The hypothesis that iron plays a major regulatory role in phytoplankton productivity. While the potential role of iron as a limiting factor in phytoplankton productivity was appreciated by researchers as early as the 1930s, it wasn't until John Martin convincingly pieced together several lines of evidence in the late 1980s that the oceanographic community gave notice to the point of planning major experiments to test it. The threads of Martin's argument included that:

• the primary source of iron to the surface waters of the oceans is from the land;
• the dissolved iron concentrations in offshore areas are extremely low, i.e. two orders of magnitude less than thought by the investigators in the 1930s;
• atmospheric dust deposition in the two major high nutrient, low chlorophyll (HNLC) areas of the oceans - the Antarctic and equatorial Pacific Oceans - are the lowest in the world; and
• laboratory experiments in which bottles filled with surface waters from NHLC regions and incubated at simulated in situ light and temperature for about a week showed that iron-enriched bottles always ended up with higher total chlorophyll that the control bottles without iron.