Differences in particulate nitrogen concentration and isotopic composition for samples collected by bottles and large-volume pumps in Gulf Stream warm-core rings and the Sargasso Sea

Abstract

Nitrogen content and isotopic composition for particles collected by standard water-sampling bottles and in situ large-volume pumps are compared. Data are presented for stations in warm-core ring 82-B in April and June 1982, newly formed warm-core ring 82-H in September/ October 1982, and the OFP site off Bermuda. For warm-core rings, relatively large differences in γ 15N and PN concentration were observed between the two techniques. For the OFP site, except for PN concentration in the upper 200 m, good agreement was observed. Consideration of the effects of different retention sizes for the filter types used by the two techniques and whether or not large particles were included in the samples brought the OFP results into even closer agreement. However, only a fraction of the discrepancies in the warm-core ring results could be accounted for. Possible explanations include artifacts associated with differences in the details of the sample collection and handling techniques or differences in the size distributions of submicron particles and their γ 15N values in rings. The much better agreement in γ 15N between pumps at the OFP site, bottles at the OFP site, and pumps in ring 82-H suggests that minimal sample handling is desired. In addition, choice of filter material appears to be critical since significant quantities of PN can pass through commonly used varieties. These difficulties underscore the need to evaluate carefully the limitations of any particle-sampling technique regarding the specific measurements to be made.