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 o f the water masses located in the N-E wake of the Kerguelen archipelago wer e studied in austral spring 1995 during the ANTARES 3/France JGOFS cruise. In agreement with satellite observations (CZCS and SeaWifs data), this crui se showed the surface waters in the wake of the Kerguelen archipelago to be rich in both chlorophyll a and dissolved iron, compared to other observati ons 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 zo nes were identified within the region of lesser or greater iron enrichment. The coastal zone was characterized by very high concentrations of dissolve d iron ( > 10 nM) and lithogenic material, but the phytoplankton biomass, d ominated 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 <mu>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 bein g dominated by large diatoms and dinoflagellates. The distribution of conse rvative tracers and lithogenic elements, as well as the phytoplankton taxon omic composition, indicate that this offshore water was not derived from a coastal origin. It more likely resulted from the mixing of the Antarctic wa ter tongue Bowing northward along the island continental slope with subanta rctic surface water flowing eastward north of the Kerguelen Islands. As reg ards 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 w ater tongue were chlorophyll a poor. This was accounted for by either a hea vy grazing pressure exerted by copepods (coastal zone) or an unfavourable l ight-mixing regime (Antarctic water tongue). In the offshore waters, by con trast, dissolved iron concentrations in the range 0.45-0.7 nM were sufficie nt to support higher phytoplankton growth under a more favourable light-mix ing regime. (C) 2000 Elsevier Science Ltd. All rights reserved.