Seasonal dynamics of plankton ecology in coastal Antarctica

The West Antarctic Peninsula (WAP) has experienced significant increases in atmospheric and ocean temperatures since the 1950s, with subsequent decreases in winter sea ice extent and duration. Concurrently, phytoplankton biomass has decreased along the northern portion of the peninsula, associated with a shift from large-celled diatoms to smaller-celled cryptophytes and mixed flagellates, and krill populations previously located north of the peninsula have shifted their range south to coastal WAP waters. Despite these changes, a comprehensive understanding of the seasonal dynamics of coastal Antarctic phytoplankton and krill remains in question. Filling this gap in our understanding is crucial for contextualizing long-term change. This dissertation is focused on understanding the austral summer seasonal dynamics of plankton ecology at Palmer Station, Antarctica, and the resulting implications for predator foraging. Chapter 1 provides overall context for the observed changes and implications for ecology of the WAP. Chapters 2 and 3 document phytoplankton seasonal succession and diversity using data from samples analyzed with an Imaging FlowCytobot. In Chapter 2, a convolutional neural network was built to automatically sort collected images of WAP phytoplankton, and in Chapter 3 the neural network was applied to two field seasons of samples collected from Palmer Station. The findings highlight remarkable similarities in phytoplankton seasonal succession between years despite significant differences in annual sea ice extent and total phytoplankton biomass. In both years, there was a tight connection between sea ice retreat and bloom initiation timing and a decrease in cell size from late spring to early autumn associated with increases in meteoric meltwater. In Chapter 4, 12 years of autonomous underwater glider data from the Palmer region were paired with a photoacclimation model to characterize phytoplankton growth and bloom phenology from summer to early autumn. Results showed a distinct phase shift at the beginning of February caused by increased wind-mixing of sedimentary iron to surface waters, which increased growth rates and cellular chlorophyll concentrations, diluted predator concentrations reducing grazing pressure, and led to a significant autumn bloom. In Chapter 5, bi-weekly acoustic surveys were conducted to evaluate seasonal changes in krill availability in adjacent Adélie and gentoo penguin foraging regions near Palmer Station. From summer to autumn, results showed a migration of adult krill inshore and an increase in diel vertical migration behavior. Additionally, there were significantly different oceanographic properties and krill swarming behaviors in Adélie and gentoo foraging regions only 10 km apart, suggesting a potential driver of differences in local penguin foraging behavior between species. Chapter 6 summarizes the main findings of the dissertation and suggests directions for future research. Ultimately, this dissertation improves our understanding of summer seasonal phytoplankton and krill dynamics, providing context for predicting how coastal Antarctic ecosystems might respond and adapt to continued environmental change.Ph.D.Includes bibliographical reference