Agricultural land use near streams frequently results in long-term disturbance to woody riparian vegetation and an alteration of reach scale geomorphic structure. Such disturbances often result in increased fine sediment input to the stream along with direct changes in channel structure. The study described here was designed to quantify stream geomorphic changes associated with agriculture and their influence on reach scale transient storage hydraulics and sediment biological function. Six small streams in the Appalachian Mountains of western North Carolina were selected to compare 3 reaches with active near-stream agriculture to 3 forested reference reaches. The study site categories differed significantly in many structural and hydraulic properties including slope, sinuosity, sediment size, and transient storage extent. However, differences cannot be attributed to land use alone. Distinct disparity in slope suggests that many of the categorical differences between stream types may also reflect valley scale structure. Despite these larger scale controls, the abundance of suspendable fines varied substantially among agricultural stream substrates, possibly due to varied land-use practices. Suspendable fine sediments and valley slope explained 91 % of variability in transient storage exchange, and abundance of inorganic fine sediments explained 77 % of variability in sediment microcosm nitrate production. This study supports conclusions that reach-scale influence of fine sediments occurred within the context of larger-scale valley structure, with implications on stream hydraulics and biogeochemistry. / Master of Science

Recent increases in atmospheric carbon dioxide caused by the combustion of fossil fuels and tropical deforestation may result in global warming. Carbon accumulation by regrowing temperate forests, in regions such as the southeastern United States, may have been extensive enough to counterbalance releases of carbon from the tropics. In the Virginia Piedmont, large amounts of carbon have accumulated in phytomass and detritus of loblolly pine (Pinus taeda) plantations and natural forests regrowing on post-agricultural fields. After 50 years, carbon in phytomass of old field plantations was 200,000 kg/ha, twice the amount accumulated by natural forests. Detrital carbon accumulations totaled over 100,000 kg/ha, but were dependent upon amounts of erosional loss prior to reforestation. The forested land area in the southeastern United States has stabilized, and forest conversion is now the primary form of reforestation. Therefore, the region's ability to continue to store carbon has been questioned. Still, the phytomass of late-rotation converted plantations stored 200,000 kg carbon/ha, twice the amount of the natural forests they replaced. In addition, while the harvest of natural forests resulted in small reductions in detrital carbon, it was rapidly restored to over 100,000 kg/ha within 30 years. Houghton et al. (1983) developed a series of models describing carbon dynamics during reforestation. My data show that patterns of carbon accumulation exhibited by regrowing loblolly pine plantations are different from their models. Therefore, modifications of the models are suggested to improve estimates of carbon storage in temperate forests. / Master of Science

<p dir="ltr">Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic chemicals known for their persistence in the environment and potential health risks. PFAS are linked to several adverse effects in human and wildlife health. The detection of PFAS in biosolids has raised concerns about their use in agricultural and land application practices. This is because some PFAS are known to enter the food system through plant uptake and some leach into groundwater. The purpose of this study was to examine the PFAS profile in soils and porewater with depth at an agricultural site with historical biosolids applications. The site selected has received biosolids at agronomic rates for corn for approximately four decades. This study utilized a total of six lysimeters, three “shallow” at 60 cm and three “deep” at 120 cm, to monitor PFAS leaching in soil. Porewater samples were collected within 1-3 days after rain events based on rainfall amount and response of the moisture sensor installed at the site. For each of five porewater sampling events, PFAS and supplemental water parameters like total organic carbon (TOC) and pH were measured. Soil cores, taken in one-foot increments before and after the 3-month study, were analyzed for PFAS, soil OC, moisture, and grain size. All samples were analyzed using high resolution mass spectrometry for 54 PFAS and in line with EPA 1633 method. Soil characteristics such as texture, moisture, and soil OC significantly influence PFAS transport and sorption capacity within the soil profile, impacting PFAS distribution across soil depths. PFAS in the soil profile decreased with increasing depth and directly correlated with soil OC. Long chain PFAS were strongly retained in the top 60 cm and minimally distributed to the porewater. Short-chain PFAS proportionally dominated porewater samples, with elevated concentrations observed in shallow porewater driven by increased saturation (perched water) from a low permeability clay layer. Unsaturated conditions enhance PFAS retardation through air-water interface partitioning in addition to soil particle sorption mechanisms. In this study, less than 0.1% of PFAS leach from the vadose zone of a biosolid impacted plot annually, underscoring the longevity of PFAS in the soil profile and importance of understanding PFAS transport dynamics for effective environmental management.</p>

The biogeochemical processes responsible for production and emission of ammonia from stored liquid dairy manure are governed by environmental factors (e.g. manure temperature, moisture) and manure characteristics (e.g. total ammoniacal nitrogen concentration, pH). These environmental factors and manure characteristics vary spatially as a result of spatially heterogeneous physical, chemical, and biological properties of manure. Existing process-based models used for estimating ammonia emission consider stored manure as a homogeneous system and do not consider these spatial variations leading to inaccurate estimations. In this study, a one-dimensional compartmental biogeochemical model was developed to (i) estimate spatial variation of temperature and substrate concentration (ii) estimate spatial variations and rates of biogeochemical processes, and (iii) estimate production and emission of ammonia from stored scraped liquid dairy manure. A one-dimension compartmentalized modeling approach was used whereby manure storage is partitioned into several sections in vertical domain assuming that the conditions are spatially uniform within the horizontal domain. Spatial variation of temperature and substrate concentration were estimated using established principles of heat and mass transfer. Pertinent biogeochemical processes were assigned to each compartment to estimate the production and emission of ammonia. Model performance was conducted using experimental data obtained from National Air Emissions Monitoring Study conducted by the United States Environmental Protection Agency. A sensitivity analysis was performed and air temperature, manure pH, wind speed, and manure total ammoniacal nitrogen concentration were identified as the most sensitive model inputs. The model was used to estimate ammonia emission from a liquid dairy manure storage of a dairy farm located in Rockingham and Franklin counties in Virginia. Ammonia emission was estimated under different management and weather scenarios: two different manure storage periods from November to April and May to October using historical weather data of the two counties. Results suggest greater ammonia emissions and manure nitrogen loss for the manure storage period in warm season from May to October compared to the storage period in cold season from November to April. / Ph. D. / Dairy manure is a byproduct of dairy farming that can be used as a fertilizer to provide essential plant nutrients such as nitrogen, phosphorus, and potassium. However, manure can only be applied to crop lands in a certain time of the year during growing seasons. Further, discharge of dairy manure into natural environment is prevented by the environmental regulations. Therefore, manure storage structures are used to store liquid dairy manure until time permits for land application or use for other purposes. During the storage, liquid dairy manure goes through biological, chemical, and physical processes and release manure gases that are linked to deteriorate human and animal health and contribute to environmental pollution. Ammonia is one of the manure gases released to atmosphere from stored liquid dairy manure. Furthermore, release of ammonia from stored manure reduce nitrogen content and reduce fertilizer value of stored manure. Implementing control measures to mitigate ammonia emission is necessary to prevent ammonia emission and reduce nitrogen loss from stored manure. Deciding and applying of appropriate control measures require knowledge of the rate at which ammonia emission occurs and when ammonia emission occurs. Use of process-based models is one of the less expensive and reliable method for estimating ammonia emission from stored liquid dairy manure. Process-based model is a mathematical model that simulates processes related to ammonia production and emission from stored manure. Even though, there are several process-based models available for estimating ammonia emission from stored liquid dairy manure, these models do not fully represent the actual processes and conditions relevant to production and emission of ammonia. For instance, spatial variation of temperature and total ammoniacal nitrogen concentration within stored manure is not considered in existing process-based models. Therefore, in this study a new compartmental process-based model was developed for estimating these spatial variations and production and emission of ammonia from stored liquid dairy manure. The model uses weather data and manure management information as inputs for estimating ammonia emission and nitrogen loss. The performance evaluation of the compartmental process-based model revealed that air temperature, manure pH, wind speed, manure total ammoniacal nitrogen concentration are important model inputs for estimating ammonia emission from stored liquid dairy manure. The model was used to estimate ammonia emission from a dairy farm located in Rockingham and Franklin counties in Virginia. Results suggest greater ammonia emissions and manure nitrogen loss for the manure storage period in warm season from May to October compared to the storage period in cold season from November to April.

Forests face an unprecedented range of disturbances from climate change, introduced pests and pathogens, and novel species, which frequently interact causing severe consequences to forest communities and ecosystem function. Understanding the mechanisms by which forests recover from disturbance and maintain stability of function is not only an issue of ecological interest, but one of pressing human need, as forest functioning is involved in maintaining a suite of ecosystem services that provide for humanity, including the global carbon and water cycles. Using an experimental manipulation of tree species diversity within an oak-dominated temperate northeastern deciduous forest, this dissertation broadly asks: how do forest community biomass and diversity influence stability and magnitude of forest community growth and water use? All three chapters are based on data from the same forest, where four treatments had previously been established by trunk girdling, which kills a tree by severing the cambium and sapwood, but leaves it standing, similar to the effect of many pathogens on trees. The treatments represent a range of species richness (2-10 species), biomass (5.5 × 104 kg/ha to 7.1 × 105 kg/ha), and level of disturbance, with experimental plots losing anywhere from zero to 94% of their living biomass. Chapter 1 focuses on competitive release after the loss of a dominant species using an annual census of trees over the last 10 years. Community level growth rates showed that diversity positively influenced biomass recovery rate. Chapter 2 addresses the role of the tree community’s biomass and diversity on soil water content using soil moisture sensors, which have recorded data hourly for two years, as well as the trees’ water stress, by using foliar stable carbon isotope ratios. Here, diverse communities have higher and more stable levels of soil water as well as trees that are less water stressed. Using the same long term data as Chapter 1, Chapter 3 assesses whether growth in the tree communities has been more stable over the past 10 years, and investigates whether this can be explained by shorter term fluctuations in tree growth measured by automated point dendrometers. While more diverse communities are more stable in their growth rates over time, this was strongly dependent on how much of the original community had been mechanically girdled. Species showed complementarity in phenology of tree growth at the seasonal scale, but our models could not directly link this intra-annual complementarity in more diverse communities to the stability seen over 10 years. Taken together, results from these three chapters suggest that diversity plays a role in mediating recovery of function from disturbance, which has implications for both the global carbon and water cycles.

The expansion and proliferation of reactive nitrogen (N) sources, predominantly fertilizer application and fossil fuel combustion, has enriched the earth with N and acidified ecosystems. Acid rain is a primary vector of both N fertilization and acidification, initiating a cascade of consequences that alter biogeochemical cycling and global biological structure and function. Studies on N and acid influences are however rarely linked despite their common source. We used a wide, chronic gradient of N deposition (5.5 – 31 kg N ha⁻¹ yr-1) to explore patterns in carbon (C) and N cycling in light of recognized biogeochemical responses to acidic deposition. Specifically, we examined the response of key controls on dissolved C and N stability because soluble pools are involved in decomposition and nutrient recycling, the formation of soil organic matter (SOM), and the translation of elements through the biogeochemical continuum from atmospheric to soil to water. Results suggest that N deposition led to reduced organic matter C/N, enhanced net nitrification, and greater DON generation; and, these patterns were associated with changes in C composition. Conversely, physiochemical processes in the mineral soil seemed to control organic matter dynamics, with effects on N processing. Moreover, pH dependent controls on DOC stability were evidenced by changes in DOC concentration, chemical complexity and recalcitrance. These horizon-specific, differential responses to acid rain indicate that changes in the forest floor N economy were responsible for increased surface water NO3-N concentrations, whereas enhanced organomineral stability of DOC caused a significant increase in DOM concentrations in export. / Master of Science

<p dir="ltr">The origins and deposition of nitrate in dust traps in Mojave Desert are examined in this thesis. Two main hypotheses are tested: (1) most of the dust in the traps comes from local soil, implying that the nitrate content is primarily derived from the soil; and (2) wet deposition is the primary source of nitrate found in the environments, implying that precipitation processes play an important role in nitrate accumulation. To test these hypotheses, we collected data from 11 dust trap from locations in of the US Geological Survey's long-term investigation of dust composition and influx rates. Dust and soil samples were analyzed for ions to determine their origins and the contributions of local vs distant sources. Our findings show that the fraction of soil-derived nitrate (<i>f</i>_soilNO₃^-) is consistently low at all traps, hardly reaching 0.03, whereas the atmospheric nitrate percentage (<i>f</i>_atmNO₃^-) is usually close to or equal to 1. This shows that atmospheric sources play a substantial role in the nitrate levels detected in dust traps. Nitrate contributions are also significantly influenced by sedimentary and geological settings, such as the distinctions between alluvium and playa regions. Playas, which are composed of silt and clay, may have higher nitrate concentrations than alluvial plains, indicating that external dust inputs are significant. The second hypothesis's results show that nitrate deposition in the study area is primarily from dry sources, with dry deposition values ranging from 0.68 to 10.84 NO₃⁻/kg/ha/yr, averaging 4.12 NO₃⁻/kg/ha/yr, and wet deposition values averaging 1.09 NO₃⁻/kg/ha/yr. This observation challenges the hypothesis that wet deposition is the primary source of nitrate. The dominance of dry deposition is further supported by low amounts of precipitation and a weak correlation between precipitation and dust deposition. This study concludes that although local soil has a role in nitrate levels in dust traps in the study site, it is not the primary source, external sources and dry deposition account for the majority of nitrate in the dustpan</p>

Macronutrients (carbon (C), nitrogen (N), and phosphorous (P)) are substances that all organisms require to survive, grow and reproduce. While C, N, and P are naturally present in all aquatic ecosystems, human activities have significantly increased their concentrations and changed their dynamics in rivers. This lead to widespread degradation of water quality. Meaningfully reversing those consequences requires understanding how anthropogenic activities have changed macronutrient dynamics. This approach is still hindered by the availability of long-term data which covers periods of pollution and recovery. In this thesis, I investigated long-term changes by collecting, combining, and interpolating time series of many different ecological variables. Particularly, I developed a conceptual model that linked a 65-year long time series of in-stream N concentrations in the Elbe to changed anthropogenic N sources, which I, in turn, explained with a characteristic succession of human needs. To understand the sources and the sinks of N in rivers, I developed a mass balance model that quantifies how much N has been retained by organisms in the Elbe over the last 42 years. Using an inverse Bayesian model approach, I estimated the daily importance of bacterial and algal activity over 36 of these 42 years. Apart from N, macronutrient ratios (C:N:P) are a crucial determinant of the integrity of aquatic ecosystems. To comprehend the effects of anthropogenic activities on C:N:P ratios, I analyzed C, N, and P data from 574 German catchments spanning a large gradient of agricultural and urban activities. Over the last 65 years, I discovered multiple regime shifts in the N dynamics of the Elbe. Before ∼ 1970, the Elbe experienced constant N concentrations across the seasons. Afterwards, a distinct seasonal pattern emerged with high concentrations during winter and low concentrations during summer. After the collapse of the German Democratic Republic in 1989, water quality in the Elbe improved drastically as many pollutant sources were removed. This manifested in declining annual mean N concentrations, but the summer and winter concentrations diverged further. I explain this with a changing ratio of agricultural and urban N sources, which affect the in-stream N concentrations differently across the seasons. Furthermore, improved water quality led to decreased bacterial and increased algal activity in the Elbe. Higher bacterial activity led to the higher N removal rates from the stream but also caused low oxygen concentrations in the Elbe and increased CO2 emissions. Across Germany, I found that most catchments, agricultural or not, are enriched with nitrogen compared to C and P. Especially the relatively low availability of C will reduce the capacity of rivers and adjacent ecosystems to remove the excessive N via biological processes. Overall, this thesis contributes to the understanding how anthropogenic activities change macronutrient availability in rivers over multiple decades and how riverine macronutrient cycling responds to different anthropogenic pressures. Nutrient management strategies usually do not consider seasonality and stoichiometry. This thesis suggests that those metrics have clear ecological implications and should be integrated into holistic macronutrient management strategies.:Summary 13 2 Introduction 16 2.1 The importance of macronutrients in aquatic ecosystems . . . . . . . . . 16 2.2 Anthropogenic impacts on macronutrient concentrations in rivers . . . . 18 2.3 Impacts of increased macronutrient concentrations on rivers and adjacent ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.4 Research questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3 Drivers of multi-decadal nitrate regime shifts in a large European catchment 34 3.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.3 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.4 Data and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.4.1 The Elbe catchment . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.4.2 Time series data . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.4.3 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.5 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.5.1 Mean concentrations and seasonality metrics . . . . . . . . . . . . 45 3.5.2 The role of in-stream nitrate retention . . . . . . . . . . . . . . . 50 3.5.3 Limitations of this study . . . . . . . . . . . . . . . . . . . . . . . 50 3.5.4 Environmental implications . . . . . . . . . . . . . . . . . . . . . 50 3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.7 Author contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4 From iron curtain to green belt: Elbe River shift from heterotrophic to autotrophic nitrogen retention over 35 years of passive restoration 62 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.2 Data and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4.2.2 Study site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 4.2.3 Two-station mass balance . . . . . . . . . . . . . . . . . . . . . . 68 7 4.2.4 Metabolism estimations . . . . . . . . . . . . . . . . . . . . . . . 69 4.2.5 Channel geometry estimations . . . . . . . . . . . . . . . . . . . . 70 4.2.6 Data preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4.3 Estimating the N demand of metabolic processes . . . . . . . . . . . . . . 70 4.4 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 4.4.1 DIN retention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 4.4.2 Dissolved oxygen saturation and metabolism . . . . . . . . . . . . 75 4.4.3 Linking metabolism and DIN retention . . . . . . . . . . . . . . . 79 4.4.4 Ecological implications . . . . . . . . . . . . . . . . . . . . . . . . 82 4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 4.6 Author contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5 Stoichiometry on the edge - Humans induce strong imbalances of reactive C:N:P ratios in streams 92 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 5.2 Data and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5.2.1 Data selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5.2.2 Derivation and visualization of C:N:P ratios . . . . . . . . . . . . 97 5.2.3 Classification of catchment stoichiometry . . . . . . . . . . . . . . 98 5.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 5.3.1 Spatial variability of median reactive C:N:P ratios . . . . . . . . . 99 5.3.2 Intra-annual variability of reactive C:N:P ratios . . . . . . . . . . 101 5.3.3 Implications for biogeochemical processing . . . . . . . . . . . . . 105 5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 5.5 Author contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 5.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 6 Discussion 116 6.1 Generality of findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 6.2 Relevance for eutrophication . . . . . . . . . . . . . . . . . . . . . . . . . 119 6.2.1 Seasonality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 6.2.2 Stoichiometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 6.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 6.4 Future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 6.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 8 7 Publication record 125 8 Acknowledgements 127 S Appendices 128 S.1 Supplementary material for the article: Drivers of multi-decadal nitrate regime shifts in a large European catchment . . . . . . . . . . . . . . . . 128 S.1.1 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 S.1.2 Analytical methods for nitrate measurement . . . . . . . . . . . . 133 S.1.3 Parameter estimation for the mixed source succession model (MSSM)133 S.1.4 The role of in-stream nitrate retention . . . . . . . . . . . . . . . 136 S.1.5 Parameter uncertainty of MSSM . . . . . . . . . . . . . . . . . . . 138 S.1.6 Sensitivity analysis of MSSM . . . . . . . . . . . . . . . . . . . . 138 S.1.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 S.2 Supplementary material for the Chapter: From iron curtain to green belt - Elbe River shift from heterotrophic to autotrophic nitrogen retention over 35 years of passive restoration . . . . . . . . . . . . . . . . . . . . . 144 S.2.1 Segment geometry estimation . . . . . . . . . . . . . . . . . . . . 144 S.2.2 Gaussian error propagation . . . . . . . . . . . . . . . . . . . . . 144 S.2.3 Interpolation of the hourly dissolved oxygen time series . . . . . . 145 S.2.4 Metabolism model implementation and validation . . . . . . . . . 146 S.2.5 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 S.2.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 S.3 Supplementary material for the article: Stoichiometry on the edge - Humans induce strong imbalances of reactive C:N:P ratios in streams . . . . 157 S.3.1 Data selection criteria . . . . . . . . . . . . . . . . . . . . . . . . 157 S.3.2 Converting C, N and P time series to C:N:P ratios . . . . . . . . 157 S.3.3 Calculation of the dist metric . . . . . . . . . . . . . . . . . . . . 158 S.3.4 Possible contributions of dissolved organic nitrogen (DON) and phosphorus (DOP) . . . . . . . . . . . . . . . . . . . . . . . . . . 158 S.3.5 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 S.3.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

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. / Doctor of Philosophy / Wetlands control the movement of nutrients and carbon at local, regional, and global scales. There is a large body of knowledge demonstrating the importance of wetlands to the transport of dissolved water constituents, such as dissolved organic matter (DOM) and nutrients. However, there is little information on what controls surface water chemistry in these wetland landscapes and less is known about belowground water chemistry. In this study I examined the role of water level, wetland shape, and time (i.e., year, month of the year, and season) on surface and groundwater chemistry in wetlands. I found that water chemistry was different between surface and groundwater and that differences were primarily due to seasons or months in surface water wetlands, while water level and flooding of organic matter-rich soil layers controlled groundwater chemistry. These results indicate that there are differences in water chemistry between surface water and groundwater that are controlled by unique drivers. These results also suggested that biological processes such as animal presence may influence wetland chemistry. To understand the role of animals in wetland chemistry, I studied the effect of wood frog (Lithobates sylvaticus) tadpole waste on nutrient concentrations in wetlands and found large tadpole groups are significant recyclers of nitrogen and phosphorous, which were used by microbes as nutrients, leading to enhanced leaf litter break-down in wetlands. These findings imply that tadpoles have an important role in wetland ecosystems by creating locations of enhanced nutrient and carbon cycling and that conservation of amphibian species may also preserve ecosystem processes in wetlands. Additionally, my initial study suggested that hydrology influences DOM concentrations in wetlands. I used high-frequency chemistry sensors to detect fluorescent dissolved organic matter (fDOM) concentrations, which represents a fraction of DOM. I found that relationships and patterns in fDOM concentration were complex, and that season and wetland shape were important in wetland DOM dynamics. Overall, this dynamic behavior across seasons and between wetlands indicates that wetland response to water levels can drive differences in water chemistry between wetlands and is important in our understanding of wetland response to storm events. The information gained from these studies is important in understanding how large wetland landscapes function and control movement of nutrients and carbon. Further, my research has uncovered the role of animal species in controlling nutrient and carbon cycling in wetland environments as well as the complex response of fDOM to water level changes in individual wetlands.

Deep water formation in the Labrador Sea, a marginal sea within the subpolar region of the North Atlantic Ocean, is vitally important to the ventilation of the global ocean interior with atmospheric gases including oxygen (O₂) and carbon dioxide (CO₂). To better understand the current mechanisms of ocean ventilation, and improve predictions of future deoxygenation and anthropogenic carbon uptake, the complex relationships between physical processes, chemical properties, and biological activity must be unraveled. Ocean biogeochemical models (OBMs) can offer a more complete picture of the ocean state than the limited snapshots provided by observations. The overarching goal of this dissertation is to use a data-constrained OBM to examine the processes controlling O₂ and CO₂ variability in the central Labrador Sea. In Chapter 2, I present the optimization of a data-assimilative regional OBM which simulates the physical and biogeochemical state of the North Atlantic Ocean from 2002 to 2017. The optimization process includes (1) removing the model spin-up to initialize the biogeochemical simulation from GLODAPv2.2016b 1° × 1° and other climatological estimates, (2) adjusting parameterized phytoplankton quantum efficiency, and (3) using a Green’s Functions approach to tune OBM parameters against O(105) in-situ biogeochemical measurements collected by BGC-Argo floats and research hydrography. I find significant model-data misfit reduction in the subpolar North Atlantic which demonstrably improve Labrador Sea modeled O₂, surface ocean pCO₂, and chlorophyll-a against independent satellite data and observation-based products. Using this data-constrained model, I then investigate the seasonal and interannual variability of central Labrador O₂ and surface ocean pCO₂. The high-frequency SeaCycler mooring dataset provides unique insight into the convective region of the central Labrador Sea over 2016. I use SeaCycler data to better understand the model simulation and, in turn, use the model to expand these biogeochemical insights in space and time. In Chapter 3, I present an oxygen budget of the central Labrador Sea over 2016–2017 by decomposing modeled dissolved O2 into its advective transport, diffusive transport, biological, and air-sea flux terms. We find that the competing effects of air-sea exchange and diffusive mixing are so balanced that there is minimal O₂ storage in the upper 150 m. In Chapter 4, I examine modeled and observation-based estimates of surface pCO₂ against in-situ SeaCycler data. Our analysis examines the seasonal and interannual variability of pCO₂ and reveals key biases in the non-thermal component of pCO₂, which is the dominant driver of modeled and estimated surface pCO₂ variability in the central Labrador Sea. Across all chapters, my dissertation works to bridge ongoing modeling and observational efforts to expand our understanding of ocean biogeochemical processes.