Variations in nitrogen isotopic composition between sinking and suspended particles: implications for nitrogen cycling and particle transformation in the open ocean

Abstract

Significant and consistent differences in nitrogen isotopic ratio (measured as δ 15N relative to atmospheric N 2) are observed between suspended and sinking particles in the Sargasso Sea. Suspended particles in the euphotic zone have an annual average of −0.2% while particles sinking out of the euphotic average 3.7%. This latter value is equivalent to the average of δ 15N value for the sources of new nitrogen to the euphotic zone. The δ 15N of NO 3 − below the euphotic zone is similar, confirming that this is the major source. The downward particle flux acts to preferentially export 15N out of the euphotic zone, with the result that suspended particles are depleted in 15N relative to new nitrogen sources. The observed difference in δ 15N is evidence that the net transformation of suspended into sinking particles in this region is a result of active processes such as macrozooplankton feeding, as opposed to passive physical effects. Suspended particles below 200 m are 6% higher in δ 15N relative to those within the euphotic zone. This observation reflects, in part, the origin of suspended particles below the euphotic zone from the larger, faster sinking particles which are enriched in δ 15N. Since these suspended particles are also higher in δ 15N relative to sinking particles by 3%, isotopic fractionation must occur during the disaggregation of sinking particles and/or the subsequent degradation of suspended particles.