Stevens
Bethany L. F.
Stevens
Bethany L. F.
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ThesisPicophytoplankton of the Northeast U.S. Shelf: community composition and dynamics(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2023-09) Stevens, Bethany L. F. ; Neubert, Michael G. ; Sosik, Heidi M.Marine picophytoplankton are the most abundant primary producers in the ocean and are expected to be favored by the ongoing effects of climate change. Predicting the response of marine ecosystems to these changes requires mechanistic knowledge of picophytoplankton ecology. This thesis uses a combination of long-term monitoring, cruise data, population models, and high-throughput sequencing to investigate the dynamics of picophytoplankton across scales of space and time that are relevant both to the physiology of the individual cells and to the structure of the Northeast U.S. Shelf (NES), a productive and economically important coastal ecosystem. To identify the drivers of seasonal changes in picophytoplankton abundance, I first estimate daily division and loss rates for a nearshore community of picoeukaryotes over a 16-year period. I compare their cell concentrations, vital rates, and responses to environmental variables to those of the cyanobacteria, Synechococcus. Next, to reveal how these dynamics relate to changes in community composition, I analyze 9-years of monthly metabarcoding data and characterize taxonomic variability within the picoeukaryote assemblage. In the second half of this thesis, I explore spatial environmental variability and test the extent to which data from the nearshore observatory are representative of the picophytoplankton communities across the NES. I analyze flow-cytometry data collected from 22 regional research cruises, estimate daily Synechococcus and picoeukaryote division rates from underway data, and describe the distinct depth distributions of the two groups from subsurface samples. The major findings of this thesis are that, across the NES, the picoeukaryotes divide at much higher rates than the more abundant Synechococcus and are subject to greater top-down control from grazing or viral lysis. Both groups are light limited in the fall, temperature limited in the spring, and undergo earlier spring blooms in warmer offshore waters. For Synechococcus, the relationships between cell concentration, division rate and environmental parameters are consistent across the continental shelf, while the picoeukaryote community appears to be nutrient-limited farther from shore. Together, this work creates a detailed picture of the various controls on picophytoplankton abundance within a dynamic coastal ecosystem and advances our understanding of how picophytoplankton communities respond to environmental change.
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ArticleTemperature regulates Synechococcus population dynamics seasonally and across the continental shelf(Association for the Sciences of Limnology and Oceanography, 2023-05-12) Stevens, Bethany L. F. ; Crockford, E. Taylor ; Peacock, Emily E. ; Neubert, Michael G. ; Sosik, Heidi M.Hourly, year‐round flow cytometry has made it possible to relate seasonal environmental variability to the population dynamics of the smallest, most abundant phytoplankton on the Northeast US Shelf. To evaluate whether the insights from these data extend to Synechococcus farther from shore, we analyze flow cytometry measurements made continuously from the underway systems on 21 cruises traveling between the Martha's Vineyard Coastal Observatory (MVCO) and the continental shelf break. We describe how seasonal patterns in Synechococcus , which have been documented in detail at MVCO, occur across the region with subtle variation. We find that the underlying relationship between temperature and division rate is consistent across the shelf and can explain much of the observed spatial variability in concentration. Connecting individual cell properties to annual and regional patterns in environmental conditions, these results demonstrate the value of autonomous monitoring and create an improved picture of picophytoplankton dynamics within an economically important ecosystem.
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ArticleTemperature dependence of parasitoid infection and abundance of a diatom revealed by automated imaging and classification(National Academy of Sciences, 2023-07-03) Catlett, Dylan ; Peacock, Emily E. ; Crockford, E. Taylor ; Futrelle, Joe ; Batchelder, Sidney ; Stevens, Bethany L. F. ; Gast, Rebecca J. ; Zhang, Weifeng Gordon ; Sosik, Heidi M.Diatoms are a group of phytoplankton that contribute disproportionately to global primary production. Traditional paradigms that suggest diatoms are consumed primarily by larger zooplankton are challenged by sporadic parasitic “epidemics” within diatom populations. However, our understanding of diatom parasitism is limited by difficulties in quantifying these interactions. Here, we observe the dynamics of Cryothecomonas aestivalis (a protist) infection of an important diatom on the Northeast U.S. Shelf (NES), Guinardia delicatula, with a combination of automated imaging-in-flow cytometry and a convolutional neural network image classifier. Application of the classifier to >1 billion images from a nearshore time series and >20 survey cruises across the broader NES reveals the spatiotemporal gradients and temperature dependence of G. delicatula abundance and infection dynamics. Suppression of parasitoid infection at temperatures <4 °C drives annual cycles in both G. delicatula infection and abundance, with an annual maximum in infection observed in the fall-winter preceding an annual maximum in host abundance in the winter-spring. This annual cycle likely varies spatially across the NES in response to variable annual cycles in water temperature. We show that infection remains suppressed for ~2 mo following cold periods, possibly due to temperature-induced local extinctions of the C. aestivalis strain(s) that infect G. delicatula. These findings have implications for predicting impacts of a warming NES surface ocean on G. delicatula abundance and infection dynamics and demonstrate the potential of automated plankton imaging and classification to quantify phytoplankton parasitism in nature across unprecedented spatiotemporal scales.