Iron biogeochemistry in the waters surrounding the Crozet Islands, Southern Ocean

• Main Author: Planquette, Hélène
• Format: Thesis
• Language: English
• Published: 2008
• Subjects: Southern Ocean; Austral; Crozet Islands
• Online Access: https://eprints.soton.ac.uk/63132/; https://eprints.soton.ac.uk/63132/1/Planquette_2008_PhD.pdf

The aim of this thesis was to improve our understanding of the natural iron fertilisation processes that can alleviate the High Nutrient Low Chlorophyll conditions normally associated with the Southern Ocean. The annual phytoplankton bloom occurring north of the Crozet Plateau (46°26’S - 52°18’E) provided a good opportunity to study these phenomena during CROZEX, a multidisciplinary study performed in austral summers 2004/2005 and 2005/2006. Particular emphasis has been placed on the sources of Fe to the upper water column, and on the different processes that drive the distribution of iron, such as mixing with deeper waters, advection of Fe rich waters from the islands, or particles and atmospheric inputs. A flow-injection analyser with DPD catalytic spectrophotometric detection (FIA-DPD) was first set up for the determination of total dissolved Fe (DFe, ? 0.2 µm) and careful assessment of data demonstrated the high quality of the information obtained in this study. Analytical measurements of DFe were performed in twenty vertical profiles from the North to the South of the islands that show evidence of a range of processes influencing the iron distributions. Most significantly, an enrichment of DFe (>1 nM) has been identified at close proximity to the islands, which suggests that the plateau and the associated sediments are a source of iron. Waters further north also appear to be affected by this input of both coastal and shelf origin, although dissolved iron concentrations decrease as a function of distance to the north of the plateau with a gradient of 0.07 nM.km-1 as a result of dispersion and mixing. This gradient was then combined with short-lived Radium isotopes profiles, allowing the determination of a horizontal advective flux of Fe. Estimates of atmosphere and vertical fluxes of Fe to surface waters were also calculated. It was then possible to estimate a pre-bloom concentration of ~ 0.44 nM, which is sufficient to drive the inferred level of the new production in the bloom area. Labile iron has been ...