Stanford University

*** Ph.D. Thesis/ Oral Defense ***

Evaluating the frequency, magnitude, and biogeochemical impacts of under-ice phytoplankton blooms

Courtney Payne

Tuesday, April 18, 9:00 am

Green 365

Department of Earth System Science

Advisor: Dr. Kevin Arrigo

            In the Arctic Ocean, changing patterns of sea ice extent and thickness have increased light availability in the surface ocean during the ice-covered portion of the year, substantially altering patterns of phytoplankton net primary production (NPP). While NPP in the Arctic Ocean was previously considered insubstantial until the time of sea ice breakup and retreat, the observation of massive under-ice (UI) phytoplankton blooms in many of the Arctic seas reveals that the largest pulse of NPP may be produced prior to sea ice retreat. However, estimating how much NPP is generated during the UI part of the year is challenging, as satellite observations are hampered by sea ice cover and very few field campaigns have targeted UI blooms for study. This thesis uses a combination of laboratory experiments, biogeochemical modeling, and an analysis of satellite remote sensing data to better understand how the magnitude and spatial frequency of UI phytoplankton blooms has changed over time in the Arctic Ocean, as well as to assess the likely biogeochemical consequences of these blooms. In Chapter 2, I conduct a laboratory experiment to understand the functional relationship between co-limiting light and nutrient conditions on phytoplankton growth. In Chapter 3, I present a one-dimensional ecosystem model (CAOS-GO), which I use to evaluate the magnitude of UI phytoplankton blooms in the northern Chukchi Sea (72°N) between 1988 and 2018. In Chapter 4, I used the same model configuration to investigate the role of UI bloom variability in controlling sedimentary processes in the northern Chukchi Sea. I subsequently deployed this model in the southern Chukchi Sea (68°N) to understand latitudinal differences in UI bloom importance across the region (Chapter 5). Finally, in Chapter 6, I used satellite remote sensing to determine how UI bloom frequency changed across the Arctic between 2003 and 2021. Together, the results of this dissertation suggest that UI phytoplankton blooms can substantially contribute to total NPP and can drive reductions in food availability and nitrogen loss. However, this work also demonstrates that UI blooms, which have likely been an important source of NPP across the Arctic since at least the 1980s, are likely an ephemeral feature, with their prevalence likely to decline in coming years as sea ice retreat shifts earlier.