Often called ‘ocean deserts’, subtropical gyres have low biological productivity but, due to their immense size, contribute significantly to carbon sequestration regulating global climate. In gyres, biological productivity strongly depends on nitrogen supplied by microbes called diazotrophs, capable of fixing molecular nitrogen (N2) into bioavailable nitrogen forms. Earth system models predict increasing uncertainty in biological productivity towards the end of the 21st century. Such uncertainty responds to N2 fixation parametrisation in models and is largely driven by the scarce observations available in the Indian Ocean (IO), representing only 1% of the N2 fixation data available globally. Moreover, our current understanding of N2 fixation is mainly based on nutrient availability (phosphorus and iron) impacts on diazotrophs. However, my previous research shows that other controls including ocean circulation and diazotroph/non-diazotroph interactions are key in shaping N2 fixation inputs locally. None of these controls have been comprehensively examined over the vast extension of the IO, nor throughout seasons. We will conduct oceanographic expeditions covering the full extension of the IO gyre at its minimum and maximum expansion seasons. Gyre expansion will be tracked with satellite and in situ hydrographic and current speed measurements. The impact of chemical (nutrients), physical (ocean circulation), and biological (species interactions) controls on N2 fixation will be comprehensively measured, using at-sea experiments and up-to-date isotopic and molecular analyses. Moreover, seasonal variability will be monitored over a full year with mooring lines anchored at the centre, northern and southern edges of the gyre, equipped with DNA samplers and a newly designed automatic device measuring N2 fixation rates. These datasets will link cellular to ecosystem processes, bridging the gap between ocean desert expansion and N2 fixation in the world’s least explored gyre.

Photosynthesis in the ocean converts approximately 100 Gt of carbon dioxide (CO2) into organic matter every year, of which 5-15% sinks to the deep ocean. The depth to which this organic matter sinks is important in controlling the magnitude of ocean carbon storage, as changes in this flux attenuation depth drive variations in atmospheric pCO2 of up to 200 ppm. Efforts to produce global maps of flux attenuation have yielded starkly contrasting global patterns, blocking our understanding of ocean carbon storage and our ability to predict it. The bottleneck is our ignorance of the spatiotemporal variability of the processes that control flux attenuation. ANTICS will directly address this knowledge gap by using an innovative synthesis of cutting-edge in situ imaging, machine learning and novel data analyses to mechanistically understand ocean carbon storage. Use state-of-the-art imaging technologies, I will collect data on size, distribution and composition of organic matter particles and measure their sinking velocity in the upper 600 m across the Atlantic. I will design a neural network model that allows the conversion of in situ images into carbon fluxes, and develop analysis routines of particle size spectra that quantify the processes causing flux attenuation: remineralisation, physical aggregation/disaggregation, fragmentation/repackaging by zooplankton. By statistically linking these outputs to seasonality, depth, primary production and temperature, I will be able to determine which processes dominate under specific environmental conditions. This step change in our understanding will allow ANTICS to resolve flux attenuation spatially and temporally. I will use this pioneering knowledge to validate and inform the parametrization of the marine biogeochemical component of the UK’s earth system model used for carbon cycle forecasting in the next IPCC assessments.

Turbulent mixing controls the pace of ocean ventilation, the rate at which the ocean interior is filled up with water, heat and chemicals from the ocean’s surface. This process governs the ocean's ability to store atmospheric heat and greenhouse gases, exerting a profound impact on Earth's climate and moderating the rate of human-induced climate change. However, the role of mixing in ocean ventilation remains poorly understood, due to the scarcity of direct observations and the inability of numerical models to capture small-scale turbulent dynamics. Our limited understanding can be rationalised around three major questions: 1) How does the interplay between advective and diffusive processes control ocean ventilation? 2) What is the role of mixing in the water-mass transformations sustaining the ocean’s overturning circulation? 3) What is the relative importance of the two primary regimes of ocean turbulence (the mesoscale and microscale) for the ventilation of climate-critical tracers? REMIX-TUNE addresses these questions through an innovative approach founded on two pillars: 1) Deploying the first large fleet of autonomous profiling floats equipped with microstructure turbulence sensors in key ocean ventilation regions; and 2) pioneering a novel theoretical approach to quantify mesoscale and microscale mixing using float microstructure observations and existing hydrographic data from the Argo programme. With this strategy, REMIX-TUNE will generate the first comprehensive, observation-based global database quantifying the role of mixing in ocean ventilation. This understanding will then be used to develop a new framework to assess and ground-truth the representation of mixing in the next generation of ocean-climate models. Thus, REMIX-TUNE will elicit a step change in our rationalisation of ocean mixing, and its integration into numerical models, and will leave a lasting impact on mixing research by revolutionising the methodologies employed in the field.

The role of how we manage our seas is changing rapidly. The past approach was that of single species management, devoid of many environmental aspects, and without direct input from the fishing industry. That approach has been found wanting with many stocks fished at unsustainable levels. The way forward has been to recognise that we need to manage using an ecosystem based approach while at the same time maintaining direct interaction with the fishing industry in management decisions. Such an ecosystem approach requires a more mechanistic understanding of the role the physical environment plays whilst also taking into account multiple species interactions. It is also clear, as more demand is put upon the space use of our marine systems (i.e. offshore renewable developments, Marine Protected Areas and fishing effort becomes managed more on a spatial basis), a better understanding of the role of marine habitat is essential. This is particular difficult as marine habitat can move in both space and time. However, any fisherman can tell you that there are some locations (and times) that are almost always better than others for good catches. The understanding as to why that may be is a fundamental challenge. In earlier studies involving both the PI and Co-PI, in the North and Celtic Seas, we have found that in locations where there are disproportionately high numbers of seabirds, marine mammals or fishing effort, there are also spatially limited patches of high levels of sub-surface primary production. The patches appear to be associated with topographically-driven internal waves whose physical effects may be responsible for localised increases of primary production and the physical aggregation of smaller marine organisms. Therefore these features could be responsible for the predicable areas of foraging due to complex links in the entire food web (Hypothesis A) or just enhance the ability of predators to capture prey (fish) in these locations (Hypothesis B). The sub-surface signature of these areas has meant that they have been overlooked in studies which have identified important surface features, such as thermal fronts, known to contain aggregations of many marine species. These patches therefore represent a newly-identified class of spatially important locations in shallow seas. In this project we propose to test between Hypothesis A & B using a new survey framework that incorporates all of those factors and defines the critical characteristics of the marine habitat where multi-species (predator-prey) interactions and fisheries operations are more likely to occur. This survey framework is based, to some degree, on the tried and tested method that fishermen have used for centuries: i.e. to observe seabird and marine mammal behaviour as a guide to locations of fishing grounds. Our aim is to take that practice forward with a multi-disciplinary approach between marine ecologists (U. Aberdeen), biological and physical oceanographers (POL), statutory agencies (FRS, CEFAS, JNCC) as well as fishermen (CFPO) and a range of skilled marine animal and fisheries observers (JNCC, MRAG). This project also represents a unique opportunity, to more fully utilise a NERC Oceans2025 Project. In this interdisciplinary approach we will combine the continuous physical and biological oceanographic features that will be studied in the Oceans2025 program with the additional collection of acoustic sonar backscatter, visual observations of seabirds and marine mammals, vocalisations of cetaceans, fishery observations, and longer term fisheries and marine animal survey data. With these combined data products, we will identify and define the characteristics of these sub-surface patches and the bio-physical mechanisms that may lead to the facilitation of predator-prey interactions. This research will greatly enhance the type of mechanistic understanding needed for implementing the ecosystem approach to sustainable fisheries management.

Ocean biogeochemical cycles and ecosystems are an important part of the 'Earth system' - the set of interlinked physical, chemical and biological processes, which shape the environment at the Earth's surface. These biogeochemical cycles are not only important for the oceans themselves (their composition and the kinds of creatures that live in them) but also for the climate of the planet, through their fundamental influence on the composition of the atmosphere (in particular, 'greenhouse' gases such as carbon dioxide, and other climatically important gases such as di-methyl sulphide). Historically, global ocean biogeochemical models have used simple representations of biological processes that are constrained tightly by the physical and chemical environment, using assumptions such as single-nutrient limitation and constant Redfield ratios - utilization and release of elements in constant proportions. As our knowledge has grown, the shortcomings of this approach have become increasingly apparent, giving rise to progressively more elaborate models of the ecosystem - from models that include a single explicitly modelled plant (phyto-) and animal (zoo-) plankton to increasingly, a variety of different functional types of plankton that mediate different geochemical transformations. While these more complex models have the potential to reproduce more faithfully ocean biogeochemistry and how it will respond to changes in climate and ocean circulation, the increased complexity brings with it the penalty that many more parameters must be known in order to specify the system. It is not necessarily clear how to validate such models - that is, to tell how well they are working - or what is the optimum complexity of model required to address a given problem. We are proposing a consortium of several groups involved in biogeochemical modelling in the UK. Currently, the groups work separately, each on models occupying a different place on the spectrum of complexity sketched above. In MARQUEST they will co-operate, comparing the predictions of their models and analyzing the causes of their differences and similarities. We will also examine more fundamental modelling approaches to the planktonic ecosystem, with the aim of clarifying what we can expect from the current types of model. New research outputs from MARQUEST will include: the development of new methods of validating models, making use of remote sensing ocean colour data, in-situ data sets and the observations ongoing in major European programmes such as Carbo-Ocean and Euroceans: comparison of different ecosystem models run in the same circulation codes: development of a module to simulate the coastal ecosystems, but useable in global ocean biogeochemical simulations, and an accurate physical simulation of the North Atlantic guided by data assimilation into which ecosystem simulations can be embedded. This will enable detailed comparison of ecosystem models with observations over recent decades, including a hindcast of the variation in air-sea fluxes of gases - of great use for helping to constrain both land and ocean components of the sink for anthropogenic carbon dioxide. We will also make best estimates of the evolution of the CO2, oxygen and di-methylsuiphide fluxes from ocean to atmosphere over the next 50 and 100 years.