Margaret Leinen PhD, Kevin Whilden, and Dan Whaley. “A Response to Concerns about Ocean Iron Fertilization Raised by Greenpeace”. Climos, Inc. 15 May 2008 – “Producing a strong export response to iron enrichment requires both initial HNLC conditions and a low background abundance of mesozooplankton, which allows diatom biomass to initially accumulate faster than ambient mesozooplankton can consume it [Landry et al., 2000]. Continuous iron fertilization will not produce efficient sequestration of carbon because as the mesozooplankton become abundant they can continuously graze and recycle a large proportion of the newly produced diatom biomass in the surface layer. This increased grazing rate prevents the accumulation of the diatom biomass needed for efficient export.”(p. 10) We think it is important however, to include the very next sentence of the Barber and Hiscock 2006 paper: “Therefore, efficient engineered carbon sequestration requires episodic Fe enrichment with a return to the ambient picoplankton-dominated assemblage between enrichments.”
Thus Barber and Hiscock were making a point about which strategies for OIF would be successful, not that OIF would not be successful due to eventual mesozooplankton grazing. Allsopp and her coauthors [Allsopp, 2007] acknowledge this later in their paper saying “in order to favour carbon sequestration, therefore, it would probably be necessary to repeat iron enrichments periodically, wherein picophytoplankton were again allowed to become dominant between fertilization events” (p. 9). We see no difficulty with this model of fertilization.