Iron, primary production and carbon-nitrogen flux studies during the JGOFS North Atlantic bloom experiment

Abstract

Primary production was measured every other day towards the end (18–31 May) of the 1989 North Atlantic spring bloom. Rates varied with light and averaged 90.4 mmol C m−2 day−1 at the 47°N, 20°W station. Productivities measured south of Iceland (59°30′N, 20°45′W) were somewhat lower, averaging 83.6 mmol C m−2 day−1. Carbon and nitrogen fluxes were estimated using free-floating, VERTEX type particle trap arrays. To obtain mean rates representative of the North Atlantic spring bloom, flux data from three trap deployments were combined and fitted to normalized power functions: mmol C m−2 day−1 = 14.35 (z/100)−0.946, mmol N m−2 day−1 = 2.34(z/100)−1.02, with depth z in meters. Regeneration rates were: mmol C m−2 day−1 = 0.136(z/100)−1.946, mmol N m−2 day−1 = 0.0239(z/100)−2.02. The carbon export rate from the upper 35 m for the entire NABE study period (24 April to 1 June) was 39 mmol m−2 day−1. This value divided by the averaged productivity for the entire study (86 mmol N m−2 day−1) gave an F-ratio of 0.45. Concentrations of Cu, Fe, Ni, Pb and Zn were determined in water samples provided by JGOFS NABE scientists involved with primary productivity measurements. Although little contamination was observed for Cu, Ni and Pb, relatively large amounts of Zn (10 nmol kg−1) were found in some cases. In subsequent studies it was learned that this quantity of Zn can depress productivity rates by 25%. North Atlantic dissolved Fe concentrations were similar to those occurring in the Pacific (surface = 0.07; deep = 0.5–0.6 nmol kg−1). Although no evidence of Fe deficiency was found in enrichment experiments, the addition of nmol amounts of Fe did increase CO2 uptake and POC formation by factors of 1.3–1.7. In this region, most of the phytoplankton's Fe requirement is probably met via the lateral transport of Fe from distant continental margins.