It depends on limitating factor. Fe can be limitating as well or can be toxic. But the principle of Libih-Shelford says that always only one factor is limit for the development of biological process. This review article would help: The role of iron in phytoplankton photosynthesis, and the potential for iron-limitation of primary productivity in the sea // Photosynthesis Research39: 275-301, 1994. Attached.
As well I have perfect book: Marine Geochemistry_2006 by Horst D. Schulz &
Matthias Zabel with special Chapter, but it is closed for open distribution, so contact me over RG mailing if need this Chapter: 7 The Reactivity of Iron.