The ocean holds fifty times as much carbon as is in the atmosphere. Biological processes contribute to storing carbon in the ocean on climate-relevant timescales (hundreds to thousands of years). Marine phytoplankton (drifting microscopic plants) use sunlight and carbon dioxide in the upper ocean to form their bodies which are rich in carbon. When phytoplankton die they might clump together and sink into the ocean interior, or they could be eaten by zooplankton (tiny animals) which produce fecal pellets that can sink rapidly. Once this organic carbon is deeper in the water, bacteria might colonise the particles and break them down, or they could be broken apart by zooplankton feeding on them. These processes all act to reduce the amount of organic carbon reaching the deep ocean, however the deeper it goes the longer it will remain out of contact with the atmosphere. This "biological carbon pump" helps to regulate our climate, and without biology in the ocean it has been shown that atmospheric carbon dioxide levels could be nearly double what they are today. Earth system models have differing, but all fairly simple, representations of the biological carbon pump due to a lack of understanding of how the processes contributing to particle formation and respiration function. The suite of models that contribute to the Intergovernmental Panel on Climate Change reports do not agree on the magnitude or direction of change for ocean carbon storage under future climate scenarios. This means we have low confidence for our future projections of ocean carbon storage, which is further impeded by a growing discrepancy between models and observations. In this project we will examine how much carbon has been respired during the transit from the upper ocean, and in what ways. We will measure the important processes of particle fragmentation and aggregation, microbial respiration, and zooplankton vertical migration and respiration. We will do this using a process cruise and autonomous underwater vehicles. We seek to answer the question: How is organic matter transformed and respired by biotic interactions in the mesopelagic, how does that vary with depth, location and season, and what are the consequences for ocean carbon storage? The ultimate goal is to generate new detailed understanding of important processes that influence the rate and depth of interior respiration which we will scale up to provide the globally-resolved information needed to develop the next generation of biogeochemical models.

The open ocean ecosystems which dominate the surface of our planet are all dependent on the generation of new organic matter by single celled organisms which are collectively termed phytoplankton. These organisms use light, nutrients and carbon dioxide to grow through a process termed primary production. In addition to forming the base of the marine food web, the collective primary production by these organisms is ultimately responsible for ocean biology keeping atmospheric carbon dioxide levels around 30-40% lower than they would otherwise be, thus exerting a significant impact on global climate. Understanding how primary production may vary in the future is thus important for predicting the ongoing response of both ocean ecosystems and carbon cycling to climate change. The abundance and activity of phytoplankton in the upper ocean is always a balance between growth rates (determined by the availability of resources) and loss rates including through grazing by organisms collectively termed zooplankton and mortality due to viruses and direct sinking. However, the factors determining both growth and loss dynamically vary both across the different regions of the ocean and throughout the annual cycle in complex and interacting ways. We currently try and capture the knowledge necessary to predict future changes in primary production using numerical models of these interacting processes. However, our current state-of-the-art models differ substantially in their predictions of future change due to the differing ways they represent a variety of these key processes. Focusing on an important region of the ocean for biological carbon storage, the mid-high latitude North Atlantic, our proposal aims to make exciting new year-round observations of primary production and the controlling factors using a combination of satellite, ship-based and novel robotic platforms. We will augment these observations with detailed experimental work undertaken at sea, alongside targeted numerical modelling, in order to generate an improved understanding of the balance between controls on growth and loss and, crucially, establish how this varies over the dynamic seasonal cycle. Data from our observations and experiments will allow us to establish key drivers of the magnitude and seasonal changes in primary production and link these to the overall controls on the efficiency of ocean carbon storage across a broad region of the North Atlantic Ocean. In addition to providing new understanding, our research will generate improved data sets of rates of growth and loss, providing more rigorous constraints for numerical models and hence pointing the way towards more confident predictions of future primary production and carbon cycle responses to climate change.