Temporal and spatial variations in the natural abundance of 15N in PON from a warm-core ring

Abstract

Vertical patterns of the natural abundance of 15N (δ 15N) for suspended particulate organic nitrogen (PON) for warm-core ring 82-B were found to found reflect vertical patterns in the utilization of NO 3 − by phytoplankton and the presumed diagenesis of sinking PON on long and short time scales. Temporal variations in the δ 15N of PON were strongly associated with the onset of stratification and the removal of NO 3 − from the euphotic zone. In April, before stratification, euphotic zone values ranged from −2.6 to +3.3 per mil. During stratified conditions, in June, euphotic zone δ 15N values were between +4.0 and +11.0 per mil. This shift in the δ 15N of PON is attributed, in part, to a decrease in the contribution of isotopic fractionation by phytoplankton NO 3 − assimilation. It is concluded, however, that the input of Shelf Water or Slope Water PON of high δ 15 approximately + 10 per mil) into the ring also could have made a significant contribution to the shift in δ 15N values between April and June. The contribution of Shelf Water- or Slope Water-derived PON to ring PON could be as high as 50% in the euphotic zone. A shift in the δ 15N of PON at depth in Ring 82-B (400 to 800 m) probably occured in response to the change in euphotic zone δ 15N values via the downward particle flux. It is hypothesized that PON at this depth interval had a residence time of about one month.