In the surface waters of oceans, the two most limiting nutrients for primary productivity are

Nitrogen and Iron

Dissolved nutrients is one of the most important property of sea water and has a huge effect in life in the ocean. The most critical is nitrogen among the nutrients. This  plays a major role in the stimulation of the primary production by the planktons in the ocean. This can be considered as limiting because most marine life cannot live without this nutrients. Nutrients in the ocean are cycled to keep the balance known as biological pumping.