Nitrogen transport by vertically migrating diatom mats in the North Pacific Ocean

Abstract

PHYTOPLANKTON production in the surface waters of stratified oceans is fed mainly by nitrogen that has been recycled within the euphotic zone1. The nitrogen that is lost from surface waters as organic matter exported to the deep ocean must be balanced by inputs of new nitrogen to the upper ocean2,3. Sediment trap studies2 have shown that the 15N/14N ratio (δ15N) of the exported organic matter is higher than that of the suspended particulates, and suggest that the rich nitrate pool below the euphotic zone is the source of 'new' nitrogen for the upper ocean. Yet steep vertical concentration gradients suggest that diffusive upward transport of nitrate is extremely limited, raising the question of how the nitrate reaches the surface waters. Here we present evidence that abundant diatom (Rhizosolenia) mats migrate vertically between surface waters and deep nitrate pools in the central North Pacific Ocean. Rising mats contain significantly larger internal nitrate pools than sinking mats. Mat δ15N is similar to that of the sub-nitricline nitrate, and consistently heavier than that of near-surface particulate organic matter. We conclude that Rhizosolenia mats may transport the equivalent of 50% of the new nitrogen requirements into the surface waters of the North Pacific gyre.