RRS Discovery Cruise 321, 24 Jul-23 Aug 2007. Biophysical interactions in the Iceland Basin 2007

Abstract

D321 was the first of three National Oceanography Centre ‘process study’ research cruises to be run by the Ocean Biogeochemistry and Ecosystems research group under the NERC Oceans 2025 research programme. The scientific work began by carrying out some of the extended Ellett Line stations on the way out to our study region; which centred around the historical JGOFS Ocean Weather Station India site (~ 59° N, ~ 19° W) and the northward turn of the extended Ellett line at 20° W. The Iceland Basin, like much of the N. Atlantic subpolar gyre, is characterised by a ‘bloom and bust’ seasonal cycle. Spring stratification triggers a major diatom dominated bloom event. This bloom is short lived, limited by silicate (orthosilicic acid) exhaustion (Brown et al., 2003). Two community succession pathways commonly follow the demise of the diatom bloom; typically through dinoflagellate and/or coccolithophore production. The spring bloom is dominated by eddy scale (several 10's of km) patchiness, driven by the upward and downward pumping effect of eddies on the newly forming spring stratification. However these eddy structures have another, more important, impact on phytoplankton production (Allen et al., 2005). In the release of potential energy, eddies effect a real three dimensional exchange of water across the thermocline bringing new dissolved nutrients from deeper waters up into the photic zone and transporting biogenic particles into the deep ocean. Thin ribbon like structures around the edges of eddies are clearly seen in ocean colour satellite images. The four repeated surveys carried out during D321 observed the evolution of an ‘eddy dipole’ in a background ocean full of eddies and other turbulent motions. Daily, near real-time, satellite images and in-situ vessel mounted acoustic current profiling were used to determine the movement of the eddy centres and the dipole central jet. Targeted nets and water collection within the various components of the eddy dipole enabled the assessment of its biological impacts.

Since 1996 NOCS and SAMS have been occupying an extended version of the Ellett Line that runs all the way to Iceland. The Extended Ellett line is important oceanographically because it completes the measurements of the warm saline water flowing into the Nordic Seas from the eastern North Atlantic. It also monitors around half of the returning deep and cold current, the overflow water (the rest returns to the Atlantic via the Denmark Strait to the west of Iceland).

There is little added cost, either in time or financially, in making a number of biogeochemical measurements using water samples from the hydrographers' CTD stations. Thus, recently, the scientific interests in the Extended Ellett line have become more multidisciplinary; the 2006 occupation was no exception. Samples were filtered for POC, HPLC studies and trace aluminium concentration determination, in addition some extra time was found for a number of zooplankton net hauls.

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