Zooplankton play a key role in the cycling of dissolved organic matter (DOM) and inorganic nutrients. The factors that affect these processes, however, are not fully understood. I measured the effects of various diets on DOM and inorganic nutrient production by the copepod Acartia tonsa and the heterotrophic dinoflagellate Oxyrrhis marina, and explored the mechanisms of nutrient release from copepods. Copepods feeding on a mixed diet, the preferred diet of most copepods, had significantly lower dissolved organic carbon (DOC), ammonium (NH4+), and total dissolved nitrogen (TDN) release rates compared to feeding on a carnivorous or herbivorous diet. Thus, copepod feeding strategy can control the magnitude and composition of regenerated nutrients supplied to bacteria and phytoplankton. Secondly. I determined the effects of non-bloom and bloom concentrations of non-toxic and toxic cultures of harmful algal bloom (HAB) species Prorocentrum minimum and Karlodinium veneficum on grazing and production of DOM and inorganic nutrients by A. tonsa and O. marina. All algal diets deterred grazing, which likely resulted in starvation and subsequent catabolism of grazer body tissue. Additionally, DOM was typically a higher proportion of total dissolved nutrients released by zooplankton while feeding on the toxic algal culture, suggesting algal nutrient quality or direct toxic effects played a role in the differential nutrient release. Low ingestion rates coupled with high nutrient release rates could lead to feedback mechanisms that could intensify HABs. Finally, the various mechanisms of A. tonsa nutrient release, including sloppy feeding, excretion, and fecal pellet leaching, were isolated. Excretion and sloppy feeding were the dominant modes of DOC and NH4+ release, while sloppy feeding and fecal pellet leaching were dominant modes of urea release. A large proportion of ingested PON was lost as dissolved NH4+ and urea from copepods via all release mechanisms. These results have implications for the rapidity and location at which the regenerated nutrients are recycled in the water column. My dissertation results emphasize the importance of diet and release mechanism on the production of nutrients, particularly DOM, by zooplankton, which are important in understanding the recycling and transfer of nutrients and organic matter in marine food webs.

The chemical, physical, and biological factors affecting the bioavailability of humic nitrogen (N) to coastal phytoplankton were examined. Historically, humic substances have largely been considered biologically refractory, and humic-N is though to be unavailable biologically without prior oxidation via photochemical cleavage or remineralization by bacteria. This is due in part to the high aromaticity and low N content of humic substances. This dissertation investigates whether these assumptions are valid, and whether humic substances may be a more important source of N to the coastal phytoplankton community than previously believed. The research consisted of four main parts. First, changes in the structure and N content of humic substances were monitored by forming humics in the laboratory and following the changes in structure and chemical composition as they aged. It was found that as humics age, they become more aliphatic and fulvic-like. It was also determined that the commonly used XAD-8 extraction technique may underestimate the N content of aquatic humics by stripping ammonium (NH4+) from the humic structure. Second, whether or not this underestimation of humic-N has an effect on previously reported rates of photochemical N liberation from humic compounds was investigated. It was found that while the potential for the underestimation of photochemical release exists, previously reported rates are close to correct, largely due to physical and chemical interactions of humic substances with the surrounding environment. Third, the bioavailability of humic-N was examined using a suite of coastal phytoplankton strains. In short, all coastal strains exposed to humic substances could take up humic-N in short-term incubations; an open ocean strain tested did not take up humic-N. Furthermore, younger humics appeared to be more labile than those aged for time periods greater than three months. Finally, the significance of the salinity-mediated release was investigated and found to be a potentially important transport mechanism of NH4 + to the mid-saline regions of the estuary. In conclusion, the combination of chemical, physical, and biological processes occurring in the estuarine and coastal ecosystems suggests that humic substances are highly dynamic, biologically active compounds and not the biologically recalcitrant molecules portrayed in current literature.

The influence of groundwater discharge on the hydrology and biogeochemical cycling of nitrogen in a fringing intertidal wetland was studied by characterizing groundwater discharge, determining N-cycling rates in cores, and examining nitrate reduction in situ using 15N enrichment and natural gradient tracer techniques. Groundwater discharge was estimated by three independent methods: Darcy's Law, a water/salt mass balance, and a subsurface tracer test. Seasonal patterns of discharge predicted by Darcy's Law and the mass balance were similar. Discharge maxima and minima occurred in April and September, respectively. The water/salt mass balance provided the more reasonable estimate of groundwater flux at high flows, and the Darcy technique was better at estimating low flow at our site. The high discharge seasonally purged porewater from the marsh to the estuary, and marsh processing of groundwater solute loads would occur only during this period. Mineralization, nitrification, potential denitrification (DNF), and potential dissimilatory nitrate reduction to ammonium (DNRA) rates were estimated in cores during periods of high and low groundwater discharge. All N-cycling processes occurred in sediments <1.5 meters deep. Natural abundance isotope measures, and core experiments indicated that coupled nitrification-denitrification was a sizeable sink for mineralized N. Mineralization, nitrification, and DNRA rates were 6--12x greater during Spring high discharge. DNF rates, were 10x higher during Fall low discharge. Despite accelerated mineralization and nitrification during high discharge, the DNF:DNRA ratio was <1, indicating that more of the N cycled through nitrification was retained as ammonium rather than exported as dinitrogen through coupled nitrification-denitrification. Nitrate reduction pathways in the marsh were studied in situ by creating a nitrate plume enriched in 15N. Isotopic enrichment of the ammonium, PON, dissolved nitrous oxide, and dissolved dinitrogen pools initially accounted for 14--36% of the observed nitrate loss. Adjustment of these estimates with potential losses through gas evasion, and ammonium turnover, accounted for nearly all of the N missing from the mass balance. The adjusted mass balance indicated that 68% of the nitrate load was denitrified, and 30% was assimilated and retained in the marsh.

No description available.

Submarine groundwater discharge (SGD) is any flow of water along the continental margins from the seabed into the coastal ocean, and it represents an important source of nutrients and trace metals to the coastal ocean. The chemical composition of SGD is strongly influenced by biogeochemical reactions that take place within the subterranean estuary (STE), the subsurface mixing zone of fresh and saline waters. Understanding the reactions that take place within the shallow STE is critical to evaluating the composition of SGD, and therefore SGD-driven chemical fluxes. In this dissertation, I seek to determine the biogeochemical processes controlling the behavior of the redox-sensitive metals (RSMs) Mo, U, V, and Cr in a shallow subterranean estuary in Gloucester Point, VA (USA). These RSMs tend to form soluble oxyanions under oxidizing conditions but react to form more insoluble or particlereactive (i.e., more likely to adsorb to sediments) species under reducing conditions. In this STE, advection of water through the STE and the apparent respiration of organic matter drives the formation of a “classic” redox sequence typically observed in diffusion-dominated fine-grained sediments, with sequential zones with depth of high nitrate, dissolved Fe, and sulfide. While the general redox structure and RSM distributions in the STE remained consistent over time, concentrations and mixing behavior varied over the study period. Concentrations of DOC, humic carbon, and sulfide were higher in the summer, whereas Fe and Mn concentrations were higher in winter. This contrasting behavior may be due to sulfate and metal reducing bacteria responding differently to seasonally variable factors (such as temperature or substrate availability). Mo and U were supplied to the STE by surface water, and both showed nonconservative removal. Removal of Mo was correlated with sulfide concentrations, but unlike sulfide concentrations, did not show seasonal differences. This was likely due to sulfide concentrations consistently in excess of the 11 µM threshold required to quantitatively react with and remove dissolved Mo. However, U showed greater removal in the summer, possibly driven by greater activity of U-reducing microbes. Dissolved V concentrations co-varied with DOC (with both greater in summer), indicating that V is likely complexed with dissolved organic matter. In contrast, Cr was correlated with both humic carbon and dissolved Fe in different parts of the STE. Over half of total dissolved DOC and Fe occurred in the colloidal phase, demonstrating the importance of colloidal transport in the STE. The relative proportion of RSMs in the colloidal phase increased in the order Mo < U < V < Cr, with up to 75% of Cr existing in the colloidal size fractions, suggesting the importance of colloidal transport for RSMs. Incubation experiments conducted under aerobic and anaerobic conditions showed that RSM concentration change on the order of hundreds of nM can take place in hours to weeks, within water residence times in the shallow STE. Furthermore, removal and mobilization rates between redox zones with distinct microbial populations. The mechanistic approach used this work demonstrate how spatial and temporal variability of dissolved concentrations in the STE depend on redox zonation and microbemediated reactions. Findings from this work provide a basis for evaluating how changing environmental conditions may alter RSM fluxes.

The Ross Sea, Antarctica seasonal phytoplankton bloom is one of the largest in the Southern Ocean. This project focuses on the biological pump, which removes carbon from the surface ocean to the deep ocean through the settling of particulate organic matter, the advection of dissolved organic carbon, and active flux due to vertical migration of zooplankton. The objective of this study was to focus on three interrelated components of the biological pump including sedimentation, photosynthetic rates and grazing. The study was conducted in coordination with the Interannual Variability in the Antarctic-Ross Sea program, which covered the time period between 2001--2005. Simple, one-dimensional budgets were made using in situ nitrogen and silica concentrations and published climatologies. There was significant interannual and seasonal variability in phytoplankton bloom composition and concentrations of organic matter. During February 2004, a large secondary bloom of diatoms occurred, and nitrate removal was 8-fold higher than during other years in the study period. Principal components analysis was utilized to examine patterns in the large data set. Through visualization of the loadings and scores of the principal components, the primary controls of the concentrations of biomass and organic matter were seasonality, phytoplankton community composition and temperature, which explained 68.1% of the variance of the data set. There was also a significant negative relationship between the percent abundance of Phaeocystis antarctica, a dominant phytoplankton group, and temperature. Vertical flux measurements at 200 m using sediment traps showed that fecal pellet carbon during certain periods (February 2004, 2005) represents a large percentage of the total carbon flux from the surface, which suggests that mesozooplankton were actively grazing and packaging phytoplankton into sinking pellets. Photosynthesis/Irradiance measurements were the first to show that colonial P. antarctica may have higher growth rates early in the growing season, which may be one reason why large P. antarctica blooms occur earlier that diatoms. Lastly, preliminary results utilizing a novel fluorescently labeled algae technique showed colonial P. antarctica can be grazed by zooplankton and enter the food web before sedimentation.

We present research aimed at determining the role of soil structure in the organization of microbial communities in soil. We have focused on the microbial communities involved in functional roles within the nitrogen cycle, particularly nitrogen fixation. We have studied how different aggregate structures of varying size and stability provide different environments for these communities. Initially, we compared the nitrogen-fixing communities between aggregate structures obtained from soils from arctic, temperate and tropical environments. We also compared the communities involved in nitrogen fixation, nitrate reduction and ammonia oxidation from adjacent agricultural plots under different tillage regimes. We continued to monitor the community composition of nitrogen fixers at the aggregate scale through the following year and season after a tillage disturbance. We further narrowed down our scale by comparing the total diversity and functional diversity of an individual microaggregate and compared it with others. We then looked that the effect of disturbance at larger time scales in forest soil plots of old and secondary forest growth over one hundred years after recovery. We have determined that very different functional community structure can be found across all aggregate fractions in soils from all over the world, as well as from adjacent plots. Tillage has a major effect on the community composition of communities involved in the nitrogen cycle evidenced at all aggregate scales. This disturbance also affects the overall activity of nitrogen fixers measured in soil. However, the community recovers a year after the tillage disturbance, both in terms of community composition and activity of nitrogen fixers. Individual microaggregates are different from each other in terms of overall community structure, but the functional community composition seems to be homogeneous and representative of the composition observed in a pooled microaggregate fraction. Prolonged agricultural activities significantly altered the communities obtained from secondary growth forests when compared to adjacent old forest soils. Different aggregate structures provide environments for very different microbial communities involved in the nitrogen cycle. However, even the most stable of these structures is affected by tillage at the level of community composition and activity. This work describes not only the extent of this community and niche disturbance and their ability to recover, but also the extent of functional diversity at the micro-scale of the most complex of natural environments.

Recent successful drilling operations at Lake El'gygytgyn, NE Russia have recovered sediment cores back to 3.6Ma, representing the longest time-continuous sediment record of past climate change in the terrestrial Arctic. Analysis of the hydrogen isotopic ratio (δD) of specific organic biomarkers allows reconstruction of past hydrological conditions, thereby providing a powerful tool for reconstructing past Arctic climate changes. Compound specific isotopic analysis of sedimentary lipids from this remote basin provides new insights into the climate evolution of the Arctic, capturing the mechanisms and dynamics of the last two glacial-interglacial transitions, potentially enhancing the accuracy of modeled future climate change projections and presenting an opportunity to estimate past polar amplification of climate change. The results of this research document the first continuous, high fidelity continental record of reconstructed δD in precipitation from terrestrial plant leaf waxes in the High Arctic spanning the last 120 ka. The hydrogen isotopic composition of lipid biomarkers were determined from previously obtained Lake El'gygytgyn sediment cores and compared with other multi-proxy evidence of past climate change within the lake basin. The modern isotope hydrology and controls on the δD lipid signal were first established within the El'gygytgyn Basin from modern precipitation, stream and lake waters, ice cover as well as modern vegetation, water column and lake bottom surface sediments in order to provide a modern context to properly constrain and interpret paleoclimatic proxy data. Reconstructed δD records of paleoprecipitation and temperature at Lake El'gygytgyn lead other northern hemisphere climate records (e.g. North Greenland Ice Core Project, NGRIP δ18O records) and are in phase with other continental and Antarctic climate records, suggesting early high northern latitude continental warming prior to established glacial-interglacial transitions. The data set generated here leads to multiple avenues of future work and provides critical insights into Arctic paleoclimate and paleohydrology, contributing to our understanding of high latitude environmental change over geological timescales. Collectively, the results of this dissertation research will provide a context for paleoclimate reconstructions and future organic geochemical and stable isotope analysis. Future application of compound-specific H isotope analyses to long drill cores (recovered in 2009; ∼315m of sediment) will potentially provide a quantitative high-resolution record of paleoclimatic and paleoenvironmental changes spanning the last 3.6 Ma.

No description available.

No description available.