R. Michael Gordon

What controls dissolved iron concentrations in the world ocean

Dissolved ( value in the data set at a depth near 750 m, where variability is at a maximum. The minimum concentrations are found at stations in the remote central Pacific and the maximum values occur at stations adjacent to the continental margin. The major source of iron in the deep sea is generally aeolian deposition. Integrated (surface to 500 m) concentrations of iron at each station are only weakly correlated with the aeolian iron deposition flux, however. This contrasts with other elements such as lead that also have strong atmospheric sources. These observations lead us to conclude that the nutrient-like profile is maintained by a mechanism that reduces the scavenging rate of dissolved iron at concentrations less than 0.6 nmol kg- ’ This mechanism may be complexation by strong iron binding ligands, which have been found in both the Atlantic and Pacific at concentrations near 0.6 nM. This apparent solubility would act to diminish inter-ocean fractionation. It would allow a nutrient-like profile to develop before scavenging began to remove iron. In order to test the concept, we developed a numerical model to make quantitative predictions of dissolved iron concentrations from place to place. The dissolved iron source in the ocean interior is remineralization from sinking particulate organic matter. Scavenging removes dissolved iron only at concentrations greater than the apparent solubility. The only geographically variable parameter in the model is the export flux of carbon from the surface layer, which carries iron with it. The model generated dissolved iron profiles, based on measured or estimated values of the carbon export flux, are in remarkable agreement with the observed profiles at all stations from the North Atlantic through the Southern Ocean to the North Pacific. 0 1997 Elsevier Science B.V.

Vertex: phytoplankton/iron studies in the Gulf of Alaska

VERTEX studies were performed in the Gulf of Alaska in order to test the hypothesis that iron deficiency was responsible for the phytoplanktons failure to remove major plant nutrients from these waters. In view of the observed Fe distributions and the results of phytoplankton Fe enrichment experiments, it was concluded that Gulf of Alaska atmospheric Fe input rates are sufficient to support moderately high rates of primary productivity; however, not enough Fe is available to support the high growth rates that would lead to normal major nutrient depletion. Enhanced Fe input does occur along the Alaska continental margin, where normal NO3 surface depletion is observed. Coccolithophorids appear to be best able to cope with low Fe conditions; however, they cannot compete with diatoms when Fe is readily available. Iron may be more important than available N in determining global rates of phytoplankton new production. Offshore Pacific Ocean water, replete with major nutrients, appears to be infertile without supplemental iron from the atmosphere or continental margin.

Northeast Pacific iron distributions in relation to phytoplankton productivity

Dissolved and particulate Fe concentrations, measured at three deep ocean stations on a 1600 km inshore-offshore VERTEX transect, were compared with those found at four shallow California continental margin stations. The three VERTEX profiles shared similar features: very low dissolved Fe levels (<0.1 nmol kg−1) in surface waters, increasing amounts with depth, and maxima (∼1.0–1.3 nmol kg−1) in association with the oxygen minimum. In contrast, concentrations as high as 9 nmol kg−1 of dissolved Fe were found at the shallow margin stations, in association with elevated levels of Mn (17 nmol kg−1) and Co (200 pmol kg−1). Inshore and offshore Fe distributions were evaluated in relation to the phytoplanktons requirement for this essential element. Local shelf diagenetic Fe input appears to be adequate for phytoplankton growth even in environments where increased demand results from the upwelling of major nutrients. However, the Fe laterally mixed out into the oceans interior within the oxygen minimum and supplied to the surface via vertical mixing processes provides only a few percent of open ocean phytoplankton demand; the other 95% must be provided by atmospheric input. We also consider environments in which Fe supplies may be limiting phytoplankton growth; i.e. surface waters of the Subarctic and Antarctic, where major nutrients are never depleted. We postulate that atmospheric Fe input rates are not high enough to meet the elevated phytoplankton demand resulting from offshore major nutrient upwelling. As a result, major nutrient depletion occurs only along continental margins and ice edges, where Fe supplies should be adequate. Atmospheric dust concentrations were one to two orders of magnitude higher in glacial times than those in the present and last interglacial periods. The lower glacial atmospheric CO2 levels, which resulted from the increased biological utilization of major nutrients at high latitudes, may have been stimulated by the enhanced availability of atmospheric Fe.

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.

Iron deficiency and phytoplankton growth in the equatorial Pacific

Several experiments were conducted in the equatorial Pacific at 140°W during the Joint Global Ocean Flux Study, equatorial Pacific, 1992 Time-series I (TS-I, 23 March–9 April), Time-series II (TS-II, 2–20 October) and FeLINE II cruises (10 March–14 April), to investigate the effects of added Fe on phytoplankton communities. Seven series of deckboard iron-enrichment experiments were performed, with levels of added Fe ranging from 0.13 to 1000 nM. Time-course measurements included nutrients, chlorophyll a and HPLC pigments. Results of these experiments showed that subnanomolar (sub-nM) additions of Fe increased net community specific growth rates, with resultant chlorophyll a increases and nutrient decreases. Community growth rates followed Michaelis-Menten type kinetics resulting in maximum rates of 0.99 doublings per day and a half-saturation constant of 0.12 nM iron. The dominant group responding to iron enrichment was diatoms.

On the formation of the manganese maximum in the oxygen minimum

Abstract A simple model that accounts for the formation of the Mn maximum in the oxygen minimum is presented here. In this model, Mn is proposed to cycle in a constant proportion to carbon, as do nitrogen and phosphorous. Superimposed on the Mn-carbon cycle is the removal of Mn(II) via scavenging onto sinking particles and transport by vertical diffusion. Scavenging is assumed to follow the rate law observed in the laboratory for Mn(II) oxidation. Manganese (II) concentrations were calculated with the model at stations in the Pacific and Atlantic Oceans and compared with measurements of dissolved Mn. All parameters in the model were based on laboratory measurements or field observations. The model reproduced Mn(II) maxima of the correct concentration and at the correct depth. This agreement was observed at a range of oxygen concentrations. The calculations demonstrate that the Mn maximum can form because of a reduction in the pseudo-first order scavenging rate constant ( k ′) within the oxygen minimum. The value of k ′ will decrease in regions of the water column with low oxygen and pH ( k ′ = k 0 [O 2 ] OH − 2 ). These regions will accumulate higher dissolved Mn(II) concentrations before the rate of Mn(II) removal, k ′ [Mn(II)], equals the input from remineralization of POC and a steady state is reached. An additional source of Mn, such as flux from continental margin sediments or dissolution of Mn oxides, is not necessary to account for formation of the Mn maximum.

Iron-enrichment bottle experiments in the equatorial Pacific: responses of individual phytoplankton cells

Abstract Iron-enrichment bottle experiments were monitored using flow cytometry to investigate the hypothesis that phytoplankton in the equatorial Pacific are iron-limited. Iron-enriched Synechococcus, ultraphytoplankton, nanophytoplankton, pennate diatoms, and coccolithophorids had higher fluorescence and/or forward light scatter per cell than control cells; for Prochlorococcus the trends were the same although the differences were not significant. This suggests that most phytoplankton cells were physiologically affected by the low iron concentrations in this region. However, only pennate diatoms showed significant increases in cell concentrations due to iron enrichment. The sum of chlorophyll fluorescences of individual cells measured by flow cytometry yielded patterns similar to those of extracted bulk chlorophyll, whth increases of up to 10-fold in iron-enriched bottles but as most 3-fold in control bottles; pennate diatoms accounted for most of the increase in chlorophyll in iron-enriched bottles.

We still say iron deficiency limits phytoplankton growth in the subarctic Pacific

The failure of Banse (1990) to use a reasonable initial particulate organic nitrogen (PON) value resulted in erroneously high, and physiologically impossible, estimates of phytoplankton growth rates. To correct this situation, we used an initial PON value of 1 μmol PON L−1 for both experimentals and controls; rates similar to those expected under prevailing light and temperature conditions were obtained. This reinterpretation of our data again demonstrates the dramatic effects that are observed when small quantities of iron are made available to the phytoplankton inhabiting offshore subarctic Pacific waters.

Primary productivity and trace-metal contamination measurements from a clean rosette system versus ultra-clean Go-Flo bottles

Abstract Primary productivity rates, measured during the 1992 United States Joint Global Ocean Flux Study (U.S. JGOFS) Equatorial Pacific (EqPac) process study with a new Trace-Metal clean rosette system (TM rosette) designed to be trace-metal clean, agreed within 5% with those determined using ultra-clean procedures that were previously shown to be trace-metal clean. The TM rosette system did not inhibit phytoplankton primary productivity rates. Using the TM rosette system, there was no contamination of Co, Ni, Cu, Cd or Pb, and only slight contamination of Fe and Zn, relative to ultra-clean collection. However, the slight contaminations were below levels that affect primary productivity rates. Therefore, systematic phytoplankton inhibition by trace-metal contamination appears to have been successfully eliminated with water collected using the TM rosette system.