Assays and mixed culture experiments prove that the supply of chelating substances is frequently the most crucial aspect of phytoplankton nutrition in sea water. Experimental results, theoretical considerations and tentative calculations show that this supply is controlled by the stability of the trace metal chelates and in the sea may be mediated by certain unknown minor components of the dissolved organic matter.

The results of Parts I and II are considered together and show how seawater quality is closely related to the chelation of trace metals, mainly iron, and various other factors previously listed in the summary to Part I.