Glider observations of productivity in the Alboran Sea (GOPITAS)

Lead Research Organisation: University of East Anglia
Department Name: Environmental Sciences


Marine plants contribute about half of the global net primary production and thus sustain fisheries and world food supplies. Current climate change caused by anthropogenic greenhouse gas emissions is likely to affect this production through changes in temperature, ocean circulation, pH, nutrient and light availability. Understanding what drives production is therefore a key problem of marine science. Here, we propose a pilot study involving UEA's three ocean gliders, to make a significant contribution to improving this understanding by observing the physical, chemical and biological processes driving production in the Alboran Sea, western Mediterranean. In the temperate and polar oceans, winter overturning provides nutrients for surface production, but in the subtropical gyres this mechanism is too weak to overcome nutrient limitation. The gyres of the Alboran Sea behave like the subtropical gyres, but are much smaller and therefore easier to study. In such regions, smaller-scale processes are likely to contribute significantly, e.g., wind/turbulence interactions at the mesoscale (10-100 km) and submesoscale (1-10 km). Resolving these small-scale processes through traditional ship-board surveys is expensive and technically challenging. Recently developed autonomous platforms and sensors can significantly enhance traditional ship-based work. For example, a fleet of >3000 Argo floats now take regular temperature and salinity profiles of the upper 2 km of the world's oceans ( and help improve our understanding of oceanic heat budgets and circulation. Biological and chemical sensors add further dimensions to these technologies. In particular, oxygen sensors can measure net community production, i.e. the balance between oxygen-producing photosynthesis and oxygen-consuming respiration. Continuous measurements of key parameters and processes have thus become possible on a global scale. Floats can only vertically in the water column and are otherwise drifting passively. Gliders have been developed to partly overcome the limited manoeuvrability of floats. These autonomous vehicles can be interactively piloted in the vertical as well as horizontal direction and acquire depth profiles of marine physical and biogeochemical parameters with high resolution in space and time. They can 'see' where satellites cannot penetrate the surface, work for months at a time and are much cheaper than traditional oceanographic cruises. We propose the use of three gliders for simultaneous measurements of physical, chemical and biological parameters in the Alboran Sea, a small seasonally oligotrophic gyre system in the western Mediterranean Sea, adjacent to two frontal zones. Our overall goal is to establish how to best use gliders to improve our understanding of processes sustaining biological production, on all temporal and spatial scales. The GOPITAS pilot study will bring a biogeochemical component to the international REP10 Alboran Sea experiment organised by the NATO Undersea Research Centre (NURC) involving up to 15 gliders. The Small Grant is sufficient to enable GOPITAS thanks to the generous support in kind from our project partners, including free access to the ships HMS Roebuck (UK Navy) and NRV Alliance (NATO) as well as deployment and technical support from the glider manufacturer (iRobot). This will be one of the first deployments worldwide of three biogeochemical gliders simultaneously.
Description During the period of the grant from 31 March 2010 to 30 March 2012, we achieved most of the five original objectives, as detailed below. Submission of data to BODC is still outstanding, but is fore-seen by December 2013.

Seaglider SG510 ("Orca") was deployed by BO Mytilus on 1 June 2010 and recovered on 21 September 2010 after 113 days at sea. The mission comprised 18 zonal transects of about 50 km across the shelf slope of the Iberian upwelling system west of Galicia (Spain) at 42.1º N and 9 to 10º W and an excursion to chase an eddy about 100 km NW of the main study area (figure on the right). The glider completed 1611 dives - a world record -and travelled about 1600 km. Data from the last 8 days of the mission were lost because of a file storage error.

Objective 1 "to determine temporal and spatial variability of key physical and biogeochemical quantities". 1346 dives to between 140 and 1000 m yielded depth profiles of temperature, salinity, oxygen, chlorophyll a (figure below; in mg m-3 vs. Julian Day, coloured dissolved organic matter (CDOM) fluorescence and backscatter at 600 nm. Every zonal transect took about 4 days, which allowed detailed observation of two major upwelling periods during the first two months of the mis-sion. Internal tides were evident in the depth-averaged currents and the actual glider trajectories through the water.

Objective 2 "to study the influence of mesoscale and submesoscale processes on biological productivity". It is too early to say what the influence of these processes is, but biological net community production (NCP, estimated from changes in the O2 inventory) and net primary production (NPP, inferred from chlorophyll) clearly increased during upwelling events (figure on the left; in mg m-2 d-1 C). NCP lagged NPP, which reflects characteristic gas exchange time for mixed layer O2 of 2-4 weeks.

Objective 3: to establish calibration protocols for biogeochemical and optical sensors

The sensors were manufacturer-calibrated prior to and after the deployment. On three dates during the mission they were also compared to collocated ship-based CTD casts, which revealed a drift in the oxygen optode on the glider (perhaps related to bio-fouling, see below). In situ calibration therefore seems to be desirable in future work.

Objective 4: to develop and test optimal sampling strategies for glider deployments in a region straddling both mid-gyre and frontal regimes

The mission comprised 18 transects of the western Iberian shelf and shelf slope. Often the south-ward surface current pushed the glider south, away from the nominal transect. By performing dives to 1000 m we were able to avoid the surface current near the western turnaround point, but did not always manage to reach the eastern turnaround point. However, overall we were happy how well the glider coped in this dynamic region.

Objective 5: to test sensor integrity and stability, and establish whether bio-fouling can be mini-mised by diving to 1000 m rather than 500 m

Biofouling was pronounced in this biologically active upwelling region and could not be avoided by performing regular dives to 1000 m. The photo on the right shows algal growth on the rear fairing and oxygen optode.
Description Development of new oceanographic sampling strategies using autonomous underwater vehicles
First Year Of Impact 2011
Sector Environment