That the nitrate to phosphate ratio in the interior of all of the major ocean basins is highly similar is possibly due to the residence times of these elements in the ocean relative to the oceans circulation time, roughly 100 000 years for phosphorus and 2000 years for nitrogen. The fact that the residence times of these elements are greater than the mixing times of the oceans (~ 1000 years) can result in the ratio of nitrate to phosphate in the ocean interior remaining fairly uniform.