Sara J. Tanner

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

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 phytoplanktons Fe requirement is probably met via the lateral transport of Fe from distant continental margins.

Developing standards for dissolved iron in seawater

In nearly a dozen open- ocean fertilization experiments conducted by more than 100 researchers from nearly 20 countries, adding iron at the sea surface has led to distinct increases in photosynthesis rates and biomass. These experiments confirmed the hypothesis proposed by the late John Martin [Martin, 1990] that dissolved iron concentration is a key variable that controls phytoplankton processes in ocean surface waters. However, the measurement of dissolved iron concentration in seawater remains a difficult task [Bruland and Rue, 2001] with significant interlaboratory differences apparent at times. The availability of a seawater reference solution with well- known dissolved iron (Fe) concentrations similar to open- ocean values, which could be used for the calibration of equipment or other tasks, would greatly alleviate these problems [National Research Council (NRC), 2002]. The Sampling and Analysis of Fe (SAFe) cruise was staged from Honolulu, Hawaii, to San Diego, Calif., between 15 October and 8 November 2004 to collect data and samples that were later used to provide this reference material. Here we provide a brief report on the cruise results, which have produced a tenfold improvement in the variability of iron measurements, and announce the availability of the SAFe dissolved Fe in seawater standards.