other model, indicating overwhelming support for the DPM). The superiority of the DPM supports its underlying hypothesis that the main source of error in ECMs is their failure to account for parameter variability rather than structural error. Analysis of the fitted DPM coefficients for cropland export and instream retention revealed some of the factors controlling nitrate concentration: cropland nitrate exports were positively related to stream flow and watershed average slope, while instream nitrate retention was positively correlated with nitrate concentration. By quantifying spatial and temporal variability in sources and sinks, the DPM provides new information to better target management actions to the most effective times and places. Given the wide use of ECMs as research and management tools, our approach can be broadly applied in other watersheds and to other materials.
Get PDFSeasonal Variation in Floodplain Biogeochemical Processing in a Restored Headwater Stream
Stream and river restoration activities have recently begun to emphasize the enhancement of biogeochemical processing within river networks through the restoration of river-floodplain connectivity. It is generally accepted that this practice removes pollutants such as nitrogen and phosphorus because the increased contact time of nutrient-rich floodwaters with reactive floodplain sediments. Our study examines this assumption in the floodplain of a recently restored, low-order stream through five seasonal experiments. During each experiment, a floodplain slough was artificially inundated for 3 h. Both the net flux of dissolved nutrients and nitrogen uptake rate were measured during each experiment. The slough was typically a source of dissolved phosphorus and dissolved organic matter, a sink of NO3–, and variable source/sink of ammonium. NO3– uptake rates were relatively high when compared to riverine uptake, especially during the spring and summer experiments. However, when scaled up to the entire 1 km restoration reach with a simple inundation model, less than 0.5–1.5% of the annual NO3– load would be removed because of the short duration of river-floodplain connectivity. These results suggest that restoring river-floodplain connectivity is not necessarily an appropriate best management practice for nutrient removal in low-order streams with legacy soil nutrients from past agricultural landuse.
Get PDFBalancing watershed nitrogen budgets: accounting for biogenic gases in streams
Denitrification is critical for removal of reactive nitrogen (Nr) from ecosystems. However, measuring realistic, scalable rates and understanding the role of denitrification and other dissimilatory processes in watershed nitrogen (N) budgets remains a significant challenge in biogeochemistry. In this study, we focused on the stream reach and network scale in three Mid-Atlantic coastal plain watersheds. We applied open channel methods to measure biogenic N 2 and N 2 O gas fluxes derived from both in-stream and terrestrial nitrogen processing. A large portion of biogenic N 2 flux through streams (33–100 %, mean = 74 %) was a result of groundwater delivery of biogenic N 2 with the remaining portion due to in- stream N 2 production. In contrast, N 2 O was largely produced in-stream, with groundwater delivery con- tributing on average 12 % of the total biogenic N 2 O flux. We scaled these measurements across one stream network and compared them to hydrologic Nr export and net anthropogenic N inputs (NANI) to a 4.8 km 2 watershed. The N budget revealed that, during the study period, the biogenic N 2 flux through streams was comparable to the difference between NANI and hydrologic Nr export (i.e. the ‘‘missing’’ N). This study provides a methodological and conceptual framework for incorporating terrestrial and in-stream derived biogenic N gas fluxes into watershed N budgets and supports the hypothesis that denitrification is the primary fate of NANI that is not exported in streamflow.
Get PDFComparison of effects of inset floodplains and hyporheic exchange induced by in stream structures on solute retention
The pollution of streams and rivers is a growing concern, and environmental guidance increasingly suggests stream restoration to improve water quality. Solute retention in off-channel storage zones, such as hyporheic zones and floodplains, is typically necessary for significant reaction to occur. Yet, the effects of two common restoration techniques, in-stream structures and inset floodplains, on solute retention have not been rigorously compared. We used MIKE SHE to model hydraulics and solute transport in the channel, on inset floodplains, and in structure-induced hyporheic zones of a third-order stream. We varied hydraulic conditions (winter base flow, summer base flow, and stormflow), geology (hydraulic conductivity), and stream restoration design parameters (inset floodplain length and presence of in-stream structures). The in-stream structures induced hyporheic exchange for approximately 20% of the year (during summer base flow) while inset floodplains were active for approximately 1% of the year (during stormflow). Flow onto inset floodplains and residence times in both the channel and on the floodplains increased nonlinearly with the fraction of bank with floodplains installed. The fraction of streamflow that flowed onto the inset floodplains was 1–3 orders of magnitude higher than that which flowed through the structure-induced hyporheic zone. Yet, residence times and mass storage in the hyporheic zone were 1–5 orders of magnitude larger than that on individual inset floodplains. In our modeling, neither in-stream structures nor inset floodplains had sufficient percent flow and residence times simultaneously to have a substantial impact on dissolved contaminants flowing downstream.
Get PDFVariation of hyporheic exchange potential among urban streams and implications for stream restoration
Bidirectional (hyporheic) exchange of water between stream channels and sediments benefits stream ecosystems, yet the effects of urbanization on such exchange are poorly understood. Exchange is controlled by a set of geomorphic parameters collectively defined as “hyporheic potential,” including sediment hydraulic conductivity (K), vertical undulations of the streambed (VC), and channel sinuosity (S). We measured these hyporheic potential metrics in 10 stream reaches with varying percent impervious surface (0–47 percent) in their contributing watersheds. We performed linear regression between hyporheic potential metrics and two metrics of urbanization: percent impervious in the watershed and average riparian forest buffer width. We found that most trends between hyporheic potential metrics and both urbanization metrics were noisy, with low r2. Furthermore, hyporheic potential varied as much among our streams as in non-urban studies, and K varied as much within stream reaches as between stream reaches. These results collectively indicate that the effects of urbanization on hyporheic potential in our streams were minimal and that urbanization may not heavily constrain hyporheic exchange. Nevertheless, we did find that S increased with impervious cover in the contributing watershed, consistent with higher urban storm flows leading to sinuosity adjustments to reduce channel slope. Urbanization may therefore enhance hyporheic potential under certain circumstances, and bears further study. Furthermore, there may be value in increasing the benefits of hyporheic exchange above existing levels through “hyporheic enhancement.” Because K varied more among streams than VC, which varied more than S, efforts to enhance hyporheic exchange may have greatest effect if they manipulate K.
Get PDFFirst Report of the Successful Operation of a Side Stream Supersaturation Hypolimnetic Oxygenation System in a Eutrophic, Shallow Reservoir
Controlling hypolimnetic hypoxia is a key goal of water quality management. Hypoxic conditions can trigger the release of reduced metals and nutrients from lake sediments, resulting in taste and odor problems as well as nuisance algal blooms. In deep lakes and reservoirs, hypolimnetic oxygenation has emerged as a viable solution for combating hypoxia. In shallow lakes, however, it is difficult to add oxygen into the hypolimnion efficiently, and a poorly designed hypolimnetic oxygenation system could potentially result in higher turbidity, weakened thermal stratification, and warming of the sediments. As a result, little is known about the viability of hypolimnetic oxygenation in shallow bodies of water. Here, we present the results from recent successful tests of side stream supersaturation (SSS), a type of hypolimnetic oxygenation system, in a shallow reservoir and compare it to previous side stream deployments. We investigated the sensitivity of Falling Creek Reservoir, a shallow (Zmax = 9.3 m) drinking water reservoir located in Vinton, Virginia, USA, to SSS operation. We found that the SSS system increased hypolimnetic dissolved oxygen concentrations at a rate of ∼1 mg/L/week without weakening stratification or warming the sediments. Moreover, the SSS system suppressed the release of reduced iron and manganese, and likely phosphorus, from the sediments. In summary, SSS systems hold great promise for controlling hypolimnetic oxygen conditions in shallow lakes and reservoirs.
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