New perspectives on primary production in the Pacific sector of the Southern Ocean
Keywords: Environmental Earth System Science.

Regions of the Southern Ocean have experienced rapid shifts in temperature and ice cover in response to climate change. These changes have the potential to impact rates of net primary production (NPP) in the Southern Ocean, a process that provides food to the entire Antarctic food web and facilitates the removal of vast amounts of atmospheric carbon dioxide (CO2). In this dissertation, I examine the physical and biogeochemical drivers of NPP in the Pacific sector of the Southern Ocean, starting with highlights of the regional heterogeneity of the SO response to climate change (Chapter 1). In Chapter 2, I present the results of a satellite-based study in which I quantify changes in NPP and chlorophyll a (Chl a) in response to the observed changes in sea ice dynamics in the Pacific sector of the Southern Ocean south of 60°S. This study covers a 16-year time period (1997--2013) and incorporates both the shelf and off-shelf regions of the Pacific sector, including the highly productive Ross Sea. I observed significant secular changes in NPP from 1997 to 2013 in the off-shelf region, with NPP increasing on the eastern side and decreasing on the western side of the study area. The changes in NPP are consistent with the changes observed in sea surface temperature (SST) and open water days (OWDs), which decreased (increased) on the western (eastern) side of our study area. Finally, I examined the influence of the Southern Annular Mode (SAM) and the El Niño Southern Oscillation (ENSO), on SST, OWDs, Chl a, and NPP and observed a significant relationship between the state of the SAM and ENSO and SST, OWDs, and NPP across the study region. The response of OWDs, SST, and NPP to atmospheric forcing by SAM and ENSO was opposite for the shelf and off-shelf regions, such that during a positive phase of SAM or negative phase of ENSO (La Niña), SST, OWDs, and NPP increased on the shelf and decreased in the off-shelf region. In Chapter 3, I examine a region of elevated productivity visible in the northwestern portion of the study region focused on in Chapter 2. NPP in the Southern Ocean is limited by iron availability, and hydrothermal vents have been identified as a potentially important source of iron to Southern Ocean surface waters. Using both satellite and in situ data, I identify a recurring phytoplankton bloom in the high-nutrient, low-Chl waters of the Antarctic Circumpolar Current in the Pacific sector of the Southern Ocean. In January 2014, the bloom covered an area of 266,000 km2 with depth-integrated Chl a > 300 mg m-2, NPP rates > 1 g C m--2 d--1, and a mean CO2 flux of −0.38 g C m--2 d--1. The elevated iron supporting this bloom is likely of hydrothermal origin based on the recurrent position of the bloom relative to two active hydrothermal vent fields along the Australian Antarctic Ridge and the association of the elevated iron with a distinct water mass characteristic of a nonbuoyant hydrothermal vent plume. In Chapter 4, I examine temporal variations in NPP, sea ice, SST, and surface currents in theAustralian Antarctic Ridge bloom identified in Chapter 3. I conducted a satellite-based study to examine the interannual variability in NPP, sea ice, sea surface temperature, wind speed, and surface currents (using Lagrangian particle tracking analysis) in the region of the AAR bloom. My study covers a 22-year time period from 1997 to 2019 and separates the bloom into two regions, KR1 and KR2, named for the hydrothermal vents with which they are associated. I found that the characteristics of the KR1 and KR2 blooms in terms of mean annual NPP rate, SST, and sea ice cover were markedly different in both timing and magnitude. I also found a significant degree of interannual variability in NPP at both KR1 and KR2, despite remarkably slow and consistent surface currents across the region. I further found that NPP at KR1 and KR2 were highly correlated. However, NPP showed no relationship with either SST, sea ice conditions, or surface currents for either region of the bloom. I also examined variability in earthquake activity in the AAR region, with the goal of using it as a proxy for hydrothermal vent output, and again found no relationship with NPP. The interannual variability of the AAR bloom has implications for both carbon export and the value of the region as a foraging ground for upper trophic level organisms. Finally, I conclude that this work demonstrates that we can significantly advance our understanding of the mechanistic control on NPP by studying patterns in NPP on a regional scale as opposed to the scale of an entire ocean basin. By focusing on the Pacific sector of the Southern Ocean I was able to show that the shelf and off-shelf regions respond differently to the same atmospheric forcing, identify a bloom that had been overlooked for more than 40 years, demonstrate that the bloom was likely of hydrothermal origin, and show that interannual variability in the bloom was high and not explained by the factors that influence NPP throughout the majority of the Pacific sector