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.

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Herein, I have completed several experiments which encompass developing fiddler crabs as a model system, as well as sentinel and temperature studies to investigate biotic and abiotic factors in parasite transmission. My studies show which factors prevent, delay, or accelerate transmission and progression of H. perezi. The fiddler crab experiments by chapter are as follows: Chapter 1. I screened adult and juvenile fiddler crab populations for naturally occurring H. perezi infections at endemic and non-endemic sites. No natural infections were found in the adult or juvenile populations (Chapter 1 and 3). I completed inoculation trials with U. minax, U. pugnax, and U. pugilator, demonstrating that the parasite can survive and replicate in these species. Fiddler crabs can live for several months with patent infections. For example, I successfully transferred H. perezi from blue crab to fiddler crab and back to blue crab. Through serial inoculations I was able to serially maintain the parasite in the lab year-round. Building on the above experiments, I completed minimum dose studies which showed that a minimum inoculum of 1,000 parasite cells was required for patent infections. Additionally, I evaluated parasite progression through studies using Uca minax. These studies which used an inoculum in the ameboid trophont and clump colony stages showed that H. perezi progresses through its life-history stages in fiddler crabs as it would in blue crabs, with the filamentous trophont stage first observed in the hemolymphs smears followed by the ameboid trophont stage. Chapter 2. Intertidal environments are well known as areas of environmental extremes, and accordingly the animals that reside there have adapted to those conditions out of necessity. One abiotic factor that can have large diel variation is temperature. to address the impact of temperature variation of the marsh and subtidal habitat on H. perezi, I developed laboratory temperature experiments with nascent infections (7 °C, 15°C, 20°C, 25°C, 30°C), with patent infections (10°C, 15°C, 20°C, 30°C), and a progression series over fine scale (15°C, 17°C, 19°C, 20°C) temperature increments. These studies demonstrated that growth of the parasite is limited at the higher and lower temperatures, and that H. perezi is eliminated from the host at 30°C. This was confirmed by hemolymph smears, histology, and PCR. Chapter 3. The successful laboratory inoculations and lack of infections in fiddler crabs from endemic areas led to additional field deployments. These experiments aimed to address the dissonance of the initial results. My sentinel studies included fiddler crabs deployed in a crab pot from a pier touching bottom, deployed from the pier approximately mid-tidal height, deployed mid-marsh in mesh cages without access to bury, and deployed mid-marsh with access to bury. Fiddler crabs can obtain H. perezi infections in the marsh when caged without access to bury or when fully or partially submerged from a pier. However, they do not obtain H. perezi infections when given access to bury. Natural behaviors, such as burying along with elevated marsh temperatures likely prevent the establishment of H. perezi in the natural fiddler crab population.

The objective of this study was a comparison of developmental changes and energy utilization in eggs, unfed larvae, and fed larvae of two marine fish species that both have pelagic early stages, but differ in phylogeny and early life ecology. The bay anchovy (Anchoa mitchilli), a clupeiform, spawns in estuaries and shallow coastal waters. The black sea bass (Centropristis striata striata), a perciform, spawns offshore at 15-50 m depths. Densities of zooplankton eaten by first-feeding fish larvae are normally higher in coastal waters than offshore. An important determinant of survival of larval fishes is their ability to fulfill nutritional requirements after yolk energy is exhausted. Faster-growing larvae are less likely to be preyed upon. The manner in which energy is partitioned may indicate relative plasticity with respect to departures from optimal food composition or abundance. Differences among species might result from different feeding strategies or from adaptation to different feeding conditions. Energy utilization was assessed by constructing energy budgets (I = G + M + F&U): ingested calories, I, from feeding rates; growth calories, G, from composition and weight; metabolic calories, M, from oxygen uptake; egested and excreted calories, F&U, by difference. Three lines of evidence were found that suggest black sea bass are able to resist fluctuations in food availability better (survive and grow at lower densities): (1) Sea bass have more time to find food and develop feeding skills--50 hours between first feeding and yolk exhaustion vs 10 hours for anchovies. (2) Sea bass feed more efficiently and probably pay a lower metabolic price for their food. Over the first five days of feeding, capture success averaged 85% for sea bass and 60% for anchovies. (3) During the first five days, sea bass gross growth efficiency (12%) was twice that of anchovies (6%). Sea bass may also be more resistant to starvation from complete food deprivation. their yolk lasts longer. During starvation, their weight-specific metabolism is lower and they lose body calories at a lower rate. The bay anchovy seems to be adapted to the high food densities, and the black sea bass to the low food densities, that characterize their respective habitats. For sea bass the food supply is more difficult to exploit, and this requires greater efficiency.

Seasonal hypoxia is a characteristic feature of the Chesapeake Bay as a result of anthropogenic eutrophication from agriculture and urban development throughout the watershed. Although in recent years coordinated management efforts have successfully reduced the flux of nutrients into the Bay, the overall goal of sufficient oxygen concentrations below the pycnocline for living resources remains unfulfilled. This was particularly apparent in 2018 and 2019 when the volume of hypoxic water exceeded the long-term (35-year) average due to anomalously high riverine discharge. To quantify the impact of watershed nutrient reductions, conventional statistical methods were employed in concert with a 3-D numerical modeling approach to estimate the enhanced resiliency of hypoxia in the Chesapeake Bay to environmental conditions in recent years. A realistic 3-D numerical model hindcast from 2016-2019 was run, along with sensitivity experiments over the same interval that used organic and inorganic nitrogen concentrations representative of 1985 values. Differences between these sensitivity results and the realistic hindcast suggest that had nitrogen reductions not occurred, annual hypoxic volumes (O2 < 3 mg L-1) would have been ~50-120% greater during the average discharge years of 2016-2017 and ~20-50% greater during the wet years of 2018-2019.The relative effect is even greater for O2 < 1 mg L-1 , where annual volumes would havebeen ~80-280% greater in 2016-2017 and ~30-100% greater in 2018-2019. The exact magnitude of this effect is dependent on the terrestrial inputs used to run the numerical model and is particularly sensitive to assumptions regarding organic nitrogen loading. Numerical model results are supported by statistical analysis of observational data; however, the magnitude of change due to nutrient reductions is greater in the numerical modeling results than the statistical analysis. This discrepancy is largely accounted for by warming in the Bay that has exacerbated hypoxia and offset roughly 6-34% of the improvement from nutrient reductions. Although these results reassure policymakers and stakeholders that their efforts to reduce hypoxia have been worthwhile, they also serve as a reminder that greater reductions are needed to counteract the ever-increasing impacts of climate change.