Phytoplankton Growth in Surface Water Samples from the Central South

Phytoplankton growth in surface water samples from the central South Pacific Ocean was tested.Some samples were untreated.Others had iron, phosphorus, or silica added.The water was already high in nitrogen, so nitrogen was not added.Addition of silica did not change NPP.Addition of iron increased NPP fivefold.Addition of phosphorus increased NPP slightly.These results indicate that phytoplankton growth in this part of the ocean is primarily limited by

A) nitrogen.
B) phosphorus.
C) iron.
D) silica.
E) both nitrogen and phosphorus.