<p> To investigate how biogeochemical processes affect the concentrations and fluxes of dissolved organic matter (DOM) in forested watersheds, I studied the quantity and quality of DOM at the Arbutus Lake Watershed, Adirondack Park (NY, USA). First, to understand the biogeochemical changes of DOM for surface waters in the watershed, I studied spatial and temporal patterns of the quantity (bioavailable/refractory concentrations) and quality (&delta;¹³C, aromaticity, and low (LMW; &lt; 1kDa) and high molecular weight (HMW; > 1kDa) of dissolved organic carbon (DOC), nitrogen (DON), and sulfur (DOS). DOC and DON constituents passing through a wetland were composed of highly refractory, aromatic HMW components compared to upland streams. DOS was dominated by the refractory, LMW form. I developed a &ldquo;bioavailability-molecular size model&rdquo; showing the refractoriness of the LMW DOS compared to the HMW DOC and DON. </p><p> Second, to evaluate how a lake nested in a forested watershed affects the dynamics of DOC and DON, I analyzed a mass balance of DOC, DON and dissolved inorganic nitrogen (DIN) from 2000 to 2009. Annual DOC and DON mass balances were strikingly uncoupled, and Arbutus Lake generally acted as a sink for DOC. But a periodic source for DON was observed, indicating that internal recycling between DIN and DON might be important in affecting DON concentrations in this oligo-mesotrophic lake. </p><p> Third, isotopic signatures of DOS in the inlet catchment of the Watershed revealed that the incorporation of reduced S (e.g., SH-) derived from bacterial dissimilatory sulfate reduction contributed to generating DOS in ground and surface waters. These results provide new information on mechanisms for geochemical DOS formation in forested catchments. </p><p> Last, to better understand isotopic changes in DOS, three isotopic pretreatment methods for sulfate widely used were compared. I found an excellent agreement of the &delta;³⁴S-sulfate values among the three methods. However, some differences were observed in the &delta;¹⁸O-sulfate values associated with possible O contamination before isotopic measurements. </p><p> Overall, my studies highlighted novel approaches comparing sources and transformation of three elemental DOM fractions of DOC, DON, and DOS along various watershed components and a lake nested forested watersheds.</p>

<p> High-latitude ecosystems store large amounts of carbon in soil organic matter and are among the most vulnerable to climate change. In particular, fire severity and frequency are increasing in boreal ecosystems, and these events are likely to have direct and indirect effects on climate feedbacks via increased emission of carbon (C) from soil and changes in vegetation composition, respectively. In this study we created experimental burns of three severities in the northeastern Siberian arctic, near Cherskiy, RU, and quantified dissolved C, nitrogen (N), and phosphorus (P), and microbial respiration and extracellular enzyme activities at 1-day, 8-days, and 1-year post-fire. </p><p> Our objective was to determine how fire affects C, N, and P pools, soil microbial processes, and how these effects scale across severity and time since fire. We found labile C and nutrients increased immediately post-fire, but appeared similar to unburned controls within a week. Phosphorus alone remained elevated through 1-year post-fire. Leucine aminopeptidase activities initially increased with fire severity, but by 1-year, activities decreased with fire severity at a rate an order of magnitude faster. Fire severity suppressed phosphatase and ?-glucosidase activities at all time points. Soil respiration was reduced by half in high severity plots 1-year post-fire, while net rates of N mineralization increased by an order of magnitude. We found that changes in soil C and nutrient pools, soil respiration, and net N mineralization rates responded in a threshold-fashion to fire severity, although P was uncoupled from C and N by changing at a distinct severity threshold. Extracellular enzyme activities and edaphic variables scaled linearly with fire severity. The interaction of threshold and linear response curves to fire severity may help explain the variability across studies in soil microbial community responses to fire. Microbial communities recovering from more severe fires have the possibility to decrease future ecosystem C losses through reduced respiration. The changing fire regime in permafrost ecosystems has the potential to alter soil microbial community dynamics, the retention of nutrients, and the stoichiometry of C, N, and P availability.</p>

<p> Nitrogen (N) fixing trees are commonly used to promote forest restoration in disturbed areas because they can quickly recreate forest canopy structure. That structure in turn is hypothesized to attract animal seed dispersers and create enough shade to reduce undesirable species (particularly grasses). Yet N-fixers tend to increase soil N availability, which could facilitate the spread of nitrophilous invasive species. This dissertation evaluates the long-term consequences for understory community composition of establishing three N-fixing tree species (<i>Acacia koa, Sophora chrysophylla</i>, and <i>Morella faya</i>) after exotic grass-fueled fire in the seasonally dry subtropical woodland in Hawaii. To understand the restoration potential of these species, I compared discrete single-species stands of N-fixing trees in burned areas to both an intact native woodland and burned, open sites with no tree cover. Although N-fixing species are often assumed to be ecologically similar, trait variation among N-fixing trees in this system was strong enough to differentiate understory communities among stands of the three N-fixer species. To understand the mechanisms driving differences in understory composition among site types, particularly among N-fixing trees, I characterized the abiotic environment created by these species in terms of light and N availability, both of which were important drivers of understory community composition. High light and N availability were associated with greater exotic species cover and unique exotic species. Surprisingly, N availability was highest and N cycled fastest beneath the relatively slow-growing <i>S. chrysophylla </i> despite having much lower litter-N inputs than the faster-growing <i> A. koa</i> and <i>M. faya</i>. In this study, fast N-cycling was associated with high specific leaf area, high foliar N content and low foliar lignin:N. These traits are consistent with fast leaf economic spectrum traits in the general ecological literature, but this approach has not previously been applied to distinguish among N-fixing trees. Native Hawaiian dry forest understory recovery, particularly that of woody species, was limited throughout the burned area regardless of canopy cover. To determine what limits native shrub recovery, I sampled the seed bank and recorded natural seedling germination. I also planted native seedlings into the understory of all site types and either removed or left intact the invasive grass grasses present in the understory. I found that native shrubs were limited by both seed availability and competition with exotic grasses. Although outplant survival did not vary by N-fixer species identity, differences in the mechanisms by which each N-fixing species limited native seedling survival likely play a role in understory community assembly long-term. When restoration occurs in the context of secondary succession, prioritizing the creation of forest structure using N-fixing trees, particularly open-canopied fast-cycling species, such as <i>S. chrysophylla</i>, could make full community recovery more difficult by promoting rather than suppressing exotic grasses.</p><p>

<p> Government agencies assess the biological integrity of streams and the chemistry of groundwater to monitor anthropogenic impacts on freshwater ecosystems. Some of the impacted streams lack obvious stressors that can be linked to the impacts. In the case of elevated nutrients and ionic concentrations found in impacted stream sites within the Malibu Creek Watershed, research from the Las Virgenes Municipal Water District pointed towards the naturally occurring leachate from the Modelo formation, which contests previous studies suggesting an influence from anthropogenic activity. Differentiating between natural sources of trace element concentrations and point/non-point source contamination could aid in resolving water quality issues since monitoring occurs near anthropogenic development. Water, bedrock, and diatoms were sampled from spring, seep, and stream sites underlain by Tertiary sedimentary bedrock throughout the study area. Multivariate analyses revealed an association of groups driven by salinity along with a correlation between geochemistry and diatom assemblages. Mixing models and radar charts identified potential rock types as sources for analyte concentrations. Sites that were underlain by the Modelo formation presented brackish water conditions and high nutrient concentrations, which potentially influenced the diatom species compositions found at the spring/seep sites as a result of weathering.</p>

<p> In this study, the Weather Research and Forecasting Model (WRF) is coupled with the Advanced Canopy-Atmosphere-Soil Algorithm (ACASA), a high complexity land surface model (LSM). Although WRF is a state-of-the-art regional atmospheric model with high spatial and temporal resolutions, the land surface schemes available in WRF are simple and lack the capability to simulate carbon dioxide, for example, the popular NOAH LSM. ACASA is a complex multilayer land surface model with interactive canopy physiology and full surface hydrological processes. It allows microenvironmental variables such as air and surface temperatures, wind speed, humidity, and carbon dioxide concentration to vary vertically. </p><p> Simulations of surface conditions as well as reference and actual evapotranspiration from WRF-ACASA and WRF-NOAH are compared with surface observations for year 2005 and 2006. Results show that the increase in complexity in the WRF-ACASA model not only maintains model accuracy, it also properly accounts for the dominant biological and physical processes describing ecosystem-atmosphere interactions that are scientifically valuable. The different complexities of physical and physiological processes in the WRF-ACASA and WRF-NOAH models also highlight the impacts of different land surface and model components on atmospheric and surface conditions. </p><p> Lastly, unlike the simple big-leaf WRF-NOAH model with no carbon dioxide simulation, the high complexity WRF-ACASA model is used to quantify the carbon dioxide exchange between the biosphere and atmosphere and to examine the importance of atmospheric carbon dioxide concentration on surface processes on a regional scale. A new carbon dioxide (COCO₂) tracer is introduced into the WRF-ACASA coupled model to allow atmospheric CO₂ concentration to vary spatially and temporally according to surface plant physiological processes. The comparison between the two model simulations with and without a COCO₂ tracer shows that the impact of atmospheric COCO₂ concentration and transportation are important, and therefore these should not be neglected when simulating COCO₂ flux at regional scales. Overall, this study shows that the high complexity WRF-ACASA model is robust and able to simulate the surface conditions and COCO₂ fluxes well across the region, particularly when given accurate surface representations. </p>

<p> Benthic macrofauna can play an important role in facilitating some of the microbial mediated processes of nitrogen cycling in estuarine sediments. Declines in benthic macrofauna, like polychaete worms, have been attributed to long-term increases in bottom water hypoxia in Chesapeake Bay. Utilizing a large monitoring dataset including benthic macrofaunal abundance, biomass, and concurrent measures of environmental parameters, I examined how environmental conditions regulate the densities of opportunistic polychaetes in a mesohaline estuarine system. This analysis points to a benthic community dominated by euryhaline, opportunistic polychaete worms (<i>M. viridis, S. benedicti, H. filiformis, A. succinea</i>) which have well adapted but varying responses to hypoxia and other stressful conditions. Results of two laboratory experiments with the opportunistic polychaete <i>Alitta (Neanthes) succinea</i> were used to quantify the short-term influence of density and size of surface-feeding polychaetes on sediment-water fluxes of inorganic nitrogen under varying oxygen conditions. Polychaete enhancements of O₂ and nitrogen fluxes were strongly correlated with total animal biomass. Solute fluxes were stimulated by presence of both larger and smaller worms, but per capita effects were greater for the deep-burrowing larger polychaetes. Utilizing a unique large-scale monitoring dataset collected in the Chesapeake Bay, I employed Classification and Regression Tree (CART) and multiple linear regression (MLR) analyses to assess the relationship between benthic biomass and NH₄⁺ efflux within different regions of the estuary by season. In addition to labile organic matter, oligohaline and mesohaline tributary temperature and salinity control the rate of nitrogen cycling and benthic macrofaunal biomass. In deeper regions of mesohaline tributaries and the mainstem Bay, dissolved oxygen was found to be the dominating parameter regulating sediment nitrogen pathways as well as the structure of the benthic macrofaunal community. With increased macrofaunal biomass, spring regressions indicated an enhancement of NH₄⁺ efflux. In contrast, fall regressions indicated the enhancement of fixed nitrogen removal from sediments. Summer data lacked a significant relationship, but high NH₄⁺ effluxes under hypoxic/anoxic conditions suggested dissolved oxygen is the primary driver of summer nitrogen cycling. This study, using field and laboratory data, concludes that a complex balance between seasonal and regional dissolved oxygen, temperature and salinity conditions shape not only the benthic community but also the relationship between macrofaunal biomass and sediment nitrogen flux in this eutrophic estuarine system.</p>

Thesis (M.Sc.) - Carleton University, 2006. / Includes bibliographical references (p. 142-146). Also available in electronic format on the Internet.