Utilization of nitrogen and phosphorus by primary producers in warm-core ring 82-B following deep convective mixing

Abstract

Rates of utilization for NO 3 −, NH 4 +, urea, and PO 4 − by primary producers and the abundances of particulate C, N, and P were determined for the euphotic zone of warm-core ring 82-B in April and early May 1982. The ring had formed in late February, and our study took place between a period of deep convection and the formation of a stable seasonal thermocline. Deep convection enriched the surface waters with NO 3 − and PO 4 −, which remained in sufficient quantities to preclude nutrient limitation of primary production during the course of the study. Concentrations of HN 4 + and urea were always less than or equal to the limit of detection (0.03 μmol kg −1). The fraction of total nitrogen taken up as NO 3 − (ƒ ratio) was high (0.62–0.66) in the core region of the ring. During the course of this study, the concentration of NO 3 − decreased near the surface, allowing a NO 3 − gradient to develop, while the concentration of particulate N (PN) and the rate of N uptake increased. However, these processes were not in balance. Only about 16% of the NO 3 − based production remained in the euphotic zone. The PN:Chl a ratio was higher at the bottom of the euphotic zone than at the top. This indicates that detrital PN was being formed faster than it was exported by either sinking or mixing processes. The overall reduction in euphotic zone NO 3 − concentrations was about one third the measured consumption rate. Midway through the study a storm was apparently responsible for redistributing NO 3 − in the euphotic zone, and perhaps mixing NO 3 − up from greater depths. After that event, the mean NO 3 − uptake rate was about twice the rate of NO 3 − disappearance. Upward flux of NO 3 − associated with the relaxation of depressed density surfaces as the ring lost kinetic energy could have supplied NO 3 − at a rate equivalent to 40–110% the mean NO 3 − uptake rate for the period. The highest rates of NO 3 − uptake were observed at the end of the study, and were in excess of the calculated rates for upward flux. This may explain the increasing magnitude of NO 3 − gradients observed durinng the final few days of this study.