The influence of sea ice and hydrography on the timing, distribution, and intensity of phytoplankton blooms in the rapidly changing Chukchi Sea (Arctic Ocean) [electronic resource]
Keywords: Environmental Earth System Science.

This dissertation examines the influence of sea ice and hydrography on phytoplankton blooms in the seasonally ice-covered Chukchi Sea in the western Arctic Ocean, where rapidly changing environmental conditions are dramatically altering marine ecosystems. Motivated in large part by the discovery of a massive under-ice phytoplankton bloom in 2011 during the NASA-funded Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment (ICESCAPE) program, this research seeks to elucidate phytoplankton bloom dynamics in the Chukchi Sea. Signaling a potentially major ecosystem transition, the unexpected discovery indicated that recent reductions in sea ice thickness and age have transformed the under-ice light environment through the presence of expansive melt ponds that transmit sufficient light for photosynthesis, resulting in a shift in bloom timing to earlier in the year, prior to sea ice retreat. Through studies of the spatial and temporal distribution of under-ice blooms, physical drivers of phytoplankton bloom development, and characteristics of the under-ice environment that contribute to extraordinary phytoplankton growth beneath the ice, the objective of this work is to improve our understanding of this highly productive yet relatively understudied region. My approach in this research combines satellite remote sensing of ocean color and sea ice with observational and experimental fieldwork as part of four polar oceanographic expeditions aboard USCGC Healy. In Chapter 1, I use satellite imagery to map evidence of under-ice blooms over the 15-year satellite record in the Chukchi Sea and distinguish them from marginal ice zone blooms that begin after sea ice retreat. Results of this work demonstrate that under-ice blooms have been prevalent for more than a decade prior to shipboard observations and are likely widespread across the region. To understand what factors control phytoplankton bloom distributions and magnitude, in Chapter 2, I relate phytoplankton abundance to hydrography and show that nutrient-rich winter water pathways in the Chukchi Sea are consistently associated with phytoplankton blooms both under the ice and in open water. The culmination of this study is a new conceptual model for phytoplankton blooms in the Chukchi Sea that links the flow of highly productive winter water pathways to the presence of biological hotspots. In Chapter 3, I use novel early-season measurements of spring under-ice phytoplankton, nutrients, and hydrography to understand physical controls on the development of phytoplankton blooms in ice-covered waters. Results of this work illustrate that blooms form in stratified waters beneath fully consolidated sea ice, while enhanced convective mixing beneath sea ice with leads impedes the development of under-ice blooms through vertical mixing that reduces light availability in the upper mixed layer. Characteristics of the under-ice light environment are further explored in Chapter 4, in which I present results from shipboard incubation experiments that illustrate the effects of visible light and ultra-violet radiation on phytoplankton growth and physiology. This study indicates that the under-ice environment is ideally suited for phytoplankton bloom development due to reduced damaging visible and ultra-violet radiation in surface waters beneath the ice as compared to open water. The integrative work of these four chapters greatly advances our knowledge of the effects of climate change on biological processes in the Arctic Ocean and establishes the foundation for investigating the implications of these changes for the ecosystem as a whole.