A biogeochemical study of the island mass effect in the context of the iron hypothesis: Kerguelen Islands, Southern Ocean

In the context of the iron hypothesis, the biogeochemical characteristics of the water masses located in the N-E wake of the Kerguelen archipelago were studied in austral spring 1995 during the ANTARES 3/France JGOFS cruise. In agreement with satellite observations (CZCS and SeaWifs data), this cruise showed the surface waters in the wake of the Kerguelen archipelago to be rich in both chlorophyll a and dissolved iron, compared to other observations in the HNLC oceanic waters of the Southern Ocean. This gives support to the hypothesis that natural fertilization due to iron inputs occurs in the waters surrounding the Antarctic Islands. However, the mesoscale structure of the water masses in the study area was complex, and three contrasted zones were identified within the region of lesser or greater iron enrichment. The coastal zone was characterized by very high concentrations of dissolved iron ( > 10 nM) and lithogenic material, but the phytoplankton biomass, dominated by small species, was low. An intrusion of cold Antarctic surface water, rich in silicic acid, was separated from the coastal zone by a shelf-break front. This water tongue contained chlorophyll a concentrations as. low (< 0.3 g 1(-1)) as in the iron-poor open ocean. A third water body, offshore, was characterized by the highest concentrations of phytoplankton (cell abundance and chlorophyll a) and biogenic silica, the community being dominated by large diatoms and dinoflagellates. The distribution of conservative tracers and lithogenic elements, as well as the phytoplankton taxonomic composition, indicate that this offshore water was not derived from a coastal origin. It more likely resulted from the mixing of the Antarctic water tongue Bowing northward along the island continental slope with subantarctic surface water flowing eastward north of the Kerguelen Islands. As regards the interactions between dissolved iron and phytoplankton biomass and growth in surface waters, two typical situations were encountered. Although iron-rich (dissolved Fe > 2 nM), both the coastal zone and the Antarctic water tongue were chlorophyll a poor. This was accounted for by either a heavy grazing pressure exerted by copepods (coastal zone) or an unfavourable light-mixing regime (Antarctic water tongue). In the offshore waters, by contrast, dissolved iron concentrations in the range 0.45-0.7 nM were sufficient to support higher phytoplankton growth under a more favourable light-mixing regime. (C) 2000 Elsevier Science Ltd. All rights reserved.