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 [REF].
While such arguments can potentially explain why the ratios are fairly constant, they do not address the question why the N:P ratio is nearly 16 and not some other number.