MBARI SOLAS(Surface Ocean-Lower Atmosphere Study) Publications: Johnson et al. (2003) Surface ocean-lower atmosphere interactions in the Northeast Pacific Ocean Gyre: Aerosols, iron, and the ecosystem response, Global Biogeochemical Cycles, 17 (2), 1063
Click below links for .txt files:
Iron is a nutrient element required by phytoplankton for nitrate utilization and chlorophyll production. There is not enough dissolved iron in sea water to allow phytoplankton to consume all of the available nitrate and reach their maximum growth potential. Atmospheric deposition is now recognized as a significant (although episodic) source of external iron to surface waters (Archer and Johnson, 2000; Jickells and Spokes, 2001). The North Pacific gyre is one region where aerosol deposition is considered important. There is much interest in how atmospheric iron deposition may potentially regulate production and carbon export in oligotrophic (low nutrient, low chlorophyll) areas of the ocean. The figure (right) is NOAA Satellite imagery showing dust deposits originating from China and the Africa.
The wind driven transport of dust from Asia results in immense regions of the North Pacific Ocean receiving large quantities of dust deposition during predominantly seasonal events. Extensive chemical analysis of these deposits indicates an average Fe concentration of between 5 and 10% (Young et al., 1991) . Solubility experiments have shown that ~10 % (Fung et al., 2000; Maring and Duce, 1987) of this Fe fraction dissolves in seawater and is presumably available to phytoplankton. Dust deposited throughout the Northern Pacific Ocean may therefore increase the productivity of these waters.
Asian dust storms generally occur in late spring (Apr.-June). Within days, the dust plume is out over the North Pacific. The figure below shows the track of a large dust storm that was followed with the SeaWIFS Satellite in 1998.
Despite the interest in aeolian iron deposition and its subsequent affect on ocean productivity, very few shipboard studies have coincided with dust events. To answer the question “Does the deposition of Fe from continental dust lead to enhanced biological production?”, we must first determine if the deposition of dust leads to increased concentrations of dissolved Fe.
In April of 2001, between our two transits, a large dust storm occurred. This even was captured on satellite imagery and dust can be seen originating from the Asian mainland and carried over Japan. This carried dust aerosols as far as the east coast of the US.
Dissolvable aluminum and iron were continuously measured by FIA during the R/V WESTERN FLYER transits and both show evidence of dust deposition (shown below, within red bars). Soluble iron deposited by dust is removed chemically by the formation of insoluble oxyhydroxides and removed biologically by phytoplankton. Aluminum however, is much less reactive and since surface water Al concentrations generally appear to be driven primarily by dust deposition (Measures and Vink, 2000), the concentration of Al can be used as an estimate of dust depositions which in turn can be used to calculate the estimated amount of Fe initially deposited . We found a 3.5 nM increase in Al on the second transit which would equate to an Fe increase of ~0.8 to 1.4 nM. The measured increase of ~0.7 nM Fe indicates that 0.1 to 0.7 nM had been removed. This relatively large range is due to the uncertainty of the relative solubilities of Al and Fe.
Vertical profiles of dissolvable Fe were also taken at 9 stations. Elevated iron concentrations were found to a depth of 100 m. These changes occurred in a ratio that is close to the crustal abundance ratio of the metals, which indicates a soil aerosol source. A vertical profile of iron concentrations at the Hawaii Ocean Time-series (HOT) site, which lies on the transect line near Hawaii, was measured in late April, 2001 by Wu et al. (2001). That profile has elevated surface iron concentrations that match those measured by us in May. An input of 24 µmol dissolved Fe/m2 must have occurred during the 30 day period from late March to late April (0.8 µmol dissolved Fe/m2/d). Fe concentrations east of 150°W were consistent with previous VERTEX (Martin and Gordon, 1988) measurements.
Click below links for .txt files:
The underway CTD system showed no significant hydrographic differences between the two transits once out of the influence of the California coastal regime (shown below). This is further evidence that the observed Fe and Al increase was due to dust and not water mass changes.
The cruise track is shown again superimposed on contours of the mean soil aerosol concentration calculated over the North Pacific from March to May 2001. Concentrations were calculated with the Navy Aerosol Analysis and Prediction System (NAAPS).
The dust event that occurred in mid-April was recorded at Mauno Loa Observatory (MLO). However, mean aerosol iron concentrations observed during this period (11 ng Fe/m3) were substantially less than the monthly mean values reported at the same site in the 1990’s. The MLO measurements and the NAAPS model results indicate low aerosol concentrations near Hawaii and a large meridional gradient with maximum concentrations in the boundary layer north of 30° N. However, the change in surface water iron and aluminum concentration was highest south of 25° N, near Hawaii. This suggests that the iron and aluminum concentration changes were the result of local processes, rather than a reflection of the broad-scale distribution of aerosol. If aerosol metal concentration was the proximate control on metal concentrations in surface waters, then both iron and aluminum should have increased the most to the east of 145° W and not to the west. The results presented here point towards a combination of higher solubility and higher aerosol scavenging rates than are typically assumed.
It is also possible that the surface ocean Al and Fe gradients reflect the effects of vertical mixing over winter. The depth of winter mixing varies greatly along the transect (Levitus et al., 1994), with significant increases to the east of 150°W as shown below. The residence time for dissolved aluminum in surface waters is estimated to be about 5 years (Orians and Bruland, 1986), which is substantially longer than that of iron in surface waters. It may be that regular, deep winter mixing over multiple years along the central portion of the transect entrains sufficient deep water with low aluminum concentrations to obscure higher rates of aerosol input.
Are North Pacific gyre waters Fe limited? The ADIOS project (Asian Dust Input to the Oceanic System) found a 60% increase in productivity immediately following a dust deposition event, which only persisted for a few days (Young et al., 1991). We however, did not observe a significant increase in chlorophyll concentrations between the two transects (shown below). It may be, as noted by the ADIOS group “when the iron-rich dust first arrived, primary producers were iron-limited, and subsequently, primary productivity was stimulated to the extent possible before it was probably limited by other nutrients”.
Nitrate concentrations were quite low across both of the transects and near our detection limit. Mean phosphate concentrations in the mixed layer are about 0.1 µM lower in May when compared to March values. The decrease in phosphate was accompanied by a slight (<0.1 µg/L) increase in chlorophyll concentration during the May cruise. If the Redfield ratio for nutrient uptake were maintained, a 0.1 µM phosphate draw down should be accompanied by consumption of >1 µM fixed nitrogen. Given the low initial nitrate concentrations, nitrogen fixation would likely have had to supply the remainder of the new nitrogen required to support uptake of 0.1 µM phosphate. Low abundances of the nitrogen fixing phytoplankton Trichodesmium suggests little evidence for a significant nitrogen fixation signal and the very small reduction in phosphate may be an artifact.
Iron is clearly a limiting nutrient in the equatorial region (Coale et al., 1996; Fitzwater et al., 1996). However, dissolved iron concentrations at HOT have seldom been less than 0.2 nM in any of the samples collected from the upper 100 m during the past decade (Rue and Bruland, 1995; Wu et al., 2001, this work). The effects of iron on community growth rates in shipboard, iron incubation experiments with samples from the equatorial Pacific can be expressed in terms of a Michaelis-Menten model with a half-saturation constant of 0.12 nM (Fitzwater et al., 1996). Iron will be strongly limiting to growth rates when its concentration is less than the half-saturation value. If the value of the half-saturation constant in subtropical gyre waters near HOT is similar to that observed along the equator, then typical iron concentrations >0.2 nM are not likely to be strongly limiting.
Coale, K. H., et al., A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean, Nature, 383, 495–501, 1996.
Fitzwater, S. E., K. H. Coale, R. M. Gordon, K. S. Johnson, and M. E. Ondrusek, Iron deficiency and phytoplankton growth in the Equatorial Pacific, Deep Sea Res., Part II, 43, 995–1015, 1996.
Fung, I. Y., S. K. Meyn, I. Tegen, S. C. Doney, J. G. John, and J. K. B. Bishop, Iron supply and demand in the upper ocean, Global Biogeochem. Cycles, 14, 281–295, 2000.
Jickells, T. D., and L. J. Spokes, Atmospheric iron inputs to the oceans, in The Biogeochemistry of Iron in Seawater, edited by D. R. Turner and K. A. Hunter, pp. 85–118, John Wiley, New York, 2001.
Johnson et al. Surface ocean-lower atmosphere interactions in the Northeast Pacific Ocean Gyre: Aerosols, iron, and the ecosystem response, Global Biogeochemical Cycles, 17 (2), 1063, 2003. [PDF]
S., T. Boyer, and J. Antonov, World Ocean Atlas 1994, vol.
Martin, J. H., and R. M. Gordon, Northeast Pacific iron distributions in relation to phytoplankton productivity, Deep Sea Res., 35, 177–196, 1988.
Maring, H. B. and R. A. Duce, The impact of atmospheric aerosols on trace metal chemistry in open ocean surface seawater, Earth Planet Sci. Lett., 84, 381-394, 1987
Measures, C. I., and S. Vink, On the use of dissolved aluminum in surface waters to estimate dust deposition to the ocean, Global Biogeochem. Cycles, 14, 317–327, 2000.
Orians, K. J., and K. W. Bruland, The biogeochemistry of aluminum in the Pacific Ocean, Earth Planet. Sci. Lett., 78, 397–410, 1986.
Rue, E. L., and K. W. Bruland, Complexation of iron(III) by natural organic ligands in the Central North Pacific as determined by a new competitive ligand equilibration/adsorptive cathodic stripping voltammetric method, Mar. Chem., 50, 117–138, 1995.
Wu, J., E. Boyle, W. Sunda, and L.-S.Wen, Soluble and colloidal iron in the oligotrophic North Atlantic and North Pacific Science, 293, 847-849, 2001
Young, R. W., et al., Atmospheric iron inputs and primary productivity: Phytoplankton responses in the North Pacific, Global Biogeochem. Cycles, 5, 119–134, 1991.