Seasonal controls on shelf-edge nutrient fluxes at the Malin Shelf

Nutrients are essential for phytoplankton growth in the sea. Phytoplankton take up nutrients from the surrounding water and the extent of their growth is largely dependent on nutrient availability. Nitrate and phosphate are vital nutrients and their presence in light-rich surface waters can sustain algal blooms. Phytoplankton draw down carbon dioxide from the atmosphere and in shelf seas can lock this carbon up in the sediments.

In this project we will investigate the idea that the seasonal supply of nutrients to shelf seas is controlled by physical mixing processes at the shelf edge. This will be carried out using new sensors capable of determining nitrate and phosphate in-situ which will be deployed on underwater gliders for up to 3 weeks at a time. Underwater gliders are autonomous vehicles that, after launching from close to the coast, can "fly" to a sampling site and send back data via a satellite link. We will use four glider missions over a seasonal timescale (June & Sept 2013, Jan & April 2014) in order to gain a fundamental insight into the annual supply of nutrients to the highly productive shelf seas around the Malin Shelf. Alongside the nutrient sensors on the gliders will be three other sensors, a conductivity and temperature sensor, an oxygen sensor and a sensor which can determine chlorophyll a which is used as a proxy for phytoplankton biomass in the water. For each mission we will use two gliders to simultaneously characterise both the on- and off-shelf concentration gradients. These glider missions will be integrated into a large NERC-funded consortium, FASTNEt, which aims to identify the physical processes that facilitate the exchange of water across the edge of the north-west European shelf and understand how they evolve across the seasons. FASTNEt will be deploying long-term, shelf-edge current meter moorings and another underwater autonomous vehicle, the Autosub Long Range (ALR) in the same location.

The use of sensors deployed on gliders allow us to increase the spatial and temporal coverage of data that would not be possible from traditional shipboard measurements, that cannot be carried out in stormy conditions. The combination of in-depth knowledge of the physical processes occurring at the Malin Shelf along with accurate high resolution nutrient data from the sensors means that we will be able to investigate what controls nutrient supply to the photic zone and how this impacts on the phytoplankton growth.

Additionally, we will be trialling new deployment methodologies for these underwater gliders - using fast day-boats to take them as far offshore as possible from a coastal base.

Our data will be freely available and we will share our findings with end-users (such as the UK government's Department for Environment, Food and Rural Affairs who are responsible for fisheries, and the UK Met Office) throughout the project.

NERC's investment in the development of new sensors combined with autonomous platforms has the potential both to increase the spatial and temporal resolution of measurements in the marine environment and to substantially decrease the associated cost. By demonstrating that these sensors can be deployed on gliders for extended durations will allow scientists to utilise these tools for routine monitoring and process based studies with confidence. For example, CEFAS and the Environment Agency could use this approach to acquire the data needed for the Water Framework Directive and monitoring nutrient concentrations in coastal and shelf systems.

An improved biogeochemical process understanding of the role of nutrient supply in controlling primary production in shelf seas will have wider benefits, as changes to productivity are fundamentally coupled to global climate and to the stability of marine ecosystems. The shelf seas are strongly impacted by stratification, and with future changes predicted to increase this stratification, the supply of nutrients will change with subsequent impacts on primary production and carbon cycling. The outputs of this study will therefore be of direct and lasting benefit to a wide user-community of policy-forming bodies such as International and National Governmental Environmental and Climate change departments (e.g. UK Met Office, Environment Agency, European Union, UK government bodies including DECC and CEFAS) interested in modelling the shelf seas and the way in which these systems can be managed and protected.

Our project partner, the Defence Science and Technology Laboratory (DTSL), are very interested in this project as it will provide them with information on the performance of long-range and long-endurance underwater vehicles such as gliders. The use of chemical, biological and physical sensors mounted on underwater vehicles will help them maximise the impact of science and technology on UK defense and security which is one of their main aims. DTSL are interested in looking at novel methods for deploying underwater vehicles and would gain from the expertise generated during this study.

This project is part of the large FASTNEt consortium and as such our data and findings will be communicated to their project partners (NCOF, Marine Scotland, Marine Institute - Ireland) during the planned stakeholder workshop (Edinburgh).
The use and capability of nutrient sensors deployed over long time periods on gliders will be communicated to both international and national scientific and technical audiences at Oceanology, AGU/ASLO Ocean Science and Challenger meetings. We will also form a link with the European Centre of the International Nitrogen Initiative (http://www.ini-euorpe.org/) and the "Everyone's Gliding Observatories" Initiative (http://www.ego-network.org/).

All datasets and related products produced for this project will be transferred to the British Oceanographic Data Centre (BODC) for archiving and dissemination. In addition the use of the data from this project will be adapted for teaching as students are fascinated and inspired by remotely operated underwater technology.

We will also contribute to the Public and Schools Engagement planned by FASTNEt, where a glider display, and live data feed, will be hosted at the Science Museum in London and the Scottish Ocean Explorer Centre in Oban.