Transformation of carbon and nutrients associated with microbial activity in the sandy sediments of the continental shelves

The continental shelves are arguably amongst the ocean floor environments most sensitive to climatic variation, as they are vulnerable to sea-level change. Human activity has only increased this level of disturbance, and the continental shelves are now impacted by multiple stressors, including eutrophication. Eutrophication on the shelves is a result nutrient nitrogen inputs from riverine run off and atmospheric deposition. So far, however, concurrent rises in nitrogen in the open ocean have not been as dramatic. It has been hypothesized that this is due to microbially mediated nitrogen removal in the upper layers of the sandy sediments which cover 60% of the continental shelf seafloor. At the same time however, these denitrifying processes likely represent an overlooked source of the potent greenhouse gas, N₂O. To date there are still major questions concerning the functioning of the sandy shelf buffer system under increasing anthropogenic pressure, and the imprints that turnover in shelf sands leaves on material that is transported to the open ocean and deep seafloor. We must answer these questions to understand nitrogen cycling in shelf systems and to incorporate the continental shelves more successfully into numerical Earth System models. As a disproportionate amount of primary production occurs on the continental shelves, changes in benthic-pelagic coupling on them has the potential to impact the fluxes of carbon and other elements to the deep ocean floor.  Therefore, without the shelf component, we will be unable to gain a complete picture of ocean-floor processes, their transfer of elements with seawater and their future response to environmental change.

An overarching aim of Theme 2 in Receiver is to quantify the role that sandy shelf sediments play in nutrient removal and recycling and in order to better constrain carbon and nitrogen budgets on continental shelves and their imprints on the open ocean and deep sea-floor. Within this project we  aim to:

• Determine the environmental factors and adaptations that lead to the co-occurrence, or dominance of nitrification, denitrification, anammox or dissimilatory nitrate reduction to ammonium (DNRA) in the variable redox zone of sands.
• Investigate the ecophysiology and life strategies of the nitrifying community in shelf sediments.
• Constrain the influence of benthic ammonia oxidisers on water column GDGT signals to enhance our ability to reliably reconstruct past environments and to use GDGTs as biomarkers for pelagic thaumarchaeal distribution.