Quantifying net primary production in a changing Arctic Ocean
Keywords: Environmental Earth System Science

Situated at the northernmost region of the planet, the Arctic Ocean (AO), the smallest of the world's oceans, supports a rich, but vulnerable, ecosystem. Despite seemingly inhospitable conditions, the extreme seasonal pulses of primary production by phytoplankton in the AO fuel an abundant food-web composed of both endemic and migratory higher trophic level organisms. Alas, the Arctic is warming at approximately twice the global rate in response to anthropogenic climate change and the rising temperatures in this region have already triggered profound ecological changes. In the oceans, disappearing sea ice has shifted the phytoplankton growing season earlier in the year and led to a significant increase in net primary production (NPP). In order to understand the multi-layered effects of AO biogeochemistry and ecology as the climate continues to warm, it is imperative to accurately monitor changes in the magnitude and timing of NPP. Because of the harsh conditions that make the region both difficult and expensive to access for most of the year, field measurements in the AO are relatively limited. Luckily, satellite remote sensing can supplement limited in situ measurements by imaging the ocean surface from space. However, because of the unique oceanic optical conditions and phytoplankton photophysiology, global ocean color algorithms fail to accurately estimate Chl a when applied to the AO. Hence, this dissertation work utilizes in situ bio-optical measurements to inform accurate parameterization of ocean color algorithms which are then applied to assess long term changes of AO NPP. To understand the phytoplankton photophysiological responses to environmental changes as the Arctic Ocean shifts seasonally from ice-covered to open water, we evaluated photoacclimation strategies of phytoplankton during the low-light, high-nutrient, ice-covered spring and the high-light, low-nutrient, ice-free summer (Chapter 2). Field results show that phytoplankton effectively acclimated to reduced irradiance beneath the sea ice and that abundant nutrients enable pre-bloom phytoplankton to become "primed" for increases in irradiance. I used these bio-optical measurements to characterize regional and seasonal patterns in phytoplankton photophysiology and optical conditions to examine the impact on ocean color remote sensing in the Chukchi Sea (Chapter 1) and the AO (Chapter 3). Results show that phytoplankton pigment packaging (an acclimation to low light) and high absorption by colored dissolved organic matter (CDOM), especially on the interior shelves, cause default ocean color ocean algorithms to overestimate chlorophyll a (Chl a) at low phytoplankton biomass, but underestimate at high biomass throughout the AO. By assembling the largest database of in situ measurements for these waters, I successfully parameterized multiple ocean color algorithms to optimize retrievals of Chl a, absorption by CDOM and detritus, and backscattering of particles. Using the new ocean color algorithm parameterized for the Arctic Ocean, we show that primary production increased by 57% between 1998 and 2018 (Chapter 4). Surprisingly, while increases were due to widespread sea ice loss during the first decade, the subsequent rise in primary production was driven primarily by increased phytoplankton concentration, which could only be sustained by an influx of new nutrients. This suggests a future Arctic Ocean that, as long as there are enough nutrients, can support higher trophic-level production and additional carbon export. Together, the results of this dissertation demonstrate that the unique bio-optical properties of the AO must be addressed in order to accurately employ satellite remote sensing and, when doing so, we reveal dramatic ecosystem changes in response to anthropogenic climate change.