Three regimes?

3.4 Three different regimes?

Fig. 3.3 SeaWIFS images for December 1998, 1999, 2000 and 2001

Our survey strategy must consider three possible different spatial regimes. South of Crozet, a classic HNLC regime is most likely, iron stressed, dominated by small phytoplankton and primarily regenerated production with little export. North of Crozet, there may be two different bloom scenarios. Well away from the islands, iron stress is probably lifted by iron supply from shallow sediments during the winter and we anticipate diatom dominated communities in the bloom. Considerable silicate drawdown has been observed (OISO data, Metzl, pers comm.), so either silicate or iron may run out in the shallow summer mixed layer leading to bloom collapse and significant export in December. Close to the islands the bloom continues through December (Fig. 3.3), and we hypothesize a third regime in which silicate and/or iron continue to be supplied directly to the surface layer by runoff from the islands.

Spatial surveys and repeated observations in all three regimes will measure the circulation and iron distribution to establish the sources and rates of iron supply; phytoplankton biomass, new production and community structure; zooplankton biomass and community structure; and estimates of export from new production, nutrient deficits, 234Thorium, and direct observations using shallow drifting and deep moored sediment traps.

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