This fully funded studentship is part of the £10M NERC Shelf Sea Biogeochemistry Programme. The studentship will involve seagoing fieldwork, modelling and data synthesis work. Training for any aspect of the work can be provided, but only candidates keen to engage with both practical and model and data-based work need apply.

This project aims to better understand the controls on winter nitrate concentration, which limits primary production (carbon fixation) in shelf seas. In regions such as the North Sea where water 'overwinters' on the shelf, isolated from external nutrient inputs, winter nitrate must be controlled by  the balance between its production by nitrification and its loss from the water column by denitrification (to dinitrogen gas). However, nitrification (which requires oxygen and ammonium and produces nitrate) is a necessary precursor to denitrification (which requires anoxia and consumes nitrate).  Both processes can happen in sediments but only nitrification is likely to happen in the water column, so sediment oxygen dynamics and benthic disturbance (wind, tidal, trawling etc) must be important in determining this balance and thus the capacity for carbon fixation.  Winter nitrate in the North Atlantic is about double that at the North Dogger site in the central North Sea, where considerable oxygen depletion occurs in the water column, possibly enhancing denitrification. As shelf waters warm and hypoxic conditions become more widespread, on-shelf productivity may decrease as more N is removed from the system.  This is a hitherto unexplored shelf sea climate-carbon feedback.

During fieldwork in the North Sea and Celtic seas, on-deck sediment resuspension experiments will test the hypothesis that rapid water column nitrification can occur following the co-release of ammonium and nitrifying bacteria from sediments during disturbance.  This will be tested for different sediment types at various sites.

The student will conduct a model-based 'age experiment' using the NEMO hydrodynamic model to determine the average age of water on the NW European shelf. This involves minor modifications and re-running of existing  experiments. This age parameter can then be related to winter nitrate concentration synthesised from a range of sources to estimate total denitrification and its distribution across the shelf region, which will be a novel and significant result. 

In their final year the student may choose to undertake further 3D, 1D or simple box modelling, or further lab or field experiments to gain further insight into the interactions between carbon, oxygen and nitrogen cycling on the shelf. They will be encouraged and supported to publish academic papers during the course of their PhD and to develop their own ideas and experiments. The supervisory team consists of Martin Johnson and Jan Kaiser (UEA) and Ruth Parker and Liam Fernand (Cefas). Collaborators Tiago Silva (Cefas), Fay Couceiro (Portsmouth) and Charlotte Thompson (Southampton) will provide input to modelling and field experiments. This is a CASE studentship with Cefas.

(i) Couceiro et al, 2012, Impact of resuspension of cohesive sediments at the Oyster Grounds (North Sea) on nutrient exchange across the sediment–water interface, Biogeochemistry, http://dx.doi.org/10.1007/s10533-012-9710-7
(ii) Neubacher et al 2012, The potential effect of sustained hypoxia on nitrogen cycling in sediment from the southern North Sea: a mesocosm experiment, Biogeochemistry, http://dx.doi.org/10.1007/s10533-012-9749-5
(iii) Hydes et al, 1999, The balance of supply of nutrients and demands of biological production and denitrification in a temperate latitude shelf sea -- a treatment of the southern North Sea as an extended estuary. Marine Chemistry, 68(1-2), 117–131. doi:10.1016/S0304-4203(99)00069-9
(iv) Holt et al, 2012, Oceanic controls on the primary production of the northwest European continental shelf: model experiments under recent past conditions and a potential future scenario. Biogeosciences 9, 97-112, http://www.biogeosciences.net/9/97/2012/
(v) Greenwood et al., 2010,  Detection of low bottom water oxygen concentrations in the North Sea; implications for monitoring and assessment of ecosystem health. Biogeosciences 7, 1357-1373. http://www.biogeosciences.net/7/1357/2010/