A time‐series study of the vertical structure of nitrogen and particle dynamics in the Sargasso Sea

Abstract

A time‐series study of nitrogen and particle dynamics at a site near Bermuda was made with natural nitrogen isotopic ratios as an in situ tracer. Vertical structure in δ¹⁵N values and other quantities varied both seasonally and interannually. At times during stratification, weak to moderate minima in δ¹⁵N and maxima in suspended particulate nitrogen concentration (PN) were observed in association with the top of the nitracline at the base of the euphotic zone indicative of local NO₃⁻ utilization. Net transport of PN as sinking particles enriched in¹⁵N to the lower half of the euphotic zone, however, acted to weaken or erase these features. A return flux of nitrogen from the lower to upper half of the euphotic zone was required, on a 1–3‐week time scale, to prevent depletion of suspended PN in the upper 50 m. Persistently higher particle flux at 50 vs. 100 m contrasts with the findings of the VERTEX study for the NE Pacific; the upper half of the euphotic zone in the Sargasso Sea appeared to be substantially less efficient at recycling nutrients than the euphotic zone as a whole. Destruction of sinking particles within the euphotic zone was critical for determining both the overall efficiency of nutrient recycling and the vertical variations observed.