Bloom and bust: seasonal cycles of phytoplankton and carbon flux

CO2 taken up from the atmosphere by biological processes in the ocean is transferred into the deep ocean mostly in the form of sinking particles. Without this "biological carbon pump", atmospheric CO2 would be 50% higher than it already is. Despite its importance, our understanding is limited by scarce ship-board observations, especially how particulate organic carbon (POC) fluxes vary over days to seasons. However, a growing network of underwater robots are opening the door for high-resolution observations of these sinking particles. Gliders are unmanned autonomous vehicles that make measurements over weeks to months, including optical backscatter data which can be used to estimate POC fluxes. At PAP, one of the UK's long-term observing sites, we have collected data from multiple glider missions, including in collaboration with colleagues at NASA. These data will allow important unresolved questions to be addressed: How do fluxes vary seasonally in relation to phytoplankton blooms? How episodic are the fluxes? What are the implications for characterizing global-scale organic carbon fluxes? (Bol et al., 2018; Henson et al., 2015) Filling this gap will allow better predictions of how this important planetary carbon flux is linked to productivity and how it responds to variability in the environment.

The project will initially use data obtained from gliders during past missions at PAP, such as OSMOSIS [1] and EXPORTS to determine the temporal patterns in POC fluxes, their attenuation with depth, and how this relates to phytoplankton blooms. Glider-derived optical backscatter data will be transformed into estimates of POC concentration and flux, using protocols established by co-supervisor Briggs [2]. Different metrics to characterise the fraction of POC flux reaching the mesopelagic zone will be calculated. Your initial analysis will focus on establishing the temporal patterns of phytoplankton blooms, flux and attenuation, before moving on to assessing how interannual variability in physical conditions may alter the relationship between primary production and fluxes. Depending on your interests, subsequent work may include exploiting additional autonomous technologies to study particular processes, e.g. particle fragmentation [2], other locations in more detail, satellite data to broaden the study to larger spatial and temporal scales, or using global biogeochemical models to explore the mechanisms and patterns of variability in POC fluxes. If interested, you may participate in the planning and deployment of more autonomous platforms for 2 upcoming funded projects (Bio-carbon and ReBELS), both aimed at better quantifying the carbon sequestration pathways in subpolar regions.