The Ultimate Limiting Nutrient for Oceanic Primary Production

In order to know how Earth's climate system works, we need to understand what controls oceanic primary production. This largely determines the partitioning of carbon between the atmosphere and ocean, and hence the strength of Earth's greenhouse effect. Oceanic primary production also adds oxygen to the ocean-atmosphere system as well as providing the food supply for the rest of marine food webs. Phytoplankton populations and rates of turnover affect how quickly biogenic rocks such as chalks are formed at the bottom of the ocean. A full understanding of Earth history is only possible with understanding of controls on past ocean primary production levels.

It has long been argued, from first principles, that the ultimate limiting nutrient (ULN), the one whose supply dictates ocean productivion over long timescales, is phosphorus. One reason for this is the ability of some organisms, the nitrogen-fixers, to utilize dinitrogen as their nitrogen source and flourish if nitrate runs out. Many modelling studies (e.g. [1]) have concluded that the ULN is phosphorus. However, other studies (e.g. [2]) suggest that iron, the proximate limiting nutrient for primary production across ~30% of the ocean surface, and whose scarcity often restricts nitrogen fixation [3], is instead the ULN.