Regional variability of the Southern Ocean spring bloom

Phytoplankton production in the Southern Ocean plays an important role in the biological carbon pump and sequestering carbon dioxide from the atmosphere into the oceans. It is predicted that continued anthropogenic carbon emissions will lead to further warming in the Southern Ocean, thereby affecting stratification, light and nutrient availability. The two latter factors are the primary controllers of photosynthesis and phytoplankton growth. Alterations in water column properties caused by climate change will not occur separately, thus, understanding the effects of light and iron on phytoplankton physiology is key to determining future changes to primary production. Regional and in situ phytoplankton studies are sparse in the Southern Ocean. As such, predicted changes in primary production in this area are still uncertain, even in the sign of change. The goal of this thesis is to understand how light and iron affect phytoplankton physiology in the Southern Ocean, and to assess how phytoplankton bloom phenology and net primary production are affected by environmental changes.
This thesis is divided into three chapters of original data analysis, with introductory and concluding chapters. The first data chapter investigates the co-limiting effects of iron and light on phytoplankton physiology and growth off East Antarctica during summer. This chapter used incubation experiments on board the RV Investigator to measure the effects of three different levels of iron and two levels of light on phytoplankton growth, photosynthetic efficiency, and community composition. The findings show that the effects of iron limitation on phytoplankton growth are most pronounced only when light is sufficient. Under low light, iron limitation has no effect on phytoplankton growth and physiology. As phytoplankton in the Southern Ocean have adapted to low light, they are able to survive and maximise light absorption under low iron conditions. These results are key to understanding how phytoplankton will be affected by future changes in the ocean including warming, stratification, light availability and iron inputs.
Both iron and light availability in the Southern Ocean change throughout the annual cycle. Light is the limiting factor for phytoplankton growth during the winter, especially at high latitudes. As the winter ends, light becomes available and phytoplankton start to grow, making use of dissolved iron. This phenomenon marks the start of the growing season and it is known as the spring bloom. The timing of the bloom has been a topic of debate and has received a lot of interest in the last two decades, especially in the Southern Ocean, where data is limited during the winter months. Autonomous robotic floats are now able to measure features of the water column year-round, however there is still discrepancy in the literature regarding the spring bloom initiation. Using nearly a decade of data from the Biogeochemical (BGC)-Argo float array, the second chapter of the thesis is the first study to systematically investigate phytoplankton bloom phenology derived from three variables: chlorophyll, phytoplankton carbon and nitrate. This chapter shows that the bloom begins earlier when it is calculated from chlorophyll, compared to phytoplankton carbon and nitrate. This is due to light acclimation, where phytoplankton synthesize chlorophyll before growing when light is limiting, early in the season, resulting in increased chlorophyll per cell. This highlights the importance of choosing variables appropriately and understanding phytoplankton physiology when studying bloom phenology in the high-latitude Southern Ocean.
The Southern Ocean is an annually-low but seasonally-high productivity area, which means that although there is no production during the winter, the phytoplankton productivity during the growing season is an important contribution to the biological carbon pump. Until recently, most Net Primary Production (NPP) models relied on remote sensing and sparse 14C incubation experiments. With the introduction of BGC-Argo floats, combined float data and satellite-based algorithms are now able to reproduce 14C incubation NPP measures in the Southern Ocean. The last chapter of this thesis uses 339 floats and a satellite-model to (1) estimate subsurface NPP across the Southern Ocean, and (2) investigate the influence of Deep Chlorophyll Maxima (DCMs) on the vertical distribution of NPP. This is the first study to quantify the contribution of subsurface NPP to total NPP in the Southern Ocean, and to confirm that DCMs enhance this contribution. The results indicate that NPP below the mixed layer depth accounts for ~60% of total NPP only when DCMs are present. This contribution is most significant in the summer and at lower latitudes, when and where DCMs are most common.
Climate models predict that ocean warming will continue under current carbon emissions scenarios. This will cause stronger stratification, leading to a more light-exposed and nutrient-limited surface environment, along with changes in the vertical distribution of phytoplankton and NPP. Consequently, understanding how nutrients and light availability affect phytoplankton physiology and growth is essential to be able to predict how NPP will be affected by future changes in the ocean. This thesis addresses three important knowledge gaps and improves our understanding of phytoplankton dynamics for the application to future models.