Diel, Seasonal, And Interannual Changes In Coastal Antarctic Zooplankton Community Composition And Trophic Ecology

Throughout the ocean, zooplankton transfer energy from primary producers to higher predators and transport carbon from surface waters to depth. The efficiency of these processes depends in part upon the taxonomic composition and trophic ecology of the zooplankton community. Zooplankton species abundance and distribution shifted over recent decades along the West Antarctic Peninsula during a period of rapid regional warming and sea-ice decline. Although conducted within the context of long-term change, this dissertation research focuses on zooplankton dynamics at finer temporal scales that have received less attention. I analyzed depth-stratified net samples to investigate zooplankton diel vertical migration during Antarctic summer. I further conducted twice-weekly sampling over three field seasons to characterize seasonal and interannual changes in coastal zooplankton composition. I also used field measurements, experimental incubations, and compound-specific stable isotope analysis to study the growth and trophic ecology of a dominant species – the Antarctic krill Euphausia superba – during its juvenile stage. Results indicate that several zooplankton taxa conducted diel vertical migration despite nearly continuous daylight during Antarctic summer. Other carnivorous, detritivorous, and seasonally migrating taxa were most abundant in the mesopelagic zone throughout the diel cycle. Vertically migrating zooplankton and mesopelagic residents likely have a substantial impact on vertical carbon flux, which should be better quantified in the Southern Ocean. Repeated shore-based sampling showed that seasonal peaks in coastal zooplankton biomass follow local phytoplankton blooms. Seasonal succession from larger, herbivorous species to smaller, omnivorous species was a consistent phenomenon across multiple taxonomic groups and years. Seasonal abundance patterns for various taxa shifted by a week or more between two consecutive years, consistent with a two-week shift in the timing of sea-ice breakup and the spring phytoplankton bloom. Flexible life history and trophic ecology may limit the risk of reproductive failure despite climate-induced phenology shifts. Exceptionally warm temperatures coincided with a bloom of the gelatinous salp Salpa thompsoni, and such conditions may become more common in future decades. Phytoplankton ingestion was insufficient to support rapid growth of juvenile krill during summer in coastal waters, and heterotrophic protists were a critical intermediate trophic link. Phytoplankton consumption was limited due to inefficient feeding on small cells that dominated the available autotrophic prey. Metazoan zooplankton were also important prey for juvenile krill, which are truly secondary consumers. This dissertation research revealed substantial shifts in the trophic ecology and composition of Antarctic zooplankton communities at diel to interannual scales, with implications for regional biogeochemical cycles and food web dynamics. Future work should consider these shorter time scales to detect and understand long-term change in Antarctic marine ecosystems