May 2011

On Monday 9th May....

Tommy Ryan-Keogh will present:

Title:
High Spatial Resolution Iron-Addition Incubation Experiments: Mapping Seasonal Iron Stress in the High Latitude North Atlantic

Abstract:

The Irminger and Iceland basins of the High Latitude North Atlantic (HLNA) are areas characterised by incomplete nitrate depletion in the summer and relatively low chlorophyll concentrations. The annual cycle of chlorophyll demonstrates a peak in the spring in proximity to shallow bathymetry, one of the possible major iron sources to the HLNA. It has been proposed therefore that a lack of iron sources to the HLNA may lead to the development of a High-Nitrate Low-Chlorophyll (HNLC) region in the late summer. Long-term (>24 h) and rapid (24 h) incubations, at a high spatial resolution, examined the response of the phytoplankton physiology to iron addition, enabling the detection of mesoscale variability in Fe limitation. The proxy ΔFv/Fm was calculated from the 24 h incubations, and is defined as the difference in the maximal photochemical efficiency of photosystem II (PSII) (Fv/Fm) between a nutrient addition treatment and the control treatment after 24 h of incubation. The greatest increases in ΔFv/Fm were found in the centre of the basins with less of a response over the Iceland shelf and Reykjanes ridge, which may be acting as a source of iron. These results confirm and map the extent of the development of an iron-limited HNLC ecosystem in the Irminger basin, with a seasonal development of iron and nitrate co-limitation in the Iceland basin. This is the first seasonal study of the HLNA and allows the definition of two contrasting 'biomes' within the HLNA based upon the differential responses to iron addition.