Ocean 444 and Ocean 445 -- Senior Theses

Permanent URI for this collectionhttps://digital.lib.washington.edu/handle/1773/2306

These student-designed research projects, cover facets of all four oceanographic subdisciplines - Biological, Geological, Physical and Chemical Oceanograhy.

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Impact of the Island Mass Effect on Zooplankton Community Composition Around the NAM-2 Coral Atoll in the Western Pacific Ocean
(2025-03) Knopf, Julia
Phytoplankton acts as the base of the marine food web, providing energy to higher tropic levels such as zooplankton. Zooplankton are an intermediate part of the food chain that can provide evidence of an area with high productivity. The focus of this study was to investigate the Island Mass Effect (IME) and its impact on zooplankton communities around the NAM-2 Atoll located to the southeast of Guam. Data was collected on the TN440 research cruise on the R/V Thomas G. Thompson from December 29th, 2024, to January 11th, 2025. Zooplankton nets were deployed at five stations around the unmapped atoll, NAM-2, (09º08’40’’N, 148º07’50’’ E), and three stations on the 149ºE WOCE line in the open ocean, specifically at latitudes 5ºN, 9ºN, and 15ºN. Zooplankton metrics of abundance and diversity, current direction, temperature, nutrients, and bathymetry were measured around the NAM-2 Atoll and the open ocean to observe the biological productivity of the IME. The highest abundance of zooplankton throughout all of the sites were the calanoid copepods, followed by the cyclopoid copepods, and then the chaetognaths. There was a statistically significant relationship between copepods and their predator, chaetognaths. There was no statistical significance between zooplankton abundance and diversity between the atoll and open ocean sites and among the variables of temperature, nutrients, current direction, and bathymetry. These factors likely still impacted the abundance and diversity of zooplankton. These factors, along with the predator-prey relationship between copepods and chaetognath, may still indicate NAM-2 as a biologically productive area, according to the IME. Also, the varying current directions may bring different compositions of zooplankton to the study stations. Further research is needed to discover the complete picture of this location, which could eventually lead to contributions to Guam’s fishing economy.
Short-Term Regional Temperature and Salinity Prediction Based on Deep Learning Long Short-Term Memory
(2025-03) Lin, Kara
The El Nino-Southern Oscillation (ENSO) is the most significant year-to-year climate variation, affecting weather and climate systems worldwide. However, current prediction models, both dynamic and statistical, struggle with accuracy due to the complex mechanism of ENSO. This study introduces a regional temperature and salinity prediction method using a Long Short-Term Memory (LSTM) deep learning model, which is well suited for identifying long-term patterns in sequential data. The model is applied to three specific regions using in-situ data from Argo floats: the centraleastern Pacific, the central tropical Pacific Nino 3.4 region, and the Western Pacific Warm Pool (WPWP). These regions are chosen because they play key roles in ENSO dynamics. Results show that the LSTM model performs best in the WPWP, where the average mean squared error (MSE) is low (0.03), indicating high accuracy and stability. This is likely due to lower noise in the original data. In contrast, the model performs poorly in the central-eastern Pacific, where the average MSE is much higher (7.03), suggesting instability due to high noise in original data. These findings highlight the potential of deep learning for regional climate predictions and suggest that LSTM models could improve local weather forecasting and fisheries management.
Examining Ocean Mixing Dynamics at the Namonuito Guyot and Nam 2. Atoll
(2025) Wilson, Ken
This study investigates the impact of guyots and atolls on ocean currents and mixing processes in the Caroline Islands, Micronesia. The hypothesis is that atolls enhance vertical and horizontal mixing more effectively than guyots due to their surface-reaching morphology. To test this hypothesis, vertical profiling was conducted using underway CTD (Conductivity, Temperature, and Depth) and ADCP (Acoustic Doppler Current Profiler), along with calculations of mixed layer depth, stratification, mixing rate measurements, and Thorpe scale at Namonuito Guyot and an unnamed atoll near the guyot, referred to as Nam. 2 Atoll in this study. The findings reveal significant differences in mixing dynamics between the two features. The atoll exhibited stronger mixing and a more uniform mixed layer, driven by its interaction with surface currents and waves. In contrast, the guyot showed more stratified layers and weaker mixing, reflecting its submerged nature and limited interaction with surface processes. These results provide insights into the physical processes governing these geological features and their influence on ocean circulation, highlighting the distinct roles of guyots and atolls in shaping ocean mixing dynamics.
Effects of Atolls on the Distribution and Composition of Suspended Particles in the Western Tropical Pacific
(2025) Steedman, Ashleigh
Atolls are unique geomorphic structures that influence suspended sediment dynamics and microbial communities through their interaction with wave energy, currents, and limited landmass. These factors create localized patterns of suspended particles, yet their impacts on microbial activity and nutrient cycling remain poorly understood. This study investigates how atolls influence suspended sediments' vertical distribution and composition by examining differences in organic and inorganic particle concentrations at varying depths. I hypothesize that atolls promote the accumulation of fine organic matter and microbial activity near the reef, while larger inorganic sediments dominate farther away due to hydrodynamic forces. Fieldwork was conducted near Nam2 Atoll in the western Pacific, where transmissometer data, Conductivity, Temperature, and Depth (CTD) profiles, and water samples were collected across different locations. The composition of suspended particles was determined through microscopy while the proportion between organic and inorganic materials was quantified by the Loss on Ignition (LOI) method. Results indicate a significant decrease in the organic-to-inorganic ratio with depth, with the highest organic concentrations occurring in surface waters (20–100 m) and inorganic sediments increasing at deeper layers (>500 m). Statistical analysis revealed a T-statistic was 0.4129 while the p-value was 0.6889 however, showed no significant difference in organic-inorganic ratios between shallow and deep samples. The observed organic-to-inorganic ratio trends help clarify how organic carbon is stored and transported in marine environments which can inform climate models and global carbon budget estimates.
Zooplankton Diversity and Abundance in the Western Pacific Ocean
(2025) Parthasarathy, Aditya
Zooplankton plays a crucial role in marine ecosystems by linking primary producers, such as phytoplankton, to higher trophic levels. This study explains the relationship between zooplankton abundance and phytoplankton availability at different depths and times of day in the water near Guam. Sampling was conducted aboard the R/V Thomas G. Thompson at three stations N, 147 °E; N, 149 °E; and N, 148 °E three different depths, 20, 200, and 600 m 9◦4◦14◦using a 1 m ring net with a 211 μm mesh size, and CTD was used to analyze how nutrient availability, chlorophyll-a, temperature, and diel vertical migration (DVM) shape zooplankton communities. While chlorophyll-a and zooplankton abundance showed no correlation ( = 𝑅2-0.106, p = 0.932), temperature showed a strong predictor, with surface layers (0-20 m) exhibiting a significant positive correction ( = 0.568, p = 0.031). Nutrient trends revealed no 𝑅2statistically significant depth-related accumulation (p > 0.05 for nitrate, phosphate, and nitrogen). Station 18’s low zooplankton abundance, despite midday sampling coinciding with DVM, was linked to food limitation from depleted chlorophyll and nutrients. Calanoid copepods overwhelmingly dominated all stations and depths (mean abundance: 116.4 ), underscoring 𝑚−3their resilience to variable food and nutrient availability as well as their central role in the ecosystem. Diversity indices highlighted station-specific patterns: Station 8’s deep layer (0–600 m) had the highest Shannon diversity (H’ = 2.23), while Station 18’s surface layer showed the greatest Margalef richness (D = 6.06). These results emphasize the interplay of thermal stratification, localized productivity, and behavioral adaptations in structuring zooplankton communities. Understanding ecosystem dynamics like chlorophyll and nutrients is important for zooplankton responses to environmental change.
Nitrogen Isotope Analysis of Nitrogen Cycling in Coral Host Tissue and Algal Symbionts: A Study of Acropora Spp. and Stylophora Spp. near the Fulong Reefs in Taiwan
(2025) Lichter, Arden
This study investigated the differences in δ15N values across Coral Host Tissue, Algal Symbionts, and Whole Tissue Sample in two coral genera, Acropora spp. and Stylophora spp., collected from a harbor structure at the National Taiwan Ocean University Aquatic Biological Research and Conservation Center near Fulong, Northern Taiwan in August 2024. Nine coral heads were sampled, subdivided into tissue fractions, and analyzed for stable nitrogen isotope composition using the denitrifier method, which reduces the sample’s nitrate (NO3 -) to nitrous oxide (N2O). Using nitrogen isotopes (δ15N) as a nutrient tracer, this study explored three hypotheses regarding nitrogen cycling within the coral holobiont: the Host Coral provides nitrogen to the Algal Symbionts directly through the system, the Host Coral provides nitrogen to the Algal Symbionts indirectly through the system, or the Algal Symbionts do not receive nitrogen from the Host Coral but instead fix dissolved inorganic nitrogen from the environment. Significant differences in δ15N were found between Coral Host Tissue and Algal Symbionts, suggesting that corals feed their symbionts indirectly. Genus-specific differences were also observed, with Acropora spp. exhibiting higher δ15N values across tissue types compared to Stylophora spp., likely due to variations in trophic strategies, metabolic demands, and morphological characteristics. Additionally, this study found that Algal Symbionts have a greater influence on Whole Tissue Samples δ15N than Coral Host Tissue, reflecting their varying diet and nutrient uptake patterns. This study contributes to the growing body of knowledge on coral nitrogen cycling, emphasizing how nitrogen isotopes can illuminate the complex interplay between coral hosts and their algal symbionts.
Spatial variations of sea surface pCO₂ in the tropical Western Pacific near Guam
(2025) Lee, KyungJae
The ocean sequesters carbon through solubility and biological pumps. It serves a critical role in the sequestration of the atmosphere’s carbon dioxide (CO₂). Understanding the extent of its sequestration role is an essential tool in tackling climate change policies and protecting the oceans. This study aims to understand the small-scale variabilities with increased levels of anthropogenic CO₂ on the ocean’s role as a sink of CO₂. This was done by collecting underway temperature, salinity, sea surface partial pressure of CO₂ (pCO₂), atmospheric pCO₂ and conditions continuously on the 2024-2025 University of Washington Oceanography Senior Thesis Cruise between 4 °N-16 °N, 148 °E-149 °E. Small-scale spatial features of pCO₂ concentrations will be analyzed to find correlations with other parameters (temperature, salinity, bathymetry) to find the driving process of the variations in pCO₂. Highest pCO₂ concentrations were measured around the deepest sections of the Nam-2 Atoll and the furthest south. In contrast, the lowest pCO₂ concentrations were measured around the Namonuito Guyot and directly north of the 4 °N peaks, at 5.5 °N-6.2 °N. It was found that seamounts have a great influence on pCO₂ concentrations. Since pCO2 is relatively consistent in its vertical distribution, local mixing due to currents accelerating against the seamount structures stirs higher concentrations of pCO2 into the mixed layer. This makes these areas with seamounts sources of pCO₂. This will put the atmospheric pCO₂ concentrations into even higher saturation.
North Equatorial Current Influence on Ocean Mixing Dynamics in the Western Tropical Pacific Along the 149 E Transect
(2025) Ettari, Mei
This study examines the relationship between the strength of the North Equatorial Current (NEC) and local ocean mixing along the 149 E transect from 4 N to 16 N. Data was collected in the Western Tropical Pacific aboard the R/V Thomas G. Thompson from 29 December 2024 to 10 January 2025 using an Acoustic Doppler Current Profiler (ADCP) and Conductivity-Temperature-Depth (CTD) sensor. Zonal velocity data from the upper 700 m revealed distinct eastward and westward velocity patterns. The NEC was present from 8 N to 16 N, with peak velocities between 9 N and 12 N in the upper 200 m. The North Equatorial Counter Current (NECC) was observed between 4 N and 6 N in the upper 200 m. Zonal jets were noted at 10 N to 11 N and 13 N to 14 N, with an eddy observed at 15 N. The mixed layer depth (MLD) averaged 82 m across the transect. Although no clear relationship between MLD and NEC strength was found, velocity patterns suggested significant variations in mixing and stratification. Depth profiles of buoyancy frequency (N²), shear magnitude, and Richardson number (Ri) highlighted regions of shear-driven turbulence, particularly where Ri values fell below 0.25, indicating potential vertical mixing. This effect was predominantly observed in the mixed layer, with only one station showing deeper ocean mixing. The shear in the region was not strong enough to overcome buoyancy forces, suggesting a strongly stratified water column that inhibits vertical mixing. These findings underscore the need for further research on the NEC’s impact on shear and buoyancy interactions in ocean mixing dynamics.
Investigating Mechanisms Driving Spatiotemporal Variability of Barrier Layers in the Western Tropical Pacific
(2025) Almokharrak, Jood
This study investigates the physical mechanisms driving spatiotemporal variability of barrier layers in the Western Tropical Pacific (WTP) along 149°E, with a specific focus on the La Niña phase of the El Niño-Southern Oscillation (ENSO). Barrier layers, which separate the surface mixed layer from the thermocline, regulate ocean-atmosphere interactions and influence climate dynamics. This research assesses the relative contributions of freshwater input from precipitation, and wind stress on barrier layer formation and thickness. Data were collected during a research cruise in January 2025 aboard the R/V Thomas G. Thompson from an Underway Conductivity Temperature and Density (UCTD) sensor for temperature profiles, and public-source meteorological data for atmospheric conditions (ERA5). Seven stations, spaced two degrees apart in latitude, were sampled along a transect from 4°N to 15°N. Each station provided data to analyze barrier layer thickness, with spatiotemporal variability determined by comparing different formation mechanisms across stations. Spearman Correlation analyses were used to determine dominant factors influencing barrier layer thickness and variability. We found that barrier layer thickness in the WTP shows a general positive but statistically insignificant relationship with freshwater (ρ 0.32 and p-value 0.48), and a general negative but statistically insignificant relationship with wind stress (ρ 0.18 and p-value 0.70). During La Niña conditions, these effects are expected to drive variability, with thicker layers forming in regions of high precipitation and weak wind stress. Increased freshwater input enhances stratification, while strong wind stress likely promotes surface and subsurface mixing, leading to barrier layer thinning. Understanding these dynamics has implications for improving ocean-atmospheric interaction climate models in the tropical Pacific.
Evaluation of erosional processes and determining if atolls have aided the formation of polymetallic nodules Caroline Islands, Federated States of Micronesia’s Exclusive Economic Zone
(2025-06-27) McMillen, Suni
This project aimed to identify the role nearshore sources of metallics play in the formation of polymetallic nodules in the abyssal planes of the Western Equatorial Pacific. Polymetallic nodules also known as ferromanganese nodules are valuable seafloor features that contain high levels of rare earth metals, of which most notable are manganese and cobalt. The incentive to mine these nodules for their metallics has increased due to the advancement of technology, and demand. The site location for this study was within the exclusive economic zone of the Federated States of Micronesia, and specifically Nam2 Atoll. Three sediment grab samples were collected and an analysis of the historical undercurrents in the region were conducted. X ray fluorescence spectroscopy was performed on the sediment samples. Two locations contained detectable levels of manganese and one contained cobalt. A particle settling analysis conducted on the sediment grains using Stokes law, calculated the distance of possible travel using the historical undercurrent to model the trajectory of the grains. A conservative time needed to reach the atoll, utilizing only the undercurrents, is 71.2 days. The largest grains were calculated to settle in 19 days while the smallest could stay suspended for several millennia. This study site has had evidence of turbidity currents since the Miocene, and by comparing studies it was determined that a turbidity current from Nam2 could reach approximately 400 km to sites on the abyssal plain. Thus, via a turbidity current, all grain sizes collected could reach the abyssal plains. This study suggests that Co-rich/Mn-rich sediment found at Nam2 Atoll within a wide range of grain sizes, could aid in the formation of polymetallic nodule, (PMN). This would be made possible through transport via undercurrents for smaller sizes and the effects of a large sediment gravity flow for larger grain sizes. These findings potentially increase the locations where mineable amounts of polymetallic nodules could be found globally.
The Seasonal Zonal Equatorial Undercurrent Strength variation with ENSO
(2024-03-08) Shao, Yanfeng
The Equatorial Undercurrent (EUC) is an important current both physically and biologically. It is not only key to equatorial circulation but also closely related to primary production at the Equator. Seasonal variation of EUC is caused by the seasonality of the equatorial thermocline. This study retrieves in situ EUC velocity data using an ADCP from the TN427 cruise, traveling from 5°N to 5°S at 170°W in December and January, from 2023 - 2024. During the first crossing through equator, the maximum EUC velocity was measured to be 0.42 ! " and mean EUC velocity was 0.02 ! " . Calculated transport equals to 10.77 Sverdrup. During the second crossing, the velocity is measured to be 0.59 m/s with 0.04 ! " mean velocity. The transport for the second crossing was 14.78 Sverdrup. The maximum EUC velocity at the beginning of 2024 is much lower than the value in 2023, which was caused by the increase in upwelling during El Niño. According to the most recent ENSO SSTA indices of Niño 3.4 from NOAA State of Ocean Climate, the peak of Niño 3.4 index was in December 2023 and January 2024, followed by a decrease in Niño 3.4 index. Future prediction states that an increase of maximum EUC velocity will be observed within the next few months.
Comparison of Geostrophically-Derived and ADCP Current Velocity in the Upper 600 Meters of the Equatorial Pacific
(2024) Robertson, Kayla
Geostrophic balance, a fundamental concept describing the equilibrium between Coriolis forces and pressure gradients, influences current patterns in the ocean. However, at the equator, the absence of Coriolis forces impacts current predictions using standard equations. Measurements were taken on research cruise TN427 on the R/V Thomas G. Thompson, from 28 December 2023 to 11 January 2024. On a transect along 167°W, Conductivity, Temperature, and Depth (CTD) measurements are taken at degree intervals between -5°- 5° and in half degree intervals between -1°-1°. These measurements are supplemented with Underway CTD in quarter degree intervals between -2°-2° to have an increased resolution between these latitudes. Using the Thermodynamic Equation of Seawater, dynamic height is calculated using the dynamic height anomaly equation and used as a geostrophic streamfunction to predict velocities. Measurements from CTD casts are used to calculate the vertical profile of geostrophic velocity using the relative geostrophic velocity equation and compared to Acoustic Doppler Profiler (ADCP) measurements, which have been averaged at midpoint latitudes. These two vertical profiles are compared to determine the latitude-dependent variability in Root Mean Squared error values. Results indicate that as latitudes approach the equator, discrepancies between calculated geostrophic velocities and measured ADCP velocities increase, as evidenced by a latitudedependent rise in root mean square (RMS) error values. External factors such as Equatorial Counter Current (ECC) and North Equatorial Counter Current (NECC) contribute to these discrepancies. This paper addresses the lack of Coriolis fore at the Equator by comparing calculations and measured velocity profiles to determine the error between them, identifying the accuracy of predictions made using latitude while nearing the Equator.
Assessing Potential for Chromatic Acclimation in Oxygen Deficient Zone Synechococcus
(2024) Roberts, Alex
Synechococcus are a genus of ubiquitous marine cyanobacteria that play an important role in global carbon cycling. Synechococcus are abundant in oxygen deficient zones (ODZs)—expanding areas of the ocean where oxygen concentrations drop below 10 nM O2—where they fix carbon and introduce trace amounts of oxygen through photosynthesis to fuel aerobic respiration. Synechococcus can be grouped into different pigment types based on the makeup of the phycobilisome, a photosynthetic light-harvesting complex. Marine Synechococcus are predominantly either blue-light specialists, green-light specialists, or light-generalists. Light generalists undergo a process called chromatic acclimation where their phycobilisome structure is modified in response to changing light conditions allowing maximal growth under different colors of light which provides an advantage as a photosynthetic cell is moved throughout the upper water column. In this study, I use 10 strains of Clade 1 and CRD1 Synechococcus isolates and metagenomic data to determine the abundance, diversity, and potential for chromatic acclimation of Synechococcus in the Eastern Tropical North Pacific (ETNP) ODZ. I performed growth experiments on the Synechococcus isolates under white, blue, and green light finding that seven of the strains were able to chromatically acclimate while three were blue-light specialists. Through phylogenetic analysis, I found that Synechococcus in the ETNP ODZ fall into eight distinct groups that constitute 2 – 17% of the microbial community in the upper 50 meters of the water column. These results indicate that Synechococcus capable of chromatic acclimation are present and possibly abundant in the ETNP ODZ, comprising a potentially important contribution to cryptic oxygen cycling supporting aerobic heterotrophic communities.
Zooplankton Diversity and Community Composition along 167°W in the Equatorial Pacific
(2024-03-09) Reynolds, Nicole
Zooplankton are important primary consumers in the marine food web and lead an important role in carbon cycling in the open ocean. Understanding what influences zooplankton community composition can help us understand the impacts of climate change on this delicate relationship. Data was collected from 28 December 2023, through 10 January 2024, on the R/V Thomas G. Thompson near American Samoa between 5°S and 5°N along 167°W. A closing zooplankton net with 200μm mesh was used for net tows from 200m to surface at stations between 5°S and 5°N along the 167°W longitudinal line. Zooplankton abundance was highest at the equator with 345 organisms m-3 and increased from 5°S to the equator; and decreased from the equator to 5°N in a bell-curve shape. Species diversity (Shannon-Weiner) was lowest at the equator (0.878) and highest at 1°N (1.067) and 5°N (1.059). Calanoid copepods had the highest abundance over all sites (74-88% of composition), and north of the equator, calanoid copepods and gelatinous zooplankton (larvaceans) dominated most of the species composition. There were no significant relationships between species community composition and temperature, salinity, or nutrients. Results demonstrate that higher water temperatures and different current regimes impacted abundance and species presence during the 2023 – 2024 Strong El Niño. With many processes occurring with zooplankton in the open ocean, it may be that multiple variables are impacting the resulting diversity and abundance relationships. Monitoring zooplankton composition over time is vital for monitoring the health of our oceans as it has implications for global fisheries and carbon cycling.
Impact of Silicate Limitation on Diatom Mortality in the Equatorial Pacific Ocean
(2024-03-08) Prado-Casillas, Kristine
I explored how Thalassiosira oceanica cells responded when an infectious agent from the Equatorial Pacific Ocean entered its cells under silicate limited conditions. T. oceanica is an open ocean phytoplankton, specifically a diatom, which creates its cell walls out of silicate. T. oceanica is a model organism because of its small yet fully sequenced genome and fast growth rate. Research in host-virus dynamics in diatoms is new, especially in open ocean environments. Viruses play a significant role in the biogeochemical cycle, the movement of nutrients and elements through biotic and abiotic factors, by adding organic matter of diatoms to the microbial loop. They do so through a reproductive process called the lytic cycle when enough viruses replicate within a cell and cause it to burst or lyse. In the Equatorial Pacific Ocean, it is known that iron is the limiting factor inhibiting phytoplankton growth. However, it is hypothesized that silicate is the limiting factor instead of iron in this region. A recent study has found that silicate limited regions increase diatom virus infection due to thin cell walls, allowing for viruses to enter easily. However, research on marine viruses in open ocean environments like the Equatorial Pacific Ocean has yet to be conducted. I hypothesize that a silicate limited environment will increase diatom virus infection in the Equatorial Pacific Ocean. The results from growing infected T. oceanica in well plates display that 29.7% of dying and uncertain wells in silicate limited conditions died more than in replete conditions, representing only 1.6%. Higher mortality rates of T. oceanica can impact the silicate cycle and carbon export rates in the surface oceans to decrease over time while increasing the viral shunt.
Bacterial Phenotypes of the Equatorial Pacific in the Presence of Different Phytoplankton Metabolites
(2024-03-08) Poyen, Zachary
Metabolites are compounds released by phytoplankton, and are major sources of carbon and other essential nutrients for heterotrophic bacteria living in the pelagic ocean, which form the base of marine food webs via the recycling of nutrients. The goal of my experiment was to determine how the prevalence of different phytoplankton metabolites can influence heterotrophic bacterial community composition, with a focus on the equatorial Pacific region. My hypothesis was that different bacteria would show increased growth on media enriched with certain metabolites, and decreased growth on other media enriched with other metabolites. This was achieved by collecting seawater samples from three locations along a transect of the equator, which were then concentrated and plated on agar plates enriched with some of the more common phytoplankton metabolites: Glycine Betaine (GBT), Dimethylsulfoniopropionate (DMSP), Spermidine, Glucosamine, and Sucrose. A complete nutrient option (1/2 YTSS) was used as a control. These plates were assessed for bacterial growth via Colony Forming Units (CFU’s) after a 5-day incubation period. My results showed a northward trend of increasing phenotypic diversity, as well as higher numbers of CFU’s in locations north of the equator. There was also significant variation amongst the different metabolites in regard to the species of bacteria (based on colony phenotype) that grew and the number of CFU’s present. There were some consistent phenotypes across all media types. Based on these results, the metabolites present in the water column may be an important determining factor in the composition of the heterotrophic bacterial community.
Pioreactors at Sea: Assessing Microbial Community Response to Added Dissolved Inorganic Nutrients
(2024-02-02) Page-Roth, Jackson
This study aims to understand interactions within the microbial community of the oligotrophic (nutrient-poor) equatorial Pacific. This experiment attempts to provide a more realistic representation of in-situ conditions, overcoming previous difficulties capturing the dynamic behavior of microbial communities in the field. A novel methodology utilizing a continuous media supply to incubate natural microbial communities from the field accomplishes this objective. Three growth chambers were employed, one as a continuous incubation control (no nutrients added), one as a continuous incubation with added inorganic nutrients (nitrate, phosphate, and silicate), and one as a batch incubation with the same added nutrients. A continuous incubation constantly exchanges a small volume of external media from outside the growth chamber with volume from inside the growth chamber. A batch incubation does not have any media exchanged during its incubation period. All incubations followed a 16:8 light/dark period using LEDs, simulating environmental day-night cycles. Community response was assessed by monitoring the turbidity of the incubation with continuous optical density measurements, with beginning and endpoint subsamples analyzed for total bacterial abundance and DNA content using flow cytometry. There was a distinct day-night turbidity oscillation in all incubations, with the nutrient-enriched continuous incubation being the most pronounced (average normalized optical density 10% greater during simulated daytime). At the conclusion of the experiment, the continuous incubation without added nutrients had about double the bacterial cell concentration and a greater bacterial DNA content, yet similar optical density values compared to its nutrient-added counterpart. This combination of results suggests that the added nutrients selected for larger, slower growing bacteria, such as the picophytoplankton Synechococcus, which would align with previous studies. Additionally, the different turbidity oscillations between incubation methods suggest that nutrient replenishment results in a greater microbial response to diel variability. This study not only advances our understanding of microbial community dynamics in the oligotrophic equatorial Pacific but also introduces a novel experimental method that can be applied across a diversity of marine and aquatic environments.
Comparing microplastic distribution and potential risk to zooplankton in American Samoan coastal waters and the Equatorial Pacific
(2024) Olson, Isaac
Microplastics (MPs) are a growing threat to marine organisms due to risks of ingestion and toxin bioaccumulation. Zooplankton are especially vulnerable to MPs and are important as a key piece of the trophic web. However, MP abundance, type, and effect on zooplankton communities in the southwest equatorial Pacific remains understudied. Using data from a cruise on the R/V Thomas G. Thompson between December 28th, 2023 – January 10th, 2024, this study analyzes MP abundance and type, and compares it to zooplankton relative abundance to determine MP pollution and risk to zooplankton populations around American Samoa and the Equatorial Pacific. I hypothesized that higher levels of microplastic pollution would be found near American Samoa than at the Equatorial Pacific. I further hypothesized that a correlation exists between MP abundance and relative abundance of zooplankton species at-risk of MP ingestion. Zooplankton and MP samples were collected near American Samoa and the Equatorial Pacific using a 333 μm mesh Manta net. A dissecting scope was used to identify and count zooplankton and MPs. In comparing MP type and distribution, I found that the average MP concentration at coastal stations was 5.7 x 10-3 MPs/m3, while the average MP concentration at equatorial stations was 1.3 x 10-4 MPs/m3. 88% of all MPs collected were microfibers. However, no correlation between zooplankton abundance and MP abundance was found. Further research in the region is critical due to the direct connection to waste, quality of life, industry, subsistence, and ecosystem and human health in the tropical Pacific.
Quantifying Velocity, Chlorophyll, Temperature, and Nutrients Relationships with Primary Productivity in the Western Equatorial Pacific
(2024-03-08) Nguyen, Emma
Net primary productivity (NPP) and gross primary productivity (GPP) is a major component of the carbon cycle. NPP is defined as the amount of carbon biomass produced by primary producers over a given period of time and area. GPP is the combination of NPP and respiration. The NPP exceeds 100 billion tons of carbon per year on Earth and half of it comes from the ocean through phytoplankton. The equatorial Pacific Ocean is the largest tropical ocean on Earth and subsequently the largest oceanic source of CO2 to the atmosphere. Despite its importance, NPP in the west equatorial Pacific is poorly characterized due to the lack of data. Previous research suggests that strong upwelling is associated with increased nutrient concentration in the euphotic zone leading to an increase in primary productivity. However, the western equatorial Pacific is known for weaker upwellings compared to the eastern and central equatorial Pacific. This study was conducted aboard the R/V Thomas G. Thompson from December 28, 2023, to January 12, 2024, with the goal of identifying and quantifying the critical variables that have a substantial impact on NPP in the region including temperature, chlorophyll, dissolved nutrients, and current velocity. NPP and GPP was measured using in-situ oxygen incubations. The mean GPP found in this study range from −163.82 to 26.05 mmol O2 m−2 d−1. A combination of temperature, chlorophyll, dissolved nutrients, and current velocity can explain for 54.18% of the variance in GPP. There is a missing 45.82% not accounted for that requires further studies on factors that influence GPP such as iron, which is a limiting nutrient.
Characterizing Manganese Cycling During Estuarine Mixing
(2024-03-08) Jenness, Sophie
Manganese (Mn) is a trace element essential for life. In the environment, it can exist in three states (Mn(II, III, IV)), with Mn(II) being dissolved, Mn(IV) being particulate, and Mn(III) being particulate unless stabilized by organic molecules known as ligands. This project aimed to understand how Mn cycles between its three states during mixing of the Mississippi River and the Gulf of Mexico through a mixing experiment designed to mimic the salinity gradient. The proportion of dissolved Mn(III) may have been influenced by the salinity gradient, although rates of ligand promoted reduction of MnOx did not vary along it. These have broader implications for how the transition from freshwater to seawater impacts the transport of Mn, and establish a better understanding of how the salinity gradient influences the phase of Mn. Having better insight into how Mn cycles in the Mississippi River delta is important because of the broader implications it has for the chemistry of the Northern Gulf of Mexico

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