The surface ocean is home to billions of microscopic plant-like phytoplankton which produce organic matter in the surface ocean using sunlight and carbon dioxide. When they die, they sink and take this carbon into the deep ocean, where it is stored on timescales of hundreds to thousands of years. This storage helps to keep our climate the way it is today. This process of biological CO2 uptake and storage in the deep ocean is called the 'biological carbon pump' and, in order to understand how our climate will change in the near future, we need to understand what controls this process. Until fairly recently, the biological carbon pump was thought to work almost independently from the mixing processes that occur in the oceans, such as during storms, winter or by meandering ocean currents. However, recent work suggested that these physical processes may be very important for the biological carbon pump, providing a direct pathway for carbon to reach the deep ocean, and can contribute as much carbon to depth as the sinking of dead matter alone. Therefore, we urgently need to understand how the biological and physical processes interact to transport organic matter into the deep ocean. Two reasons explain this clear oversight: Physical and biological oceanographers often work independently, so that crossdisciplinary processes can get overlooked. In addition, the location where, and times when, these processes have the most dramatic effect on ocean carbon storage are hostile environments to work in, with very high waves and strong winds that make working from ships nearly impossible. ReBELS is an exciting programme that will bring together physical and biological oceanographers to closely work together on the biological carbon pump. To overcome the logistical challenges, ReBELS will take advantage of the recent developments in technology, using state-of-the-art marine autonomous robots that will be able to sample the ocean at times where we cannot do so with ships. Our study site will be the Labrador Sea in the Northwest Atlantic. There, organic carbon stays in the deep ocean much longer than anywhere else in the world (>1000 years). Moreover, the Labrador Sea has been identified as a very important location for the climate, as it is strongly affected by increasing temperatures and melting ice. Using autonomous technology, we will measure the biological carbon pump over the course of an entire year, and quantify carbon transport and carbon storage through the different biological and physical processes. To do so, we will measure the distribution of organic matter particles throughout the water column and determine whether they are sinking or being transported by ocean mixing. We will then extend our results to the entire Northwest Atlantic using proxies that can be determined on larger scales (for example from satellites). Finally, we will work with modellers to include these important processes when predicting climate in the future.

There is a consensus: the Arctic Ocean faces a future with less sea ice. Therefore more sunlight reaches the surface waters accompanied with major implications for the ecosystem functioning, productivity, and biogeochemistry. Sunlight is essential for life in the ocean. It drives the photosynthesis of biogenic matter by phytoplankton that fuels in turn the productivity of the whole pelagic food web. Long minimized by biogeochemical modellers, simulating accurately the underwater light environment experienced by phytoplankton is pivotal to robustly predict the Arctic Ocean ecosystem response to climate change. It is particularly true for the productive shelf waters characterized by complex optical properties. A recent study predicts a substantial increase of Arctic plankton productivity in response to longer sunlight exposure caused by sea ice melt. However, it might not be a general rule everywhere in the ocean. Arctic shelves experience the highest freshwater and dissolved organic matter (DOM) discharge of any ocean and it this will increase due to Arctic warming. A large fraction of this DOM is coloured (CDOM) and therefore strongly absorbs sunlight, reducing light levels needed for phytoplankton growth. How much would this strong sunlight absorber contribute to lower plankton productivity in the rapidly changing Arctic? Embedding the riverine CDOM in biogeochemical models is required to answer this pressing question and to further produce robust climate change scenarios for the Arctic Ocean. A high-resolution ocean-sea ice-plankton ecosystem model will be used to simulate the ocean and biological conditions for years of normal and relatively low sea ice coverage (i.e. higher sunlight exposure for phytoplankton) in the Barents/Kara Sea. Riverine CDOM will be accounted in the model as a variable subject to transport to map and validate its spatio-temporal distribution in the coastal waters. A complex bio-optical model will be then developed and embedded in the physical-biological coupled model to simulate the TCDOM shading effect on the underwater light field jointly with its main light-mediated removal process (photodegradation). This will allow more accurate simulated primary production rates that will be compared with coincident satellite-derived estimates obtained using a new method developed by two collaborators of this project. This comparison will be the first for the Arctic waters and therefore constitutes an important framework for operational biological oceanography in this productive and economically important area.

The balance between the production of organic carbon during phytoplankton photosynthesis and its consumption by bacterial, zooplankton and phytoplankton respiration determines how much carbon can be stored in the ocean and how much remains in the atmosphere as carbon dioxide. The amount of organic carbon stored in the ocean is as large as the amount of carbon dioxide in the atmosphere, and so is a key component in two global carbon cycle calculations needed to avoid a global temperature rise of more than 1.5 degrees C: the calculation of the technological and societal efforts required to achieve net zero carbon emissions and the calculation of the efficiency of ocean-based engineering approaches to directly remove carbon dioxide from the atmosphere. Yet, despite its vital role, our ability to predict how ocean carbon storage will change in the future is severely limited by our lack of understanding of how plankton respiration varies in time and space, how it is apportioned between bacteria and zooplankton and how sensitive it is to climate change-induced shifts in environmental conditions such as increasing temperature and decreasing oxygen. This woeful situation is due to the significant challenge of measuring respiration in the deep-sea and the uncoordinated way in which these respiration data are archived. This project will directly address these two problems. We will take advantage of our leadership and participation in an international programme which deploys thousands of oceanic floats measuring temperature, oxygen and organic carbon in the global ocean, in an international team of experts focused on quantifying deep-sea microbial respiration, and our experience of collating international datasets, to produce an unprecedented dataset of bacterial and zooplankton respiration. We will derive estimates of respiration based on data from floats, so that together with estimates derived from recently developed methods including underwater gliders, the new database will include respiration measurements calculated over a range of time and space scales. Crucially, respiration rates will be coupled with concurrent environmental data such as temperature, oxygen and organic carbon. This dataset will enable us to quantify the seasonal and spatial variability of respiration and derive equations describing how respiration changes with the proportion of bacteria and zooplankton present and with the chemical and physical properties of the water. These equations can then be used in climate models to better predict how respiration and therefore ocean carbon storage will change in the future with climate-change induced shifts in temperature, oxygen, organic carbon and plankton community. We will take part in a hybrid hands-on and online international training course on observations and models of deep-water respiration targeted to early career researchers from developing and developed countries to showcase the useability of the respiration database and the global array of oceanic floats. We will also prepare Science Festival exhibits on observing life in the deep ocean for schoolchildren. The deliverables of the project - a unique global open-access database of respiration measurements, new equations describing the sensitivity of respiration to changing temperature and oxygen suitable for climate models and online training materials for early career researchers - are of benefit to scientists who aim to predict how a changing climate will affect the storage of carbon in the ocean, educators who train the next generation of ocean scientists and practitioners, policy makers who need to quantify nationally determined contributions to actions limiting global warming, and scientists, engineers, lawyers, governing bodies and commercial companies designing, evaluating and implementing ocean-based carbon dioxide removal technologies.

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 long-term storage of carbon in the deep ocean by sinking particles is a key piece of the global carbon cycle, playing a critical role in setting atmospheric CO2 levels, driving ocean de-oxygenation, and delivering food to deep ocean ecosystems. The aim of GLOBESINK is to improve our ability to answer the big question: "How will storage of carbon by sinking ocean particles change in the future?". The answer could have wide-reaching impacts on climate, fisheries, and biodiversity. GLOBESINK will attack this big question using a new "big data" approach. The first step will be to dramatically increase the number of sinking particle flux measurements in the ocean, by approximately 10-fold. GLOBESINK will achieve this by applying its own innovative statistical methods to 10s of millions of particle measurements made by a new global fleet of over 200 drifting underwater robots. Using this new dataset, GLOBESINK will directly calculate the total global carbon stored and how this amount has varied in time and space over the past decade. This information will be combined with key measurements of particle size, rate of particle breakup, temperature, and oxygen concentration, all hypothesised to be important drivers of carbon storage, in order to assess whether and how these drivers should be included in global climate models.