Hein J. W. de Baar

Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment

Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean

Atmospheric iron and underway sea-surface dissolved (<0.2 μm) iron (DFe) concentrations were investigated along a north-south transect in the eastern Atlantic Ocean (27°N/16°W-19°S/5°E). Fe concentrations in aerosols and dry deposition fluxes of soluble Fe were at least two orders of magnitude higher in the Saharan dust plume than at the equator or at the extreme south of the transect. A weaker source of atmospheric Fe was also observed in the South Atlantic, possibly originating in southern Africa via the north-easterly outflow of the Angolan plume. Estimations of total atmospheric deposition fluxes (dry plus wet) of soluble Fe suggested that wet deposition dominated in the intertropical convergence zone, due to the very high amount of precipitation and to the fact that a substantial part of Fe was delivered in dissolved form. On the other hand, dry deposition dominated in the other regions of the transect (73-97), where rainfall rates were much lower. Underway sea-surface DFe concentrations ranged 0.02-1.1 nM. Such low values (0.02 nM) are reported for the first time in the Atlantic Ocean and may be (co)-limiting for primary production. A significant correlation (Spearmans rho = 0.862, p<0.01) was observed between mean DFe concentrations and total atmospheric deposition fluxes, confirming the importance of atmospheric deposition on the iron cycle in the Atlantic. Residence time of DFe in the surface waters relative to atmospheric inputs were estimated in the northern part of our study area (17 ± 8 to 28 ± 16 d). These values confirmed the rapid removal of Fe from the surface waters, possibly by colloidal aggregation.

Cadmium versus phosphate in the world ocean

Abstract Cadmium (Cd) is one of the best studied trace metals in seawater and at individual stations exhibits a more or less linear relation with phosphate. The compilation of all data from all oceans taken from over 30 different published sources into one global dataset yields only a broad scatterplot of Cd versus phosphate. However, the smaller high-quality dataset obtained by rigorous selection of only those stations with uniform Cd PO 4 -ratio in the deep waters, provides a consistent global description of the deep (> 1000 m) waters. The deep Cd PO 4 -ratio increases from about 0.18 × 10−3 in the subarctic North Atlantic to about 0.33–0.35 × 10−3 in the northern Indian and Pacific Oceans, in accordance with increasing phosphate content, i.e. age, of the deep water. The increasing Cd PO 4 -ratio with age (and phosphate) of the deep water masses is a function of the coupling between biogeochemical cycling and deep water circulation. Changes in the latter, for example during a glacial period, inevitably lead to significant shifts in the Cd PO 4 relationship of seawater. There is a statistically significant bimodality of deep Atlantic versus deep Antarctic/Indo/Pacific waters, suggesting that the deep Atlantic is a distinct biogeochemical province for Cd cycling. This distinction is likely caused by the high inventories of both Cd and phosphate in Weddell Sea source waters. For each of both populations, a given concentration of phosphate yields a predicted value of Cd within ±100 pM (Atlantic) and ±200 pM (Antarctic/Indo/Pacific), respectively, at the 95% confidence level. If one ignores the bimodality, then for a given phosphate the corresponding Cd might be predicted within ±150 pM at the 95% confidence level; the validity of this is currently being verified by studies of South Atlantic waters which may or may not provide the missing link between both populations. Currently, the global distribution of the Cd PO 4 -ratio in surface, thermocline and deep waters is consistent with preferential biogeochemical removal of Cd versus phosphate from surface waters. The net result for Cd/PO4 is not dissimilar to the preferential surface removal of 12C over 13C driving the deep distribution of the dissolved 12 C 13 C -ratio, although for Cd/PO4 the underlying mechanism is obviously very different and not well understood.

Ocean iron fertilization - Moving forward in a sea of uncertainty

It is premature to sell carbon offsets from ocean iron fertilization unless research provides the scientific foundation to evaluate risks and benefits.

Low dissolved Fe and the absence of diatom blooms in remote Pacific waters of the Southern Ocean

The remote waters of the Pacific region of the Southern Ocean are the furthest away from any upstream and upwind continental Fe sources. This prime area for expecting Fe limitation of the plankton ecosystem was studied (March–April 1995) along a north–south transect at ∼89°W. At the end of the austral summer the upper wind-mixed layers were in the order of ∼100 m deep, thus mixing the algae down into the dimly lit part of the euphotic zone where photosynthesis is severely restricted. The dissolved Fe was found at low concentrations ranging from 0.05 nM near the surface to 0.5 nM in deeper waters. Along the transect (52°S–69°S), the dissolved iron was enhanced in the Polar Front, as well as near the Antarctic continental margin (0.6–1.0 nM). In between, the southern ACC branch was depleted with iron; here the concentrations in surface waters were quite uniform at about 0.21 nM. This is only somewhat lower than the 0.49 nM (October 1992) and 0.31 nM (November 1992) averages in early spring in the southern ACC part of Atlantic 6°W sections [de Baar, H.J.W., de Jong, J.T.M., Bakker, D.C.E.. Loscher, B.M., Veth, C., Bathmann, U., Smetacek, V., 1995. Importance of iron for phytoplankton spring blooms and CO2 drawdown in the Southern Ocean. Nature 373, 412–415; Loscher, B.M., de Jong, J.T.M., de Baar, H.J.W., Veth, C., Dehairs, F., 1997. The distribution of iron in the Antarctic Circumpolar Current. Deep-Sea Research II 44, 143–188.]. First, the lower ∼0.21 nM in March–April 1995 may partly be due to continuation of the seasonal trend where the phytoplankton growth, albeit modest, was removing Fe from the surface waters. Secondly, the 89°W Pacific stations are further downstream continental or seafloor sources than the Atlantic 6°W section. In the latter case, the ACC water had passed through the Drake Passage and also over the Sandwich Plateau. Indeed for Drake Passage, intermediate Fe concentrations have been reported by others. The generally somewhat lower surface water Fe at the ACC and PF at 89°W is consistent with the distance from sources and the late summer. It also would explain the very low abundance of phytoplankton (Chl a) in the region and the conspicuous absence of plankton blooms. In the subAntarctic waters north of the Polar Front there are no diatoms, let alone diatom blooms, due to low availability of silicate. Thus, it appears the biological productivity is suppressed due to iron deficiency, in combination with the severe seasonal effects of wind mixing on the light climate, as well as regional silicate limitation for diatoms.

Factors controlling phytoplankton ice-edge blooms in the marginal ice-zone of the northwestern Weddell Sea during sea ice retreat 1988 : field observations and mathematical modelling

The factors controlling phytoplankton bloom development in the marginal ice zone of the northwestern Weddell Sea were investigated during the EPOS (Leg 2) expedition (1988). Measurements were made of physical and chemical processes and biological activities associated with the process of ice-melting and their controlling variables particularly light limitation mediated by vertical stability and ice-cover, trace metal deficiency and grazing pressure. The combined observations and process studies show that the initiation of the phytoplankton bloom, dominated by nanoplanktonic species, was determined by the physical processes operating in the marginal ice zone at the time of ice melting. The additional effects of grazing pressure by protozoa and deep mixing appeared responsible for a rather moderate phytoplankton biomass (4 mg Chla m−3) with a relatively narrow geographical extent (100–150 km). The rôle of trace constituents, in particular iron, was minor. The importance of each factor during the seasonal development of the ice-edge phytoplankton bloom was studied through modelling of reasonable scenarios of meteorological and biological forcing, making use of a one-dimensional coupled physicalbiological model. The analysis of simulations clearly shows that wind mixing events — their duration, strength and frequency — determines both the distance from the iceedge of the sea ice associated phytoplankton bloom and the occurrence in the ice-free area of secondary phytoplankton blooms during the summer period. The magnitude and extent of the ice-edge bloom is determined by the combined action of meteorological conditions and grazing pressure. In the absence of grazers, a maximum ice-edge bloom of 7.5 mg Chla m−3 is predicted under averaged wind conditions of 8 m s−1. Extreme constant wind scenarios (4–14 m s−1) combined with realistic grazing pressure predict maximum ice-edge phytoplankton concentrations varying from 11.5 to 2 mg Chla m−3. Persistent violent wind conditions (≥ 14 m s−1) are shown to prevent blooms from developing even during the brightest period of the year.

Photosynthesis and Calcification by Emiliania huxleyi (Prymnesiophyceae) as a Function of Inorganic Carbon Species

To test the possibility of inorganic carbon limitation of the marine unicellular alga Emiliania huxleyi (Lohmann) Hay and Mohler, its carbon acquisition was measured as a function of the different chemical species of inorganic carbon present in the medium. Because these different species are interdependent and covary in any experiment in which the speciation is changed, a set of experiments was performed to produce a multidimensional carbon uptake scheme for photosynthesis and calcification. This scheme shows that CO2 that is used for photosynthesis comes from two sources. The CO2 in seawater supports a modest rate of photosynthesis. The HCO is the major substrate for photosynthesis by intracellular production of CO2 (HCO+ H+→ CO2+ H2O → CH2O + O2). This use of HCO is possible because of the simultaneous calcification using a second HCO, which provides the required proton (HCO+ Ca2+→ CaCO3+ H+). The HCO is the only substrate for calcification. By distinguishing the two sources of CO2 used in photosynthesis, it was shown that E. huxleyi has a K½ for external CO2 of “only” 1.9 ± 0.5 μM (and a Vmax of 2.4 ± 0.1 pmol·cell−1·d−1). Thus, in seawater that is in equilibrium with the atmosphere ([CO2]= 14 μM, [HCO]= 1920 μM, at fCO2= 360 μatm, pH = 8, T = 15° C), photosynthesis is 90% saturated with external CO2. Under the same conditions, the rate of photosynthesis is doubled by the calcification route of CO2 supply (from 2.1 to 4.5 pmol·cell−1·d−1). However, photosynthesis is not fully saturated, as calcification has a K½ for HCO of 3256 ± 1402 μM and a Vmax of 6.4 ± 1.8 pmol·cell−1·d−1. The H+ that is produced during calcification is used with an efficiency of 0.97 ± 0.08, leading to the conclusion that it is used intracellularly. A maximum efficiency of 0.88 can be expected, as NO uptake generates a H+ sink (OH− source) for the cell. The success of E. huxleyi as a coccolithophorid may be related to the efficient coupling between H+ generation in calcification and CO2 fixation in photosynthesis.

Cd, Zn, Ni and Cu in the Indian Ocean

Abstract Vertical profiles of dissolved Cd, Zn, Ni and Cu in the Northwest Indian Ocean (Arabian Sea) exhibit a nutrient type distribution also observed in other oceans. The area is characterized by strong seasonal upwelling and a broad oxygen minimum zone in intermediate waters. However, neither Cd, Zn, Ni nor Cu appear to be affected by the reducing conditions, in contrast with earlier reported observations of Mn, Fe and rare earth elements. Low Cd/PO4 slopes in surface waters of about 0.15 nM/μM are in good agreement with slopes typical of surface waters in other oceans. Deep water slopes, however, increase from 0.5 nM/μM to 0.85 nM/μM going inshore. These slopes are much higher than published for the deep North Atlantic and North Pacific Oceans, yet comparable to the high Cd/PO4 slope recently published for the Antarctic Ocean. Deep water cadmium-phosphate ratios increase with the age of the deep water from the Atlantic through the Antarctic and Indian to the Pacific Ocean. Slopes of Zn/Si (0.062 nM/μM) are about the same as found in the Pacific Ocean, deep water ratios are about 30% higher. The slopes Ni/Si (0.054 nM/μM) are in good agreement with previous reports from the Indian Ocean. The Cu profile shows evidence of surface water inputs, regeneration in intermediate and deep waters and benthic fluxes, and is further influenced by intensive scavenging, notably in surface waters.

Responses of Southern Ocean phytoplankton to the addition of trace metals

Trace metal enrichment experiments were performed under ultraclean conditions with natural oceanic plankton populations in the Southern Ocean along 6°W in October–November 1992. Five Fe-enrichment experiments were conducted, as well as a further eight experiments with single or combined addition of Fe, Mn, Co and Zn. Water was incubated from the Polar Frontal region at the beginning and again later in the middle of a bloom, from the Antarctic Circumpolar Current south of the Front, from the sea-ice edge area and from the ice-covered northern Weddell Sea. Growth responses of phytoplankton to the addition of Mn (2.5 nM), Co (0.4 nM) or Zn (2 nM) were not very clear (slight enhancement), which may partly have been due to low initial biomass. Growth enhancements by Fe (2 nM) were more pronounced, albeit only where initial biomass was relatively high and where distinct growth in the controls was observed simultaneously. Overall, rates of chlorophyll a increased and final yields increased, relative to the controls, by about 10% to more than 50%. Additions of 5 and 10 nM Fe yielded about 10% higher specific growth rates than additions of 2 nM. Combining Fe with one or more of the other trace metals did not result in significantly higher yields or growth rates as with Fe alone, even at high initial biomass. Moreover, the accumulation of cellular 55Fe over time was neither enhanced nor suppressed by Mn, Co, and Zn, further suggesting that co-limitation by the other metals did not occur. Fe is evidently the most important trace metal controlling phytoplankton development. Microzooplankton grazing was determined (dilution method) at the end of one 9 day experiment where a very clear response by phytoplankton was observed following addition of 2 nM Fe. Gross production and grazing rates (as chlorophyll) were 0.71 and 0.34 day−1, respectively, in the control and 0.89 and 0. 30 day−1 in the Fe enrichment. Apparently only algal production was stimulated by Fe enrichment. We conclude that in the Southern Ocean biomass build-up by phytoplankton is limited by decreasing availability of trace metals, notably Fe, under conditions of sufficient light and low grazing pressure.

Natural iron fertilization of the Atlantic sector of the Southern Ocean by continental shelf sources of the Antarctic Peninsula

In large parts of the Southern Ocean, primary production is limited due to shortage of iron (Fe). We measured vertical Fe profiles in the western Weddell Sea, Weddell-Scotia Confluence, and Antarctic Circumpolar Current (ACC), showing that Fe is derived from benthic Fe diffusion and sediment resuspension in areas characterized by high turbulence due to rugged bottom topography. Our data together with literature data reveal an exponential decrease of dissolved Fe (DFe) concentrations with increasing distance from the continental shelves of the Antarctic Peninsula and the western Weddell Sea. This decrease can be observed 3500 km eastward of the Antarctic Peninsula area, downstream the ACC. We estimated DFe summer fluxes into the upper mixed layer of the Atlantic sector of the Southern Ocean and found that horizontal advection dominates DFe supply, representing 54 ± 15% of the total flux, with significant vertical advection second most important at 29 ± 13%. Horizontal and vertical diffusion are weak with 1 ± 2% and 1 ± 1%, respectively. The atmospheric contribution is insignificant close to the Antarctic continent but increases to 15 ± 10% in the remotest waters (>1500 km offshore) of the ACC. Translating Southern Ocean carbon fixation by primary producers into biogenic Fe fixation shows a twofold excess of new DFe input close to the Antarctic continent and a one-third shortage in the open ocean. Fe recycling, with an estimated “fe” ratio of 0.59, is the likely pathway to balance new DFe supply and Fe fixation.