These areas are known as HNLC (high-nutrient, low-chlorophyll) zones and are thought to be caused by a scarcity of iron.

CNN: Topical waters

But it wasn't until the 1980's that the American oceanographer John Martin measured the iron content of the seawater in HNLC zones and found negligible amounts.

CNN: Topical waters

He argued that the sea gets it iron from dust swept into the sea from the land, and that wind currents weren't carrying iron to HNLC areas.

CNN: Topical waters