RSS 1.0Oceanography
Recent Submissions
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Sea ice strain, stress, and fracture activity at kilometer scalesThrough-thickness fractures including cracks, leads, and pressure ridges divide sea ice into individual plates and plate assemblies. While they remain intact, plates deform via continuous strain as they interact. Radar interferometry can identify active fractures at plate assembly boundaries and measure the continuous strains between them at high spatial resolution and spatial scales from meters to kilometers. However, interferograms are only sensitive to the one dimensional component of surface strain parallel to a radar’s lines of sight. Working with coauthors, I develop a novel analytical inverse model to estimate two-dimensional, horizontal surface motion from the one-dimensional information provided by interferograms over areas of ice experiencing spatially uniform strain. Model results accurately capture thermal strain in sheltered landfast ice and realistically estimate rigid displacements in drifting ice. In areas of non-uniform strain, we combine one-dimensional interferometric strain measurements with field observations from a sea ice camp in the Beaufort Sea to investigate relationships between strain, stress, and fracture activity. We calculate the first published estimates of the effective elastic modulus, E*, and effective Poisson’s ratio, v*, of in situ drifting sea ice under natural loading rates. We estimate E* within the range typically used in sea ice models but estimate v> > 0.5, larger than typically assumed and indicative of anisotropy in sea ice Poisson response at low strain rates. Finally, we synthesize interferometric records of strain and fracture to identify an approximately 1 km radius of influence of impact forces resulting from contact across active fractures. We also identify apparent fracture reactivation after multi-day quiescent periods, indicating prior fractures may remain weaker than surrounding ice for such periods. Together, this work outlines both new observations and new tools for future researchers to utilize in studies of sea ice mechanics and dynamics at intermediate scales in areas of high-concentration winter pack ice.
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Sea-ice bloom dynamics, putative parasitoids, and crude-oil impacts to microbiota modified by irradianceArctic sea ice serves as a critical habitat for microbial communities, supporting, in part, the Arctic food web and influencing global biogeochemical cycles. However, climate change is rapidly altering this environment leading to a shorter ice-covered period and increased light transmittance. Greater accessibility due to less ice coverage is increasing industrial and commercial activities which raises the risk of crude oil spills that threaten sea-ice microbial communities. This dissertation explores the seasonal dynamics of prokaryotes and unicellular eukaryotes during the spring algal bloom, and the impacts of potential light stress in combination with crude oil exposure on Arctic sea-ice diatoms and microbial communities with time-series data and laboratory-based experiments. Chapter 2 examines microbial community succession during the 2021 vernal ice-algal bloom near Utqiagvik, Alaska. The bloom reached was larger than previously observed in this region. An unprecedented bloom of the oil-degrading bacterium Oleispira suggested potential environmental hydrocarbon contamination. Metabarcoding and co occurrence analyses revealed that diatoms, particularly Nitzschia spp., were primary hosts for parasitoid taxa such as chytrids Cryothecomonas and, highlighting the potential for top-down control of algal populations and the maintenance of diversity. Chapters 3 & 4 investigate the interactive effects of crude oil exposure and irradiance on Arctic sea-ice diatoms. These experimental results show species-specific responses to oil, with Fragilariopsis cylindrus being highly sensitive, while Synedropsis hyperborea exhibited stimulated growth at low oil concentrations. (Meta)transcriptomic analyses in Chapter 4 revealed that oil exposure induced a switch of metabolism in diatoms from autotrophic to catabolic, particularly in pathways related to lipid degradation. The findings suggest that oil spills may favor flagellates over diatoms, shifting microbial community composition with potential consequences to biogeochemical cycles. Together, these studies provide novel insights into the importance of internal lipid reserves, alternative metabolic pathways, and microbial interactions in supporting microalgal resilience within the Arctic sympagic ecosystem.
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Fine-scale spatial patterns of gelatinous zooplankton in the Northern Gulf of AlaskaThe Northern Gulf of Alaska (NGA) is characterized by high variability across spatial and temporal scales. In the NGA, zooplankton are a crucial link between primary production and higher trophic levels. Understanding the mechanisms that structure zooplankton assemblages is important to our overall understanding of ecosystem functioning. Nonetheless, thorough description of zooplankton abundance and distribution patterns is challenging due to the inherent variability and complexity of the marine environment. The study of gelatinous zooplankton is further complicated by the limitations of traditional plankton net sampling methods that are inefficient for the collection of high-resolution spatiotemporal data and often inflict damage on these fragile bodied organisms. In the NGA, and many other ocean systems, this has historically left gelatinous zooplankton under sampled and poorly studied in comparison to cooccurring crustacean zooplankton. To address these challenges, recent advances in imaging technology and computing power were leveraged by deploying an In Situ Ichthyoplankton Imaging System Deep-Focus Particle Imager (ISIIS-DPI) in the NGA from 2022-2023. The ISIIS-DPI is a towed vehicle capable of collecting vast amounts of high-resolution imaging and oceanographic data. An analysis pipeline with convolutional neural network (CNN) architecture was employed to automate the identification of zooplankton images and expedite processing time, allowing for description of fine-scale distributional patterns of gelatinous zooplankton and their associations with surrounding biophysical drivers. Evidence is presented that ctenophore, hydromedusae, and siphonophore aggregations are concentrated around frontal features and track with the surrounding variability in their ocean environment. Several first records in the NGA of previously undetected species are also presented. These novel datasets demonstrate the previously underestimated prominence of gelatinous zooplankton in the NGA and improve our understanding of ctenophore, hydromedusae, and siphonophore abundance and distribution patterns in the context of their oceanographic environment. This work is the first adaptation of in situ imaging and machine learning technologies in the NGA and presents the opportunity to more accurately describe the role of gelatinous zooplankton in marine ecosystem function.
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Transformations and deep intrusions of particles and plankton in the global oceans: which particles sink deeper and whySinking marine particles transport carbon from the ocean’s surface to the deep ocean, thereby contributing to atmospheric carbon dioxide modulation and benthic food supply. Many studies have shown that particle size is not a good predictor of particle sinking speed or behavior. Thus, the overarching question of this dissertation: why do certain particles sink faster or deeper than others, and is there a way to predict what depth a particle will reach in the ocean? Multiple facets of the ocean’s biological carbon pump are investigated using a combination of sediment traps, in situ particle imaging, and machine learning technology. In the Gulf of Alaska, we find aggregates contributed 61% to total carbon flux, suggesting that aggregation processes, not zooplankton repackaging, played a dominant role in carbon export. The role of the physical environment on the biological carbon pump was investigated in the Southern Ocean. Fluffy aggregates and grazers were most common at the surface during a phytoplankton bloom, whereas 1-3 months after a bloom, grazers are in the mesopelagic and feces and dense aggregates are in high abundance in the bathypelagic. These results shed light on how frontal structures in the Southern Ocean influence patterns of particle export and remineralization in the mesopelagic with implications for how this influences global biogeochemical cycles. Finally, the effect of biogeochemical province and carbonate saturation state was investigated in the tropical and subtropical North Atlantic and Pacific. We find that plankton distribution and marine particle morphology in the Atlantic Ocean are more strongly impacted by aragonite and calcite saturation state, despite much shallower saturation horizons in the Pacific. This research can help better predict how the strength of carbon storage in the ocean may change with climate change, which is critical for climate modelers to predict the effects of climate change more accurately.
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Seasonal marine inorganic carbon dynamics on the Northern Gulf of Alaska continental shelfThe Northern Gulf of Alaska supports many socioeconomic and ecosystem services but is subject to increasing ocean temperatures, marine heatwaves, and freshwater runoff, harmful algal blooms, and ocean acidification. In addition, large natural variability in biological and physical drivers complicates characterization of the progression of ocean acidification and variability in seawater inorganic carbon conditions. Here, we present two years (2019-2021) of in situ moored upper-ocean partial pressure of carbon dioxide (pCO2) and pH data on the outer continental shelf of the Northern Gulf of Alaska. The observations show that subsurface pCO2, pH and aragonite mineral saturation state (Ωarag) were highly seasonal, although generally remained at a moderate level with pH varying between 7.9 and 8.2, and Ωarag > 2 throughout the year. The influence of biogeochemistry, either from in situ or vertical exchange changes, strongly drove pCO2, pH and Ωarag anomaly variations throughout the year, while temperature also exerted a strong influence on pCO2 and pH. The thermal and biogeochemical drivers generally compensated each other, lessening the amplitude of seasonal variations; exceptions generated the highest and lowest pH and pCO2 conditions of the year. In spring, primary production and seasonally cold temperatures led to the highest pH and lowest pCO2 of the year. The lowest pH and highest pCO2 of the year occurred in fall as high-frequency events when subseasonal mixing events entrained deeper, CO2-enriched water, coincident with seasonally warm temperatures. Air-sea CO2 flux calculations suggest a greater wintertime atmospheric CO2 source than previously measured. This work complements other ongoing hydrographic and nearshore monitoring, modeling, and experimental work needed to understand the regional progression and impact of ocean acidification on this variable and changing ecosystem.
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A systematic characterization of Southeast Alaska’s oceanographyContinued anthropogenic warming of the world’s surface oceans will lead to increased thermal stratification and decreased vertical nutrient supply, resulting in an expected decline in the rate of photosynthetic carbon production. Decreased primary productivity will alter the marine food web, and may impact subsistence, commercial, and recreational fisheries in Southeast Alaska (SEAK), which are important sources of food and income for local communities. Historic oceanographic data in SEAK are spatiotemporally sparse, and consistent measurements are necessary to understand present patterns and predict long term change. We analyzed the physical, chemical, and biological oceanography of SEAK from 2022-2024, initiating what is set to be consistent, longitudinal data collection in this region. We characterized local environmental parameters with systematic vertical profiles (n > 800) of temperature, salinity, and chlorophyll, alongside discrete water samples (n > 600) for macronutrient analysis. Results show spatiotemporal (both seasonal and interannual) variability in all parameters, with 2022 showing generally warmer, fresher, and more nutrient rich conditions in near surface waters (< 30 m). Evidence of nitrification was detected from the timing of peak concentrations of ammonium, nitrite, and nitrate at 30 m, suggesting that Southeast Alaska resembles a more closed system than the Gulf of Alaska. Surface nutrients were also compared to the physical and geographical parameters that may influence variability and it is evident that strongly stratified regions with high volumes of freshwater input have greater differences between surface and deep nutrient concentrations. The oceanographic setting of SEAK underpins local primary productivity, and subsequently the spatiotemporal distribution of fish and megafauna. This work represents a critical first step in the establishment of a comprehensive baseline understanding of SEAK oceanography, and may help to inform local communities about the mechanisms that impact the ecosystems upon which they rely, helping them prepare for future change.
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Climate influences emerge within two decades of observations for pelagic tunicates and pelagic snails in the northern Gulf of AlaskaMucus-net feeders are under-appreciated organisms that can have large impacts on how organic matter moves through food webs. In the Northern Gulf of Alaska and Prince William Sound they are represented primarily by pelagic tunicates (larvaceans, doliolids, salps) and pelagic snails (pteropods). Over a 20-year time-series (2001-2021) there was strong seasonality of larvacean abundance and species composition. Spring was dominated by Oikopleura labradoriensis and Fritillaria borealis, with higher diversity during autumn, due largely to the periodic presence of warm-water species. There was a distinct cross-shelf gradient with Oikopleura dioica, spreading across the lower salinity shelf during the autumn, and Fritillaria species becoming more prominent at offshore stations. Up to 45% of individual species and 19% to 28% of the tunicate community variation could be explained by physical variables (Temperature, Salinity, Chlorophyll α) and climate indices (ENSO, PDO, NPGO) during the spring and autumn, respectively. There were pronounced shifts in species composition and abundance during marine heatwaves. Limacina helicina showed a negative relationship to climate indices, with strongest correlation to the North Pacific Gyre Oscillation during spring. These long-term species-level records in subarctic waters provide improved understanding of how the mucus-net feeding community may continue to shift in a changing ocean climate.
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Lagrangian surface drifter analyses from observations and numerical modeling in the subpolar North AtlanticLagrangian surface drifters are powerful tools to study the dynamics of the ocean across a variety of spatial and temporal scales, ranging from regional to global and monthly to climatological, respectively. This dissertation investigates the utility of Lagrangian surface drifters for estimating the mechanical input of energy into the ocean by the atmosphere, and for gathering information about the underlying dynamics driving oceanic variability. The basis for the analysis was a large dataset of 88 surface drifters deployed in the subpolar North Atlantic between 2018 and 2019. In addition, numerical drifters from both idealized and realistic ocean models were used to supplement the observations. The study region is characterized by pronounced mesoscale eddy activity and, due to its proximity to the North Atlantic storm track, strong atmospheric storms causing energetic near-inertial oscillations. It is hence well-suited for the analyses presented here. We introduced a novel surface drifter instrument, the Minimet, that measures sea surface wind in situ along the drifter track. Estimates of in situ Minimet wind power input were found to be over 40% higher than those using a reanalysis wind product. This discrepancy was likely due to Minimets accurately capturing strong high-frequency wind events that were misrepresented in the reanalysis product, highlighting the utility of the Minimets for both wind power input calculations and the important validation of gridded wind products. We currently lack a basic understanding of the Lagrangian velocity frequency spectrum and how it relates to the underlying dynamics. We therefore investigated the Lagrangian spectral shape and found significant variability linked to eddy kinetic energy. Lastly, we established a direct link between the Lagrangian velocity frequency spectrum and Eulerian kinetic energy wavenumber spectrum. This link had not previously been made from single particles and together with a better understanding of the Lagrangian frequency spectrum furthers our ability to efficiently utilize Lagrangian data.
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Shifts in microbial community composition during the 2019 Pacific marine heatwave in the northern Gulf of AlaskaClimate change has led to a gradual increase of sea surface temperatures in the Northern Gulf of Alaska (NGA) interspersed with marine heatwaves (MHW) that impose a rapid but temporary perturbation of sea surface temperature. MHWs have the potential to alter marine microbial community structure, which may impact the production and transfer of carbon to higher trophic levels. The year 2019 was characterized as an MHW in the North Pacific, with sea surface temperatures in the NGA reaching ~2.5 º C above average during 2019 and ~1 º C above average during 2020, while 2021 had near-average sea surface temperatures. To characterize shifts in the NGA's microbial community, samples for DNA and flow cytometry were collected on NGA Long Term Ecological Research cruises in summers 2018-2021. Flow cytometry sample analysis revealed higher abundances of picoeukaryotes, Synechococcus, and nanoeukaryotes in the summer of 2019 relative to 2020 and 2021 on the continental shelf. The diversity of eukaryotic microbes was lower during 2018-19 than 2020-2021, with similar patterns observed within the diversity of individual eukaryotic taxa. Conversely, the diversity of prokaryotic microbes was higher during 2019 than 2020. Different environmental conditions were correlated with small cell abundance and microbial diversity. Elevated picoeukaryote abundance was associated with higher temperature and inversely correlated with chlorophyll a concentration, while Synechococcus abundance was anti-correlated with the concentration of nitrate and phosphate. Shannon diversity of 18S reads correlated with lower salinity measurements while Shannon diversity of 16S reads was not significantly correlated with any tested biological or environmental variables. These correlations indicate that increases in sea surface temperature, along with associated changes in nutrient concentrations and salinity, act as environmental drivers with the potential to shifts the NGA's microbial community structure. Such a community shift towards pico-nanophytoplankton may reduce trophic transfer efficiency and decrease the production of fisheries and other higher trophic levels in a warmer NGA.
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The impact of submarine groundwater discharge on nutrient dynamics in a sub-polar mudflatSubmarine groundwater discharge (SGD) plays a major role in the hydrological and biogeochemical cycles controlling nearshore nutrient availability. The Northern Gulf of Alaska coastline is highly diverse, ranging from rocky beaches, sandy beaches, and mudflats; SGD varies according to sediment permeability, strength of wave pumping, and slope of the water table. SGD has been previously estimated at a rocky beach in the NGA, but this thesis sought to quantity SGD in an extensive mudflat using well-established tracers (radium and radon) and determine the major sources of nutrients to the bay. The rate of SGD was comparable between the mudflat and rocky beach (233 ± 245 and 260 ± 360 cm day⁻¹, respectively), and both locations were significant sources of nitrate and silicic acid, and sinks of phosphate. Offshore water also provides a major input of nitrate and phosphate to the bay. Thus, there is no single dominant source for all macronutrients, consequently, multiple processes must be considered when predicting nutrient cycling.
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Acoustic detection and characterization of sea ice and surface waves in the northeast Chukchi SeaMonitoring the status of Arctic marine ecosystems is aided by oceanographic moorings that autonomously collect data year-round. Near Hanna Shoal in the northeast Chukchi Sea, the Chukchi Ecosystem Observatory moorings include an ASL Environmental Sciences Acoustic Zooplankton Fish Profiler (AZFP) datalogger, a multi-frequency upward-looking sonar that is programmed to collect data from across the upper 30 m of the water column every 10-20 seconds. Using six years of nearly continuous data, here we describe a statistical analysis of the datalogger's 455 kHz acoustic backscatter return signal. When used in conjunction with a selforganizing map machine learning algorithm, these data allow us to accurately differentiate between the presence of sea ice and open water and characterize surface waves. The approach detects short-duration (e.g., 15 minutes or longer) sea ice leads that pass over the mooring in winter, and sparse ice floes that pass over in summer. The ability to algorithmically identify small-scale features within the information-dense acoustic dataset enables rich characterizations of sea ice conditions and the ocean surface wave environment. Example applications include quantifying the recurrence of leads during ice-covered seasons, sparse ice in otherwise open water, statistics of ice keels and level ice, and wave height statistics. By automating the acoustic data processing and alleviating labor- and time-intensive analyses, we can maximize the use of these year-round acoustic data. Beyond applications to newly produced datasets, the approach opens possibilities for the efficient extraction of new information from existing upward-looking sonar records from recent decades.
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Lipid accumulation in three species of Neocalanus copepod in the northern Gulf of AlaskaThe Northern Gulf of Alaska experiences pronounced seasonality and inter-annual variability characterized by a significant bloom of phytoplankton in the spring. Neocalanus copepods in the NGA have evolved to match their lifecycle to the seasonality of the Gulf of Alaska and feed upon the spring phytoplankton bloom. All three of these Neocalanus species utilize diapause as an over-wintering strategy; acquiring large stores of lipid to sustain them through winter hibernation and subsequent reproduction. Zooplankton were sampled with 150 and 505 µm mesh nets from 0 to 1200 m along the Seward Line and within Prince William Sound in the Northern Gulf of Alaska during 2018-2020 to track the physiological process of Neocalanus copepods preparing for diapause. We measured lipid sac area, lipid volume and percent lipid to quantify lipid content. Neocalanus showed significant interannual variability in final lipid accumulation both at depth and in the surface during the study period. For all three species, lipid content increased with increasing stage and prosome length. Lipid content increased from spring to summer for N. flemingeri, remaining steady into fall as animals molted into adults and descended to depth for diapause. Neocalanus plumcrhus stored lipid from spring to summer before descending slightly after N. flemingeri. Neocalanus cristatus exhibited dissimilar behavior to the other two species, storing consistently low amounts of lipid, alluding to a different lifecycle. Each Neocalanus species displayed similar lipid accumulation behavior with offset timing from one another. Neocalanus exhibits an earlier developmental timing as compared to other lipid accumulating copepods giving them a competitive advantage to reach maturity in time to feed on the early phase of the spring phytoplankton bloom faster than other species. Our data provided some evidence for both the lipid accumulation hypothesis and the developmental program hypothesis being utilized in Neocalanus populations in the Northern Gulf of Alaska. This work serves as the first detailed study of body condition and lipid sac condition in Neocalanus populations throughout the water column within the Northern Gulf of Alaska.
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Investigation of variability of internal tides in the Tasman SeaSurface tides, when obstructed by bottom relief, give rise to periodic oscillations within the stratified oceanic interior. Such transformation of the depth independent (barotropic) tide into internally propagating (baroclinic) waves comprises 1/3 of the global energy losses from the surface tide. Internal waves of tidal period known as internal tides tend to have low vertical shear and hence are very stable and long lived. They have been observed to propagate essentially unchanged across ocean basins. Details of the internal tide wave life-cycle are not well known, yet turbulent dissipation powered by the slow decay of these waves is one of the key processes shaping deep ocean water properties. The Tasman Sea stands out as a natural laboratory to investigate the internal tide life cycle. In this dissertation, the generation and propagation of internal tides were examined by means of realistic simulations of ocean circulation under varying conditions, and were compared to observations obtained during the Tasman Tidal Dissipation Experiment (TTIDE). The simulations reveal that the barotropic-to-baroclinic conversion is intensified at the Macquarie Ridge near New Zealand by coupling with secondary, nonlocally produced internal tides. Because of this complexity, regionally varying hydrographic conditions drive remarkable temporal and spatial variability of internal tide generation. The internal tides that are created at the ridge constructively superpose into a spatially confined, beam-like feature (Tasman beam) that radiates across the Tasman Sea over 1000 kilometers from its generation region and reaches the Tasman shelf. The beam is described well at first order by simple plane wave propagation theory, but also exhibits non-plane wave characteristics associated with diffraction. Additional intricacy arises from development of a standing wave, the result of the beam's reflection near Tasmania. Temporal changes include hydrography-induced refraction and strong perturbations from interactions with eddies. It is concluded that in-situ mooring measurements and ship surveys of internal tides exhibit a great deal of apparent spatial and temporal variability that can be difficult to interpret. This variability can largely be eliminated in the analysis of numerical models which allow the underlying wave field energy life cycle to be quantified.
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Utility of trace element studies for improving our understanding of geochemical processes within the arctic ocean environmentThe Arctic Ocean is a dynamic region undergoing rapid change. Sea ice and meteoric water are intrinsic components of the Arctic environment that play key roles in its ecosystem, including the distributions and cycling of trace elements throughout the pan-Arctic Ocean. Meteoric water (e.g., rivers and snow deposition) contributes to the input of trace elements to surface waters, while sea ice dynamics contribute to the transport of these constituents across Arctic basins. Trace element distributions can provide insights into Arctic processes. The focus of Chapter One is on particulate (>0.2 μm) trace elements in Arctic pack ice, associated snow, and underlying surface waters collected from September-October 2015 during the US GEOTRACES Western Arctic cruise (GN01). This late-season pack ice provides a snapshot of sea ice characteristics in regions near the North Pole, within the Makarov and Canada Basins, and can estimate the impact melting sea ice may have on particulate trace element inputs to Arctic waters. Chapter Two presents on the utility of dissolved barium (dBa), a bio-intermediate element of lithogenic origin, as a tracer of meteoric water throughout the Siberian Arctic Ocean. Samples for Chapter Two were collected during the 2018 Nansen and Amundsen Basin Observatory System. The distribution of dBa in this region may provide useful insights into important shelf processes, such as tracing shelf waters along continental slopes. In Chapter 3, additional spatiotemporal geochemical parameters (δ¹⁸O and salinity) are considered alongside dBa to model how Arctic water mass fractions (meteoric, sea ice melt, and Atlantic waters) changed between 2013, 2015, and 2018 within the Siberian Arctic Ocean. This dissertation contributes to the understanding of Arctic Ocean processes through the application of trace element studies and highlights the usefulness of combining tracers to better understand this dynamic environment.
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Spatial and temporal variability of dissolved aluminum and manganese in surface waters of the northern Gulf of AlaskaThe Northern Gulf of Alaska (NGA) shelf is a productive high-latitude environment where nutrient dynamics are greatly impacted by the seasonal variability in freshwater input and water column mixing. Iron is a key nutrient on the NGA shelf that directly modulates primary production, but inputs are difficult to quantify due to high biological uptake and control exerted by Fe-binding organic ligands. Other lithogenic elements such as aluminum and manganese have the same sources as iron (rivers and sediment) and similar abiotic removal via particle scavenging, but exhibit quasi- conservative behavior in seawater allowing for their use as tracers of these sources. Thus, Al and Mn distributions can help provide insight into iron inputs and the relative importance of various mechanisms influencing nutrient dynamics in the NGA. The data are derived from spring, summer, and fall NGA LTER (long term ecological research) cruises from 2018 and 2019 that included a focused five-day Copper River plume study, several surface transects from Kayak Island to Kodiak Island, and vertical profiles at several locations sparsely distributed throughout the shelf. We find that seasonal patterns in the surface concentrations of dMn and dAl mirrored annual glacial melt cycles, with the lowest values observed in spring and higher values in summer and fall. Spatial patterns were also apparent as both metals tended to be lower offshore than inshore, and were also lower overall (by 1-2 orders of magnitude) on transects further from the outflow of the Copper River, a major source of freshwater to the NGA. Extremely high concentrations in the Copper River plume (≤1395 nM dAl, ≤128 nM dMn) and strong correlations with salinity (p < 0.0001) highlight their quasi-conservative nature, and their usefulness as tracers of freshwater input, which helps inform iron inputs from this source. Enhanced dAl and dMn concentrations within nepheloid layers in subsurface waters indicate regions where a sedimentary source of iron is likely to be important. Residence times for dAl and dMn in surface waters over the NGA shelf were estimated to be 31 days (dAl) and 42 days (dMn) on average based on summer and fall data from both years.
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Genomic signatures of optimal growth temperature in the family ColwelliaceaeThe temperature range supporting growth defines a complex physiological phenotype that depends on interactions between an organism's genome and its environment. Its implications are widespread since small changes in optimal growth temperature (OGT) can alter an organism's ability to inhabit an ecological niche. Thus, organisms with extreme thermal growth traits (e.g., psychrophilic, with OGT < 15℃, or thermophilic, with OGT 60 -80℃) may be useful for identifying promising targets when searching for life on other planets, as well as predicting population dynamics in a warming Arctic. We performed comparative genomic analyses of bacteria newly isolated from Arctic sea ice that were affiliated with Colwelliaceae, a family of Gammaproteobacteria that contains many psychrophilic strains, to identify genomic factors that might be used to predict OGT. A phylogenomic analysis of 67 public and 39 newly-sequenced strains, was used to construct an updated phylogenetic tree of Colwelliaceae, of which at least two genera were well represented. To augment the previously reported OGTs of 26 strains, we measured growth rates at −1, 4, 11, and 17 ℃ to determine the OGTs of these 39 new strains of Colwelliaceae. We found that growth rates among all isolates were comparable at −1℃, but varied widely above 10 ℃, indicating higher variability in the ability to tolerate warmer temperatures. To analyze the phenotypic differences on a genomic level, we examined indices of amino acid substitutions that have previously been linked with cold adaptation via an increase in protein flexibility. We found that these indices were significantly correlated with OGT at the whole genome level, although the sign of some correlations were opposite of the predicted positive correlation between temperature and the indices. Using these data, we fit a multiple linear regression model for OGT within the Colwelliaceae family that incorporates the three most informative amino acid indices: GRAVY, Aliphatic Index, and Acidic Residue Proportion. Additionally, a putative cold-adaptive gene cassette was identified that was likely introduced by horizontal gene transfer between two closely related clades with different OGTs. These contributions offer key insights into OGT variability and its underlying genomic foundation in the family Colwelliaceae.
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Growth and reproductive rates of calanoid copepods in the northern Bering and southern Chukchi SeasEgg production and copepodite growth rates were measured for the calanoid copepods Pseudocalanus spp., Calanus marshallae/glacialis, and Metridia pacifica in the northern Bering and southern Chukchi Seas during June of 2017 and 2018. For all taxa, instantaneous growth rates generally decreased with increasing copepodite stage, though the differences between most stages was not significant. The growth rates for Pseudocalanus spp. averaged 0.03 ± 0.002 day⁻¹, Calanus spp. 0.09 ± 0.004 day⁻¹, and M. pacifica 0.05 ± 0.03 day⁻¹. Egg production rates increased with prosome length for all species, but when standardized to body weight this trend reversed. All Pseudocalanus species had similar weight-specific egg production (SEP): 0.18 ± 0.01 for P. acuspes, 0.15 ± 0.00 for P. newmani, and 0.11 ± 0.02 for P. minutus. The SEP for Calanus was considerably lower, 0.09 ± 0.01, while for M. pacifica it was 0.11 ± 0.01. These rates suggest considerable discrepancies between growth rates and egg production weights that we propose are due to differences in life history strategies. Pseudocalanus reproduce nearly year round, they appear to invest less in somatic growth, preferring to quickly reach their adult stage where they invest heavily into reproduction. Calanus spp. have 1 or possibly 2 generations per year in this region, they invest more into somatic growth in order to ensure their population is ready for a reproductive season timed to the spring phytoplankton bloom. The more omnivorous M. pacifica is also likely limited to 1 or 2 generations, although their ability to thrive on a wider range of food sources than Calanus seems to allow for relatively higher investment in reproduction and perhaps lower investment in somatic growth. Consistent with other studies, global growth models do not match our observations particularly well, likely because they are dominated by egg production estimates at lower latitudes.
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Abundance, composition and distribution of predatory gelatinous zooplankton in the northern Gulf of AlaskaJellyfish are conspicuous yet under-studied components of marine zooplankton communities. Abundance, biomass, size, and distribution of large-jellyfish were measured during July and September of 2018 and 2019 as part of the Northern Gulf of Alaska Long-Term Ecological Research (NGA-LTER) cruises. Nearly 1000 kg dispersed among ~13,800 jellies were collected using a 5 m² Methot net. Catches were dominated by two macro-jellies, the hydrozoan Aequorea sp. and the scyphozoan Chrysaora sp. During 2018, epipelagic macro-jellies biomass averaged 1.46 ± 0.36 g WW m⁻³ for July and 1.14 ± 0.23 g WW m⁻³ for September, while during 2019 they averaged 0.86 ± 0.19 g WW m⁻³ for July and 0.72 ± 0.21 g WW m⁻³ by September. Despite similar biomass among sampling seasons within the same year, July abundances were fivefold greater than abundances in September, with July catches dominated by juvenile jellyfish over the inner shelf, while during September jellyfish adults were more prominent and most predominant at offshore stations. Comparison to over 20 years of data from standard towed nets allowed determination of the relative magnitude of the three dominant predatory zooplankton components: Scyphozoans, Hydrozoans, and Chaetognaths in the NGA. The biomass of these smaller epipelagic predators (10 mg WW m⁻³ for hydrozoans and 8 mg WW m⁻³ for chaetognaths) is a low percentage of the macro-jellies, despite their much higher numerical abundance. Acknowledging that changes in gelatinous biomass could have profound effects on fisheries, we argue that jellyfish should be quantitatively monitored in ecosystems with high fisheries productivity.
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Pacific herring juvenile winter survival and recruitment in Prince William SoundSmall pelagic fish abundances can vary widely over space and time making them difficult to forecast, partially due to large changes in the number of individuals that annually recruit to the spawning population. Recruitment fluctuations are largely driven by variable early life stage survival, particularly through the first winter for cold temperate fishes. Winter survival may be influenced by juvenile fish size, energy stores, and other factors that are often poorly documented, which may hamper understanding recruitment processes for economically and ecologically important marine species. The goal of this research was to improve understanding of recruitment of Pacific herring (Clupea pallasii) within Prince William Sound (PWS) through recruitment modeling and by identifying factors influencing winter survival of young-of-the-year (YOY) herring. Towards this end, my dissertation addresses three specific objectives: 1) incorporate oceanographic and biological variables into a herring recruitment model, 2) describe patterns in growth and condition of PWS YOY herring and their relationship to winter mortality risks, and 3) compare the growth, condition, swimming performance, and mortality of YOY herring that experience different winter feeding levels. In the recruitment modeling study, annual mean numbers of PWS herring recruits-per-spawner were positively correlated with YOY walleye pollock (Gadus chalcogrammus) abundance in the Gulf of Alaska, hence including a YOY pollock index within a standard Ricker model improved herring recruitment estimates. Synchrony of juvenile herring and pollock survival persisted through the three-decade study period, including the herring stock collapse in the early 1990s. While the specific mechanism determining survival is speculative, size-based tradeoffs in growth and energy storage in PWS YOY herring indicated herring must reach a critical size before winter, presumably to reduce size-dependent predation. Large herring switched from growth to storing energy, and ate more high-quality euphausiid prey, which would delay the depletion of lipid stores that compelled lean herring to forage. Lipid stores were highest in the coldest year of the seven-year field study, rather than the year with the best diets. With diets controlled in a laboratory setting, spring re-feeding following restricted winter diets promoted maintenance of size and swimming ability, but had little effect on mortality rates compared to fish continued on restricted rations. Declines in gut mass, even among fully fed herring, and low growth potential suggest limited benefits to winter feeding. Mortalities due to food restriction compounded by disease were highest among herring that fasted through winter months, and among small herring regardless of feeding level. Taken together, these findings illustrate the importance of achieving a critical size and high lipid stores in the critical period before winter to promote YOY herring winter survival and ultimately recruitment.
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Diversity and community structure of eukaryotic phototrophs in the Bering and Chukchi seasThe phytoplankton of the Bering and Chukchi seas support highly productive ecosystems characterized by tight benthic-pelagic coupling. In this study, we focus on the northern Bering and Chukchi seas, considering them as one ecosystem. This community has historically been dominated by diatoms; however, climate change and accompanying warming ocean temperatures may alter primary producer communities. Using metabarcoding, we present the first synoptic, high-throughput molecular phylogenetic investigation of phytoplankton diversity in the Bering and Chukchi seas based on hundreds of samples collected from June to September in 2017. We identify the major and minor taxonomic groups of diatoms and picophytoplankton, relative abundances of genera, exact sequence variants (201 for diatoms and 227 for picophytoplankton), and describe their biogeography. These phylogenetic insights and environmental data are used to characterize preferred temperature ranges, offering insight into which specific phytoplankton (Chaetoceros, Pseudo−nitzschia, Micromonas, Phaeocystis) may be most affected as the region warms. Finally, we investigated the likelihood of using shipboard CTD data alone as predictive variables for which members of phytoplankton communities may be present. We found that the suite of environmental data collected from a shipboard CTD is a poor predictor of community composition, explaining only 12.6% of variability within diatom genera and 14.2% variability within picophytoplankton genera. Clustering these communities by similarity of samples did improve predictability (43.6% for diatoms and 32.5% for picophytoplankton). However, our analyses succeeded in identifying temperature as a key driver for certain taxa found commonly throughout the region, offering a key insight into which common phytoplankton community members may be affected first as the Alaskan Arctic continues to warm.
