VTechWorks Repository :: Browsing by Author "Hotchkiss, Erin R."

Browsing by Author "Hotchkiss, Erin R."

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Assessing Flow-driven Effects on Local and Downstream Water Quality in Central Appalachian Headwater Streams Influenced by Surface Coal Mining
Schoenholtz, Stephen H.; McLaughlin, Daniel L.; Entrekin, Sally A.; Hotchkiss, Erin R.; Timpano, Anthony J.; Cianciolo, Thomas R.; Word, Clayton S. (Virginia Tech. Powell River Project, 2020-10)
Building, applying, and communicating ecosystem understanding via freshwater forecasts over time and space
Woelmer, Whitney M. (Virginia Tech, 2023-09-05)
Accelerating rates of change in ecosystems globally heighten the need for improved predictions of future ecological conditions. Freshwater lakes and reservoirs, which provide numerous ecosystem services, are particularly threatened by global change stressors and have already exhibited substantial changes to their physical, chemical, and biological functioning. Thus, to provide useful predictive tools for managing freshwater resources in the face of global change, we must improve our ability to build, apply, and communicate understanding of lake and reservoir ecosystem dynamics. To address this, I first built ecosystem understanding by conducting multiple whole-ecosystem surveys to quantify the spatial and temporal variability of biogeochemistry in two reservoirs over a year. We found that temporal heterogeneity was higher than spatial heterogeneity for most biogeochemical variables, with the stream-reservoir interface as a consistent hotspot of biogeochemical processing. Second, I applied ecosystem understanding by producing ecological forecasts of physical (water temperature), chemical (dissolved oxygen), and biological (chlorophyll-a) variables across three waterbodies using diverse modeling methods. I developed daily, weekly, and fortnightly forecasts of chlorophyll-a at two drinking water reservoirs using a Bayesian linear model, and found process uncertainty dominated total forecast uncertainty. Additionally, I produced forecasts of water temperature and dissolved oxygen in an oligotrophic lake using a hydrodynamic-ecosystem model and found that water temperature was more predictable than oxygen despite variable performance over depth and between years. Across these two forecasting studies, forecast skill relative to a null model varied among water quality metrics: water temperature forecasts outperformed the null model up to 11 days ahead, oxygen forecasts outperformed the null model up to 2 days ahead, and chlorophyll-a forecasts outperformed the null model up to 14 days ahead. Third, to communicate forecasts for decision-making, I developed an educational module for undergraduate ecology students which taught important concepts on visualization and decision science. Following completion of the module, students' ability to identify methods for uncertainty communication increased significantly, as well as their understanding of the benefits of ecological forecasting. Overall, my dissertation provides insight into how reservoirs function in global biogeochemical cycles, the predictability of multiple water quality variables, and deepens our understanding of how to communicate ecosystem science for improved management and protection of ecosystems.
Can Common Pool Resource Theory Catalyze Stakeholder-Driven Solutions to the Freshwater Salinization Syndrome?
Grant, Stanley B.; Rippy, Megan A.; Birkland, Thomas A.; Schenk, Todd; Rowles, Kristin; Misra, Shalini; Aminpour, Payam; Kaushal, Sujay; Vikesland, Peter J.; Berglund, Emily; Gomez-Velez, Jesus D.; Hotchkiss, Erin R.; Perez, Gabriel; Zhang, Harry X.; Armstrong, Kingston; Bhide, Shantanu V.; Krauss, Lauren; Maas, Carly; Mendoza, Kent; Shipman, Caitlin; Zhang, Yadong; Zhong, Yinman (American Chemical Society, 2022-09-14)
Freshwater salinity is rising across many regions of the United States as well as globally, a phenomenon called the freshwater salinization syndrome (FSS). The FSS mobilizes organic carbon, nutrients, heavy metals, and other contaminants sequestered in soils and freshwater sediments, alters the structures and functions of soils, streams, and riparian ecosystems, threatens drinking water supplies, and undermines progress toward many of the United Nations Sustainable Development Goals. There is an urgent need to leverage the current understanding of salinization's causes and consequences?in partnership with engineers, social scientists, policymakers, and other stakeholders?into locally tailored approaches for balancing our nation's salt budget. In this feature, we propose that the FSS can be understood as a common pool resource problem and explore Nobel Laureate Elinor Ostrom's social-ecological systems framework as an approach for identifying the conditions under which local actors may work collectively to manage the FSS in the absence of top-down regulatory controls. We adopt as a case study rising sodium concentrations in the Occoquan Reservoir, a critical water supply for up to one million residents in Northern Virginia (USA), to illustrate emerging impacts, underlying causes, possible solutions, and critical research needs.
Carbon dioxide stimulates lake primary production
Hamdan, Mohammed; Byström, Pär; Hotchkiss, Erin R.; Al-Haidarey, Mohammed J.; Ask, Jenny; Karlsson, Jan (Springer Nature, 2018-07-18)
Gross primary production (GPP) is a fundamental ecosystem process that sequesters carbon dioxide (CO2) and forms the resource base for higher trophic levels. Still, the relative contribution of different controls on GPP at the whole-ecosystem scale is far from resolved. Here we show, by manipulating CO2 concentrations in large-scale experimental pond ecosystems, that CO2 availability is a key driver of whole-ecosystem GPP. This result suggests we need to reformulate past conceptual models describing controls of lake ecosystem productivity and include our findings when developing models used to predict future lake ecosystem responses to environmental change.
Dissolved Organic Matter Sources from Soil Horizons with Varying Hydrology and Distance from Wetland Edge
Wardinski, Katherine Mary (Virginia Tech, 2021-09-03)
Understanding hydrologic controls on carbon accumulation and export within geographically isolated wetlands (GIW) has implications for the success of wetland restoration efforts intended to produce carbon sinks. However, little is known about how hydrologic connectivity along the aquatic-terrestrial interface in GIW catchments influences carbon dynamics, particularly regarding dissolved organic matter (DOM) transport and transformation. The organic matter (carbon) that accumulates in wetland soils may be released into water, generating DOM. DOM is mobile and reactive, making it influential to aquatic metabolism and water quality. To understand the role of different soil horizons as potential sources of DOM, extractable soil organic matter (ESOM) was measured in soil horizons collected from upland to wetland transects at four Delmarva Bay GIWs on the Delmarva Peninsula in the eastern United States. ESOM quantity and quality were analyzed to provide insights to organic matter sources and chemical characteristics. Findings demonstrated that ESOM in shallow organic horizons had increased aromaticity, higher molecular weight, and plant-like signatures. ESOM from deeper, mineral horizons had lower aromaticity, lower molecular weights, and protein-like signatures. Organic soil horizons had the largest quantities of ESOM, and ESOM decreased with increasing soil depth. ESOM quantities also generally decreased from the upland to the wetland, suggesting that continuous soil saturation leads to a decreased quantity of ESOM. Despite wetland soils having lower ESOM, these horizons are thicker and continuously hydrologically connected to wetland surface water, leading to wetland soils representing the largest potential source of DOM to the Delmarva Bay wetland system. Knowledge of which soil horizons are most biogeochemically significant for DOM transport in Delmarva and other GIW systems will become increasingly important as climate change is expected to alter the hydrologic connectivity of wetland soils to the surface water-groundwater continuum and as wetlands are more frequently designed for carbon sequestration.
The drivers of freshwater reservoir biogeochemical cycling and greenhouse gas emissions in a changing world
McClure, Ryan Paul (Virginia Tech, 2020-09-29)
Freshwater reservoirs store, process, and emit to the atmosphere large quantities of carbon (C). Despite the important role of reservoirs in the global carbon cycle, it remains unknown how human activities are altering their carbon cycling. Climate change and land use are resulting in lower dissolved oxygen (DO) concentrations in freshwater ecosystems, yet more frequent, powerful storms are occurring that temporarily increase DO availability. The net effect of these opposing forces results in anoxia (DO < 0.5 mg L-1) punctuated by short-term increases in DO. The availability of DO controls alternate redox reactions in freshwaters, thereby determining the rate and end products of organic C mineralization, which include two greenhouse gases, carbon dioxide (CO2) and methane (CH4). I performed ecosystem-level DO manipulations and evaluated how changing DO conditions affected redox reactions and the production and emission of CO2 and CH4. I also explored how the magnitude and drivers of CH4 emissions changed spatio-temporarily in a eutrophic reservoir using time series models. Finally, I used a coupled data-modeling approach to forecast future emissions of CH4 from the same reservoir. I found that the depletion of DO results in the rapid onset of alternate redox reactions in freshwater reservoirs for organic C mineralization and greater production of CH4. When the anoxia occurred in the water column (vs. at the sediments), diffusive CO2 and CH4 efflux phenology was affected, and resulted in degassing occurring during storms before fall turnover. I observed that the magnitude of CH4 emissions varied along a longitudinal gradient of a small reservoir and that the environmental drivers of ebullition and diffusion can change substantially both over space (within one hundred meters) and time (within a few weeks). Finally, I developed a forecasting workflow that successfully predicted future CH4 ebullition rates during one summer season. My research provides insight to how changing DO conditions will alter redox reactions in the water column and greenhouse gas emissions, as well as provides a new technique for improving future predictions of CH4 emissions from freshwater reservoirs. Althogether, this work improves our understanding of how freshwater lake and reservoir carbon cycling will change in the future.
The Effects of Biochar and Reactive Iron Additions on Soil Carbon and Nitrogen Retention
Conner, Jared P. (Virginia Tech, 2022-06-02)
Soil organic matter (SOM) is a critical biogeochemical pool that can be managed as part of global efforts to conserve nutrients and enhance carbon (C) sequestration. But reliably increasing SOM has proven difficult because most of the organic matter that enters soil as plant litter and organic amendments (i.e., compost, manure) is susceptible to decomposition by soil microorganisms and eventually is lost to the environment as greenhouse gases and non-point source pollution. Many soils lack the physical and/or chemical properties that enable some human-modified soils (e.g., terra preta soils in the Amazon Basin) to stabilize and retain C and nutrients in SOM while maintaining relatively high levels of productivity compared to surrounding natural soils that formed under similar conditions. I hypothesized that two of the major stabilizers of organic matter common to terra preta soils of the Amazon basin – black carbon (biochar) and poorly crystalline, reactive iron (Fe) minerals – could be applied to a fine-textured soil from Southwest Virginia to improve the accumulation and retention of C and nitrogen (N). I used a field experiment to compare the effects of three types of locally-produced biochars applied with and without an organic N fertilizer (blood meal) on soil C and N availability. I then used an incubation experiment featuring the soils from the aforementioned field experiment to examine the effects of applying Fe2+ -treated manure effluent on the retention of C and N in unamended and hardwood biochar-amended soils. I found that biochar adsorbed inorganic N in all cases, while providing a reliable, stable increase in SOM due to its recalcitrant nature. However, the manure effluent used in the incubation experiment stimulated the decomposition of mineral-associated organic matter (MAOM), with the addition of Fe2+ to the manure mitigating this apparent positive priming effect and the presence of biochar actually reversing this effect and promoting an increase in MAOM following manure application to biochar-amended soil. Overall, biochar stimulated the retention of N by decreasing the leachable inorganic N in the soil and enhanced soil C stocks. Additionally, biochar applications had the added benefit of promoting the accumulation of manure in soil as stable, microbially-processed MAOM, while co-applying Fe2+ with manure only served to inhibit the priming of native soil C.
Effects of Freshwater Salinization and Associated Base Cations on Bacterial Ecology and Water Quality
DeVilbiss, Stephen Edward (Virginia Tech, 2021-01-05)
Anthropogenic freshwater salinization, which is caused by numerous human activities including agriculture, urbanization, and deicing, impacts an estimated 37% of the contiguous drainage area in the United States. High salt concentrations in brackish and marine environments (~1,500 – 60,000 µS cm-1) influence aquatic bacteria. Less is known about the effects of freshwater salt concentrations (≤ 1,500 µS cm-1) on bacterial ecology, despite the pervasiveness of freshwater salinization. Bacteria perform many fundamental ecosystem processes (e.g. biogeochemical cycling) and serve as indicators of human health risk from exposure to waterborne pathogens. Thus, to understand how salt pollution affects freshwater ecosystems, there is a critical need to understand how freshwater salinization is impacting bacterial ecology. Using a series of controlled mesocosm experiments, my objectives were to determine how (1) survival of fecal indicator bacteria (FIB), (2) the diversity of native freshwater bacterial communities, and (3) bacterial respiration and nutrient uptake rates responded across a freshwater salinity gradient of different salt profiles. Survival rates (t90) of Escherichia coli, the EPA recommended freshwater FIB, increased by over 200% as salinity increased from 30 to 1,500 µS cm-1. Survival rates were also significantly higher in water with elevated Mg2+ relative to other base cations, suggesting that different salt sources and ion profiles can have varied effects in FIB survival. Thus, freshwater salinization could cause accumulating concentrations of FIB even without increased loading, increasing the risk of bacterial impairment. Diversity of native bacterial communities also varied across a freshwater salinity gradient, with a general increase in species richness as salinity reached 1,500 µS cm-1. Community variability (β-diversity) was greatest at intermediate salinities of 125 – 350 µS cm-1 and decreased towards the upper and lower extremes (30 and 1,500 µS cm-1, respectively). These diversity patterns suggest that osmotic stress is an environmental filter, but filtering strength is lowest at intermediate salinities causing a change from more deterministic to more stochastic assembly mechanisms. Different salt types also produced distinct bacterial community structures. Lastly, bacterial respiration doubled as salinity increased to 350 – 800 µS cm-1, revealing a subsidy-stress response of bacterial respiration across a freshwater salinity gradient. Corresponding changes in nitrogen and phosphorus uptake increased N:P ratios in ambient water, especially in mesocosms with elevated Ca2+, which could affect nutrient limitation in salinized streams enriched with Ca2+. Bacterial community structure based on Bray-Curtis dissimilarity was not correlated to pairwise changes in respiration rates but was linked to net nitrogen and phosphorus uptake after five days. Collectively, these results establish that freshwater salinization alters bacterial ecology at the individual population, whole community, and ecosystem process scales. Further, different salt types (e.g., CaCl2, MgCl2, NaCl, KCl, sea salt) had varying effects on bacteria at all levels and should be considered when predicting the effects of salinization on freshwater ecosystems. Developing more nuanced salt management plans that consider not only amount, but different types, of salts in freshwaters could help improve our ability to predict human health risk from waterborne pathogens and mitigate/ reduce salinity-induced impacts to freshwater ecosystem processes and services.
Experimental Evaluation of Three Backward Transit Time Distributions (bTTD) for Solute Storage and Release During Hyporheic Exchange
Werber, Nelson Norris (Virginia Tech, 2024-01-04)
Hyporheic exchange in streams supports many important ecosystem services but can also contribute to legacy pollution, by trapping less reactive contaminants in streambed sediments that are then slowly released back to the stream over time. In this study we evaluated three different analytical representations of the backward transit time distribution (bTTD) of water leaving the hyporheic zone, corresponding to different mechanisms for how water and solutes in hyporheic zone storage are sampled for outflow: (1) uniform sampling (exponential bTTD), plug-flow sampling (Dirac delta bTTD), and preferential sampling of either young or old water (Gamma bTTD). Using the Method of Moments, these three bTTDs were tested against data from 47 previously published hyporheic exchange experiments conducted in laboratory flumes over a range of flow conditions, sediment grain sizes, and bedform sizes and types. Based on measures of model fit and parsimony (AICc), in all 47 experiments hyporheic exchange was best represented by either the Gamma or exponential distributions. Further, values for key process variables, including hyporheic exchange flux and the Gamma distribution's shape parameter are correlated with readily measured field variables, including mean grain diameter of the streambed, streambed roughness, and mean stream discharge and velocity. This work advances understanding of hyporheic exchange processes and their representation in models of pollutant fate and transport in streams.
An experimental test of climate change effects in northern lakes: Increasing allochthonous organic matter and warming alters autumn primary production
Hamdan, Mohammed; Byström, Pär; Hotchkiss, Erin R.; Al-Haidarey, Mohammed J.; Karlsson, Jan (2021-01-08)
Climate changes are predicted to influence gross primary production (GPP) of lakes directly through warming and indirectly through increased loads of allochthonous coloured dissolved organic matter (cDOM) from surrounding landscapes. However, few studies have investigated this combined effect. Here we tested the effects of warming (elevated 3celcius) and cDOM input (three levels of humic river water addition) on GPP in autumn (2 months including open water and ice-covered periods) in experimental pond ecosystems. The cDOM input decreased whole-ecosystem GPP at natural temperature conditions mainly as a result of lower benthic GPP not fully counteracted by an increase in pelagic GPP, while warming increased whole-ecosystem GPP due to a positive response of mainly pelagic GPP at all levels of cDOM input. Warming delayed autumn ice cover formation by 2 weeks but did not affect light availability in the water column compared to ambient ice-covered treatments. Gross primary production during this period was still affected by warming and cDOM. The results stress the importance of accounting for multiple climate drivers and habitats when predicting lake GPP responses to climate change. We conclude that climate change may shift whole-ecosystem GPP through different responses of habitat-specific GPP to increasing cDOM inputs and warming.
The Flow Regime of Function: Influence of flow changes on biogeochemical processes in streams
O'Donnell, Brynn Marie (Virginia Tech, 2019-07-02)
Streams are ecosystems organized by disturbance. One of the most frequent disturbances within a stream is elevated flow. Elevated flow can both stimulate ecosystem processes and impede them. Consequently, flow plays a critical role in shifting the dominant stream function between biological transformation and physical transportation of materials. To garner further insight into the complex interactions of stream function and flow, I assessed the influence of elevated flow and flow disturbances on stream metabolism. To do so, I analyzed five years of dissolved oxygen data from an urban- and agriculturallyinfluenced stream to estimate metabolism. Stream metabolism is estimated from the production (gross primary production; GPP) and consumption (ecosystem respiration; ER) of dissolved oxygen. With these data, I evaluated how low and elevated flows differentially impact water quality (e.g., turbidity, conductivity) and metabolism using segmented metabolism- and concentration- discharge analyses. I found that GPP declined at varying rates across discharge, and ER decreased at lower flows but became constant at higher flows. Net ecosystem production (NEP; = GPP - ER) reflected the divergence of GPP and ER and was unchanging at lower flows, but declined at higher discharge. These C-Q patterns can consequently influence or be influenced by changes in metabolism. I coupled metabolism-Q and C-Q trends to examine linked flow-induced changes to physicochemical parameters and metabolism. Parameters related to metabolism (e.g., turbidity and GPP, pH and NEP) frequently followed coupled trends. To investigate metabolic recovery dynamics (i.e., resistance and resilience) following flow disturbances, I analyzed metabolic responses to 15 isolated flow events and identified the antecedent conditions or disturbance characteristics that most contributed to recovery dynamics. ER was both more resistant and resilient than GPP. GPP took longer to recover (1 to >9 days, mean = 2.5) than ER (1 to 2 days, mean = 1.1). ER resistance was strongly correlated with the intensity of the flow event, whereas GPP was not, suggesting that GPP responds similarly to flow disturbances, regardless of the magnitude of flow event. Flow may be the most frequent disturbance experienced by streams. However, streams are exposed to a multitude of other disturbances; here I also highlight how anthropogenic alterations to streams – namely, burying a stream underground – can change biogeochemical function. This thesis proposes novel frameworks to explore the nexus of flow, anthropogenic disturbances, and stream function, and thereby to further our understanding of the complex relationship between streams and disturbances.
Headwater stream network connectivity: biogeochemical consequences and carbon fate
Bretz, Kristen Alexandra (Virginia Tech, 2023-05-04)
Headwaters may be small relative to other aquatic ecosystems, but they are neither simple nor static environments. Heterogeneous stream corridors constitute the majority of river network length and regulate cycling of carbon and oxygen as they expand and contract their connections across the landscape. Though headwater streams integrate many biogeochemical signals from the watersheds they drain and provide important ecosystem services, their diverse habitats and dynamic changes in wet length have been under- examined compared to dendritic, perennial streams. This oversight complicates efforts to identify biogeochemical patterns at larger scales. This dissertation sets out to expand our knowledge of stream biogeochemical responses to variable connections both within the channel and the wider stream corridor. First, I investigated how the presence and arrangement of different habitat patches in the stream corridor affected overall emissions of carbon dioxide (CO2) and methane (CH4) from sub-watersheds of a forested mountain stream network. To do this I measured concentration and flux of both gasses along and around 4 streams, including dry reaches and adjacent vernal pools as well as flowing water. I found that emissions were highly variable over space and time; in particular, the presence of a vernal pool enhanced total carbon emissions from the stream corridor. Next, to quantify carbon cycling and export from a non-perennial headwater stream, I monitored concentrations of CO2 and dissolved organic carbon (DOC) at the stream outlet. I found that CO2 concentration had a negative relationship with stream discharge, and that exports of both CO2 and DOC were driven by storms reconnecting isolated surface water reaches. I also found that carbon biogeochemistry of intermediate flow states were unique from driest and highest-flow conditions. Finally, to explore how isolated pools in the stream channel respond to flow decrease and cessation, I measured dissolved oxygen (DO) as well as CO2 and CH4 from persistent pools of two non- perennial streams throughout an unusually dry summer and fall. I found that hypoxia was common in all isolated pools, but swings in DO were not consistent between pools even of the same stream. In using diel changes in DO to estimate metabolism, I also found that ecosystem respiration varied by stream, but gross primary production was more driven by stream surface water connectivity. Climate change is inducing many new patterns in stream hydrology with critical implications for biogeochemical activity, from reducing durations of connectivity to causing stronger storms. Improving our understanding of how surface water and landscape connectivity both influence the movement of carbon within and through streams is essential to resolving questions about the contributions of freshwaters to the global carbon cycle.
High rates of daytime river metabolism are an underestimated component of carbon cycling
Tromboni, Flavia; Hotchkiss, Erin R.; Schechner, Anne E.; Dodds, Walter K.; Poulson, Simon R.; Chandra, Sudeep (SpringerNature, 2022-11)
Diel variations in the isotopic composition of dissolved oxygen in river water reveal high rates of gross primary production and respiration rates in 14 rivers across three biomes, suggesting that the microbial loop in rivers may cycle carbon more rapidly than thought. River metabolism and, thus, carbon cycling are governed by gross primary production and ecosystem respiration. Traditionally river metabolism is derived from diel dissolved oxygen concentrations, which cannot resolve diel changes in ecosystem respiration. Here, we compare river metabolism derived from oxygen concentrations with estimates from stable oxygen isotope signatures (delta O-18(2)) from 14 sites in rivers across three biomes using Bayesian inverse modeling. We find isotopically derived ecosystem respiration was greater in the day than night for all rivers (maximum change of 113 g O-2 m(-2) d(-1), minimum of 1 g O-2 m(-2) d(-1)). Temperature (20 degrees C) normalized rates of ecosystem respiration and gross primary production were 1.1 to 87 and 1.5 to 22-fold higher when derived from oxygen isotope data compared to concentration data. Through accounting for diel variation in ecosystem respiration, our isotopically-derived rates suggest that ecosystem respiration and microbial carbon cycling in rivers is more rapid than predicted by traditional methods.
The Influence of Reduced Forest Cover and Dissolved Oxygen on the Viability of Eggs from Eastern Hellbenders (Cryptobranchus alleganiensis)
Funkhouser, Holly Ann (Virginia Tech, 2024-11-18)
Riparian deforestation is a significant threat to freshwater riverine ecosystems and sensitive fauna that depend on clean water. Sensitive aquatic species are vulnerable to the destruction of riparian forest cover which diminishes protection from pollutants, sedimentation, and solar radiation, while also depleting dissolved oxygen. In this thesis, I explore the influence of degraded riparian forest cover and its effect on dissolved oxygen on the embryonic viability of a sensitive freshwater habitat specialist, the Eastern hellbender. Hellbenders are a large-bodied, long-lived amphibian that inhabits fast flowing, cold mountain streams in the eastern United States. Over the last several decades, hellbender populations have experienced declines that are associated with low riparian forest cover, a geriatric population age structure, and high rates of nest failure. Adult male hellbenders normally provide extensive paternal care to embryos and larvae over an 8-month period. However, researchers have recently discovered that in degraded populations, hellbender nests are failing due to whole-clutch filial cannibalism by adult males. The underlying mechanism that triggers males to eat their young remains unknown, but one possibility is that eggs are not developing properly and as a result the attending male ceases to provide care. However, the embryonic viability of clutches developing in habitats with low riparian forest cover is unknown. Given the limited research on hellbender embryonic viability, I first sought to examine whether embryo viability is associated with a forest cover gradient. To accomplish this, I inherited two years of laboratory and field data, and I conducted a final third year of data collection for the study. Over these three years of data collection, I simultaneously evaluated embryo viability in a controlled captive rearing system while classifying nest failure due to whole-clutch cannibalism of sibling embryos in the field. I found significantly lower hellbender embryo viability, faster hatching times, and higher rates of underdeveloped hatchlings in hellbender populations with degraded riparian forest cover. However, I found no relationship between embryonic viability and whole-clutch filial cannibalism. I concluded that elevated specific conductance of water (i.e., dissolved ions) associated with loss of forest cover may affect the embryonic development of hellbenders, but this hypothesis has yet to be tested. To further explore the impact of degraded riparian forest cover on hellbender embryonic viability, I designed a study to evaluate the influence of depleted dissolved oxygen on embryonic development. To accomplish this, I designed and implemented a dissolved oxygen manipulation system to rear sibling embryos in high, medium, and low dissolved oxygen concentrations. I found that hellbender embryos are vulnerable to relatively modest reductions in dissolved oxygen (i.e., 5 mg/L), comparable to those found to affect embryos of some of the most sensitive species examined to date. Nests reared in low dissolved oxygen had a lower percentage of viable hatchlings, lower hatching success, higher rates of underdeveloped hatchlings, and smaller morphometrics compared to those reared at higher dissolved oxygen concentrations. I concluded that hellbenders may be particularly susceptible to reductions in dissolved oxygen because of their high degree of specialization for well oxygenated streams, and I suggest prioritizing additional research on dissolved oxygen to advance hellbender conservation. Taken together, my research established a foundation of knowledge on the sensitivities of hellbender embryos to degraded water quality caused by low riparian forest cover. Further, my work underscored the importance of riparian forest conservation for hellbender populations and for other freshwater biodiversity. Protection of riparian forest will also be critical to build resilience in streams against the existential threat of climate change.
Integrating Ecosystem Patch Contributions to Stream Corridor Carbon Dioxide and Methane Fluxes
Bretz, Kristen A.; Jackson, Alexis R.; Rahman, Sumaiya; Monroe, Jonathon; Hotchkiss, Erin R. (American Geophysical Union, 2021-09)
The heterogeneity of carbon dioxide (CO₂) and methane (CH₄) sources within and across watersheds presents a challenge to understanding the contributions of different ecosystem patch types to stream corridor and watershed carbon cycling. Changing hydrologic connections between corridor patches (e.g., streams, vernal pools, hillslopes) can influence stream corridor greenhouse gas emissions, but the spatiotemporal dynamics of emissions within and among corridor patches are not wellquantified. To identify patterns and sources of carbon emissions across stream corridors, we measured gas concentrations and fluxes over two summers at Coweeta Hydrologic Laboratory, NC. We sampled CO₂ and CH₄ along four stream channels (including flowing and dry reaches), adjacent vernal pools, and riparian hillslopes. Stream CO₂ and CH₄ emissions were spatially heterogeneous. All streams were sources of CO₂ to the atmosphere (median = 97.2 mmol m⁻²d⁻¹) but were sources or sinks of CH₄ depending on location (−0.19 to 4.57 mmol m⁻²d⁻¹). CO₂ emissions were lower during the drier of two sampling years but were stable from month to month in the drier summer. CO₂ and CH₄ emissions also varied by both corridor and patch type; the presence of a vernal pool in the corridor had the strongest impact on emissions. Vernal pool patches emitted more CO₂ and CH₄ (246 and 1.95 mmol m⁻²d⁻¹, respectively) than their adjacent streams. High resolution sampling of carbon fluxes from patches within and among stream corridors improves our understanding of the connections between terrestrial, riparian, and aquatic zones in a watershed and their contributions to overall catchment carbon emissions.
Integrative Science and Solutions for Freshwater Systems Concept Paper - A plan to build a signature-strength in Freshwater Systems
Benham, Brian L.; Czuba, Jonathan A.; Hession, W. Cully; Krometis, Leigh-Anne H.; Scott, Durelle T.; Stephenson, Stephen Kurt; Thompson, Theresa M.; Bork, Dean R.; Hester, Erich T.; Polys, Nicholas F.; Ivory, James Dee; Angermeier, Paul L.; Castello, Leandro; Dolloff, C. Andrew; Emrick, Verl III; Jones, Jess W.; McLaughlin, Daniel L.; Meyers, R. B.; Orth, Donald J.; Schoenholtz, Stephen H.; Snodgrass, Joel W.; Hotchkiss, Erin R.; Smith, Eric P. (Virginia Tech, 2017-05-15)
Virginia Tech is poised to become a global leader in the pursuit and application of new knowledge to inform management and restoration of waterbodies and their watersheds. Despite our notable strengths in specific disciplines, we have not yet facilitated nor nurtured an interdisciplinary program whereby a holistic perspective of freshwater systems can permeate into VT-shaped students and bridge the gaps among water-relevant biophysical, social sciences, and the arts. We know of no other major research university with a signature-strength in integrated freshwater systems science...
Moving beyond the stream reach: Assessing how confluences alter ecosystem function and water quality in freshwater networks
Plont, Stephen James (Virginia Tech, 2023-05-22)
In freshwater networks, the sources, movement, and cycling of carbon and nutrients are shaped both by in-stream processes and the surrounding landscape. Streams receive and transport materials from upstream and terrestrial sources that support in-stream ecosystem processes and regulate downstream water quality. Understanding how these processes within a stream alter downstream carbon and nutrient fluxes is needed to assess the functional role of lotic ecosystems on the landscape. Further, predictions of how materials cycle and move throughout freshwater networks are derived from measurements at the stream reach scale which deliberately avoid complex geomorphology such as stream confluences. As a result, the impact of stream confluences on in-stream ecosystem processes and the fate of carbon and nutrients in freshwater networks has been overlooked. In this dissertation, I seek to address the following questions: (1) How are coupled carbon and nitrogen cycles altered by land use? (2) To what extent can rates of in-stream organic carbon removal inform our understanding of the role of streams in landscape carbon fluxes? (3) How are carbon metabolism and nutrient uptake altered downstream of a stream confluence? (4) How do confluences alter the transport and fate of carbon and nutrients within a freshwater network? In Chapter 2, I showed that the fate of organic carbon and nitrate are similar in headwater streams across the United States. Organic carbon travels longer distances before being respired in agricultural and urban streams compared to reference streams, suggesting that human modifications to landscapes impact carbon cycling and transport in streams. In Chapter 3, I demonstrated how rates of in-stream organic carbon removal can be used to quantify terrestrial-aquatic linkages and showed that laboratory bioassays systematically underestimate ecosystem organic carbon fluxes compared to whole-stream metabolism measurements. In Chapter 4, I conducted whole-ecosystem manipulation experiments to assess how ecosystem processes are altered by a confluence. I found that carbon metabolism and phosphorus uptake are suppressed downstream of a confluence and that rates of organic carbon uptake are spatially variable throughout a confluence mixing zone. In Chapter 5, I examined potential reach-scale and watershed-scale drivers to explain patterns of organic matter and nutrient chemistry downstream of confluences throughout a stream network. Reaches downstream of confluences were geomorphically and biogeochemically distinct from upstream reaches, and differences in upstream and tributary reach chemistry or drainage area did not explain patterns of biologically reactive parameters at confluences. My dissertation highlights the importance of in-stream ecosystem processes in driving the cycling and downstream fate of carbon and nutrients. I show how rates of whole-stream carbon metabolism can be used to better constrain terrestrial-aquatic organic carbon fluxes. I investigate the potentially disproportionate role of ecosystem interfaces, namely stream confluences, in determining the cycling and fate of carbon and nutrients in freshwater networks. This work challenges assumptions around controls over water quality in freshwater networks and asserts that by ignoring (1) contributions of all in-stream processes to whole-ecosystem function and (2) how confluences alter those processes, we risk misrepresenting the role of running waters in determining the fluxes and fate of carbon and nutrients from the reach- to the network-scale.
Multi-scale Studies of Microbial Mats and Biocrusts: Integrating Remote Sensing with Field Investigations in Antarctica's McMurdo Dry Valleys
Power, Sarah Nicole (Virginia Tech, 2024-09-06)
Primary productivity is a fundamental ecosystem process driven by vascular plants in most terrestrial ecosystems and by microbes in more extreme ecosystems. In dense associations, microbial organisms can form visually conspicuous layers on sediment, soil, and rock surfaces, called microbial mats and biological soil crusts (i.e., biocrusts). Both microbial mats and biocrusts consist of cyanobacteria, moss, diatoms, and green algae, and also support diverse heterotrophic taxa. These communities exist in harsh environments worldwide such as hypersaline environments, tundra ecosystems, and hot and cold deserts where they are foundational taxa, providing most of the primary production and nitrogen fixation, as well as promoting cohesion and stability to soil surfaces. In the McMurdo Dry Valleys of Antarctica, microbial mats are the main source of fixed carbon in lentic and lotic environments, but their contribution to soil carbon and nitrogen cycling has not been systematically examined. In my dissertation, I investigated the relationships between microbial mats and the soil environments in which they occur. Using a combination of field surveys, soil analyses, and remote sensing, my objectives were to examine the influence of microbial mats and biocrusts on underlying soils and model the main drivers of their distribution and abundance. In Chapter 2, I investigated the relationships between underlying soil chemistry and microbial mat distribution, composition, and function in the Taylor Valley, finding that microbial mats enrich underlying soils, contributing to soil organic carbon and nitrogen. In Chapter 3, I assessed the spectral detectability of patchy biocrusts using multispectral satellite imagery to examine the environments in which biocrusts occur, finding that spectral unmixing of satellite imagery can successfully detect the presence of biocrust and its association with seasonal snow patches. As a direct continuation, in Chapter 4, I created a habitat suitability model using machine learning algorithms to determine the distribution and abundance of biocrusts in the Lake Fryxell basin. I found that biocrusts contribute a significant amount of carbon to the surface soil in the Lake Fryxell basin, with biocrust presence primarily driven by snow frequency, moisture content, and salinity. This dissertation contributes to ongoing questions about the sources of energy fueling soil food webs and regional carbon balance in the Taylor Valley, and how we can use remote sensing techniques for researching these critical soil communities in the dynamic Antarctic landscape.
Mycoplasmal conjunctivitis and the behavior of wild house finches (Carpodacus mexicanus) at bird feeders
Hotchkiss, Erin R.; Davis, A. K.; Cherry, J. J.; Altizer, S. (2005)
Parasite infections can influence host foraging behavior, movement, or social interactions. House finches (Carpodacus mexicanus) in the US are susceptible to a recently emerged strain of the bacteria, Mycoplasma gallisepticum. Infected birds develop mild to severe conjunctivitis that could affect their foraging or social behavior. We videotaped house finches with and without conjunctivitis at a bird feeding station in Atlanta, GA to determine whether birds with conjunctivitis differed in feeding duration, efficiency, total food intake, or aggressive interactions. We observed 105 house finch feeding bouts (of which 41% were of birds with conjunctivitis). Infected birds spent more time at the feeding station and had smaller average and minimum flock sizes. House finches with conjunctivitis also showed lower feeding efficiency than noninfected birds in terms of seeds obtained per attempt and number of seeds eaten per unit time. However, because of their longer feeding bouts, birds with conjunctivitis consumed similar total numbers of seeds as birds without conjunctivitis. Finally, house finches with conjunctivitis were displaced from feeder perches less frequently than noninfected individuals and 75% of all observed displacement events consisted of an infected bird displacing a noninfected bird. Differences in flock sizes and feeding behavior of birds with and without mycoplasmal conjunctivitis could influence the fitness effects and transmission of this bacterium in wild house finch populations.
Physical and Biological Drivers of Wetlandscape Biogeochemistry
Corline, Nicholas John (Virginia Tech, 2024-05-22)
Wetlands play a vital role in regional and global biogeochemistry by controlling the movement and cycling of nutrients and carbon. While individual wetlands may provide these ecosystem services, high density wetland landscapes, referred to as wetlandscapes, can have far reaching aggregate effects on elemental cycling and solute transport. Here we use forested Delmarva bays or wetlands as a study ecosystem to explore physical and biological controls on wetland chemistry within forested wetlandscapes. The Delmarva wetlandscape consists of thousands of geographically isolated wetlands on the Delmarva Peninsula, United States, which despite their proximity to each other have highly variable sizes, shapes, hydrology, vegetative cover, and biological communities. This physical and biological variation makes the Delmarva wetlandscape an ideal ecosystem to understand spatio-temporal heterogeneity and drivers of biogeochemistry. In this dissertation, I demonstrate that water chemistry within the Delmarva wetlandscape is heterogeneous both within and between surface water and groundwater systems (Chapter 2). Surface water chemistry was primarily influenced by temporal factors (season and month), followed by local hydrology. In contrast, groundwater chemistry was strongly influenced by water level below ground surface and interaction with organic soil layers. These results are important in understanding both internal wetlandscape water chemistry dynamics and export of solutes such as dissolved organic matter (DOM) to adjacent river ecosystems. Further, these results suggest that local biological and hydrological factors strongly affect surface water chemistry in wetlands. To explore these factors, I used an observational approach to determine the role of larval amphibians on wetland biogeochemistry (Chapter 3) and employed high-resolution chemistry sensors to study the effect of hydrological changes on surface water dissolved organic matter concentrations (Chapter 4). Animal waste can contribute substantially to nutrient cycling and ecosystem productivity, yet little is known of the biogeochemical impact of animal excretion in wetland habitats. A common and abundant amphibian in Delmarva wetlands are wood frog (Lithobates sylvaticus) tadpoles. I found that wood frog tadpole aggregations elevated nutrient recycling, microbial metabolism, and carbon cycling in Delmarva wetlands. These results provide evidence for the functional and biogeochemical role of tadpole aggregations in wetland habitats, with important implications for ecosystem processes, biodiversity conservation, and ecosystem management. To further explore the role of hydrology on DOM concentrations, I utilized high-resolution fluorescent dissolved organic matter sensors (fDOM) and applied river solute transport frameworks and metrics to wetland catchments. I found that there was heterogeneity in wetland response to changing hydrology and that seasonality and potentially bathymetry influences fDOM concentrations. Together, these studies inform our understanding of wetlandscape heterogeneity and DOM export, as well as biological and hydrological drivers of biogeochemistry.

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