The oceans play a major role in determining the world's climate. In part this is due to the production of oxygen and the consumption of carbon dioxide by very small, single celled organisms, which are referred to as the photosynthetic picoplankton. Marine cyanobacteria of the closely-related genera Prochlorococcus and Synechococcus are the prokaryotic components of the photosynthetic picoplankton. Current and previous work in my lab has demonstrated that the in situ community structure of these organisms is fairly complex, with specific ecotypes or lineages occupying different niches to populate the world's oceans, allowing them to grow and photosynthesise under a broad range of environmental conditions. Whilst such molecular ecological studies can effectively map the spatial distributions of specific genotypes, the factors that dictate this global community structure are still poorly defined. This is important because changes in dominant picocyanobacterial lineages indicate major domain shifts in planktonic ecosystems and by observing and interpreting their distributions and physiological states we are essentially assessing changes in the rates of biogeochemical cycles. Athough the role of macronutrients, particularly N and P has received previous attention still there is a relative dearth of data on factors controlling picocyanobacterial community composition. Certainly, little if anything is known of the role of trace metals in this process. Thus, we hypothesise that in oceanic ecosystems genetically distinct picocyanobacteria are restricted to specific niches by their ability to acquire (limitation) or regulate trace metal accumulation (toxicity). In order to address this topic we propose to investigate trace metal (and macroelement) cell quotas in i) representatives of specific marine Prochlorococcus and Synechococcus lineages and to assess the affect of light stress and macronutrient shifts on these quotas and ii) in natural picophytoplankton assemblages using prior flow cytometric sorting, ICP-MS and X-ray microanalysis techniques. In so doing we will also obtain, for the first time, a real indication of picocyanobacterial cell physiological state over large spatial scales / in effect using elemental quotas as a proxy for what environment a given cell/population of cells is experiencing in situ / and hence can realistically begin to determine those macro and trace elements that are potentially depleted in situ and which are potentially restricting growth rate and/or yield.

Submarine turbidity currents are arguably the volumetrically most important process for moving sediment across our planet. They form the largest sediment accumulations (submarine fans) on earth, and single flows can transport ten times the annual flux from all of the world's rivers. However, the most remarkable feature of turbidity currents is how few direct measurements are available from these flows, as they are notoriously difficult to monitor in action. This is a stark contrast to other major sediment transport processes, such as rivers for which we have many thousands of direct measurements. Powerful long run-out turbidity currents are especially difficult to monitor, yet it is these flows that build submarine fans. Such flows are important because they break sea-floor cables that carry > 95% of global data traffic, including internet and financial markets that underpin daily lives. The velocity of turbidity currents that reach beyond the continental slope had previously been measured in just five locations, primarily from cable breaks that only record averaged front velocities. Their sediment concentration had never been measured directly. This globally important sediment transport process is therefore poorly understood, and laboratory or numerical models for such flows are poorly validated. This PhD student will analyse a remarkable dataset comprising the first synchronous velocity and concentration profiles for turbidity currents beyond the continental slope, collected at a cost of > $1M by CASE partner Chevron and co-workers in the Congo Canyon (from 2009-2013). This is the first time that high temporal resolution (>1/min) synchronous profiles of both velocity and concentration have been measured for turbidity currents beyond the continental slope. They are also the fastest (2.5 m/s) turbidity currents yet measured by instruments. The data were collected for a major oil and gas pipeline that will need to cross the Congo Canyon. This is a challenging project as previous cable breaks show the canyon is regularly swept by powerful flows. The data comes from moorings with downward pointing Acoustic Doppler Velocity Profilers (ADCPs) that measure velocity and acoustic backscatter. Backscatter is partly dependent on grain size, but also records changes in sediment concentration. Initial results were surprising for two reasons. First, flows had surprising durations of several days, with average speeds of ~1 m/s. Interestingly, it was observed that the larger flows always had a similar duration of ~6 days. This seems to indicate the establishment of an equilibrium flow configuration over the first 150 km of the canyon. Several hypothesis have been put forward to explain this behaviour, however, none of them have yet been validated. Second, the measured turbulence intensity decreased as flow speeds increased, this counter-intuitive relation suggests damping of turbulence by elevated suspended sediment concentrations. Although it has been speculated previously that turbulence damping by sediment may be of fundamental importance, such damping has never previously been documented in direct observations from full scale flows in the field. Thus, the relation between turbulence damping and sediment concentration remains to be validated in real submarine flows. A numerical model is needed to test hypotheses that equilibrium flow configurations produce multi-day flows, and to explore how turbulence dampening may affect submarine flows. Such numerical model will benefit from the well-mapped bathymetry of the Congo canyon (from Chevron and past publications by IFREMER). In this proposal we will use a state-ofthe-art fully three-dimensional numerical model that has been developed over the last eight years by project partner Complex Flow Design AS (CFD), Norway. This model is unique due to its 3D approach and its capability to introduce sediment concentration effects into its turbulence model.

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.

Plankton in the ocean, microscopic plants (phytoplankton) and tiny animals (zooplankton) that eat the plants, are vital to marine life and to Earth's climate. They form the base of food chains that support ocean ecosystems, and remove carbon from the atmosphere and bury it in (or export it to) the ocean depths. It is currently thought that plankton are responsible for removing 6 billion tonnes of carbon from the atmosphere each year; fossil fuel burning releases about 10 billion tonnes of carbon into the atmosphere annually. Without this export of carbon in the ocean, atmospheric CO2 would be twice the current concentration. The importance of plankton to food chains and carbon export depends on the species of plankton. Larger phytoplankton are better at supporting food chains and at exporting carbon because (1) larger phytoplankton sink quicker, removing carbon away from the sea surface and contact with the atmosphere, and (2) larger phytoplankton support larger zooplankton, which are eaten by fish and which also excrete large, fast-sinking faecal pellets which quickly transfer carbon away from the atmosphere. We have discovered a new link between which types of plankton can grow and the tides flowing over a mid-ocean ridge. The ocean is layered, with warmer, less dense layers at the surface and colder, denser layers deeper in the ocean. When tidal currents flow up and down the flanks of a mid-ocean ridge, these layers are pushed up and down, causing waves on the layers called "internal tidal waves". These internal tidal waves reach up to the sun-lit upper ocean, where photosynthesis by the phytoplankton takes place. We think these waves have two important effects. (1) The waves cause mixing between the layers of ocean, bringing nutrients from deep in the ocean up to the phytoplankton; this will help extra phytoplankton growth, but crucially it is also known that extra nutrient supplies allow larger species of phytoplankton to grow. (2) The waves move the phytoplankton up and down; this provides more light to the phytoplankton, because as they are moved upward they get closer to the light at the sea surface and are able to grow more. Thus, we think that the internal tidal waves create more growth of larger plankton over a mid-ocean ridge, which means better food for marine food chains and more carbon exported away from the atmosphere. This new link may explain why ridges support such diverse ecosystems, and it also means that the ocean over ridges is far better at exporting carbon than we previously thought. We have calculated that, for the whole Atlantic Ocean, including the tidal effect of the mid-Atlantic ridge adds about 50% to current estimates of how much carbon the plankton export. This means that current understanding of the ocean's role in Earth's climate, which ignores the ridge-tide effect, significantly underestimates how much CO2 plankton remove from the atmosphere. We need to fix this because our predictions of our future climate depend on having correct descriptions of the processes that govern atmospheric CO2. We will conduct an expedition to the mid-ocean ridge in the S. Atlantic. We will measure the internal tidal waves and the upward mixing of nutrients, and the effect the waves have on light received by phytoplankton. We will measure how fast the phytoplankton and zooplankton grow in response to these waves, how the species of plankton change over the ridge, and how much carbon is exported downward over the ridge compared to the adjacent ocean basin. This will be the first time that internal tidal waves are linked to patterns of carbon export in the ocean: internal tidal waves occur wherever there are ridges or seamounts in the ocean and our results will have important global implications for our understanding of ocean food webs and Earth's climate.

The seasonal thermocline in temperate shelf seas acts as a critical interface in the shelf sea system. It is a physical barrier to vertical exchange, controlling biological growth through the summer and enabling the sequestration of atmospheric CO2. Once the spring bloom is over the seasonal thermocline separates the sun drenched but nutrient deplete surface waters from the dark nutrient rich deep water. The vertical mixing of nutrients across the seasonal thermocline acts to couple this well-lit surface zone with the deep water nutrient supply, leading to the formation of a layer of phytoplankton within the thermocline (the subsurface chlorophyll maxima). This phenomenon is estimated to account for about half of the annual carbon fixation in seasonally stratified shelf seas, and yet the controlling physics is only just being unravelled. The identification and parameterisation of the physical processes which are responsible for the vertical mixing of nutrients across the thermocline is a vital prerequisite to our understanding of shelf sea ecosystems. Our proposal is to investigate the role of wind driven inertial oscillations in driving vertical mixing across the seasonal thermocline, identifying the mechanisms and processes responsible for their generation and dissipation on both special and temporal scales. The proposal will be achieved through an observational campaign closely integrated with numerical model predictions using both 1D and 3D numerical models.