de Henricus Baar

The distribution of Fe in the antarctic circumpolar current

Abstract The large-scale distributions of dissolved and total Fe in surface and deep waters of the Antarctic Circumpolar Current exhibit strong relationships with hydrography and biological processes. The mean dissolved Fe concentrations are low in surface waters of the Antarctic Circumpolar Current (0.31–0.49 nM, with a minimum of 0.17 nM) and higher (averaging 1.11.9 nM) in the Polar Frontal region. Enhanced dissolved surface water concentrations in the Polar Frontal region are attributed to input from the continental shelf and coincide with phytoplankton spring blooms of large diatoms. The effects of sea-ice melting and iceberg melting on the Fe concentrations were relatively small. Dissolved deep-water concentrations ( > 400 m) in the Antarctic Circumpolar Current ranged from 0.4 to 2.8 nM. Circumpolar Deep Water has relatively high dissolved Fe concentrations in the Polar Frontal region (0.4–2.8 nM) compared with deep waters further to the south (0.6–1.1 nM). Similarly, total dissolvable (unfiltered) Fe concentrations in the Upper Circumpolar Deep Water tend to decrease southward from the Polar Frontal region. In the Lower Circumpolar Deep Water total dissolvable Fe concentrations are higher than in the Upper Circumpolar Deep Water due to the existing nepheloid layer and sources on the Mid-Atlantic Ridge. Dissolved and total dissolvable Fe concentrations in the Antarctic Bottom Water are higher than those of other water masses in the Antarctic Circumpolar Current, consistent with the nepheloid layer as well as diagenetic input from shelf sediments. The High-Nutrient/Low-Chlorophyll areas of the Antarctic Ocean and northeast Pacific Ocean have different major Fe input sources of similar magnitude. In the Antarctic Circumpolar Current upward transport of Fe is the main input source, whereas in the North Pacific Ocean, aerosols are the dominant source.

von Liebig's Law of the Minimum and Plankton Ecology (1899-1991)

The Law of the Minimum was originally formulated by Justus von Liebig, as one of the 50 interlinked laws concerned with agriculture. The original writings of J. von Liebig often were misinterpreted by his successors. Brandt (1899) took this one law out of its context and proposed that limitation by nitrogen is a dominant factor in plankton ecology, far beyond its original application to agriculture. This was opposed by Nathansohn (1908) who suggested instead a dynamic balance of growth and loss terms. Towards validating, or eventually falsifying Brandts hypothesis, Atkins, Harvey, Cooper and others developed the chemical methods necessary for re-defining ocean nutrient cycling and growth limitation. The major exception to these modern perspectives was the Antarctic Paradox of high nutrients and low chlorophyll which inspired Gran, Atkins, Harvey and Cooper to pioneer the concept of iron limitation. An exhaustive overview is given of efforts to define Fe in seawater and its controlling effect on in situ plankton growth, for the 1920–1984 period. Somewhat parallel work in the laboratory on single species of algae in chelation-controlled media has provided much insight, but is sketched only briefly. Martin and contemporaries developed the chemical methods necessary for defining the ocean chemistry of Fe and its role for in situ growth. These developments are sketched for the 1982–1991 period. Once again the Law of the Minimum and associated bold hypotheses served, albeit briefly, to bring a nutrient element in the forefront of research. This, and the recent awareness of CO2 as rate limiting factor, underline the conclusion that advances in sciences often hinge on advances in technology, confirming Kuhn (1962). In this case the new analytical techniques developed by Atkins, Harvey, Cooper, Martin and their associates have proven revolutionary for plankton ecology. Some observations in plankton ecology may be reminiscent of the agricultural Law of the Minimum, but this would not warrant its direct application, beyond its original context and agriculture, to plankton ecology. Rather the net rate of increase of phytoplankton is the dynamic balance of multiple growth and loss terms, together also determining the biomass at given time and space.

Ecology and biogeochemistry of the Antarctic Circumpolar Current during austral spring : a summary of Southern Ocean JGOFS cruise ANT X/6 of R.V. Polarstern

The R.V. Polarstern cruise ANT X/6, part of the international Southern Ocean JGOFS programme, investigated phytoplankton spring bloom development and its biogeochemical effects in different water masses of the Atlantic sector of the Southern Ocean: the Polar Frontal region (PFr), the southern Antarctic Circumpolar Current zone (sACC), its boundary with the Weddell Gyre (AWB) and the marginal ice zone (MIZ). The relative roles of physical stability, iron limitation and grazing pressure in enhancing or constraining phytoplankton biomass accumulation were examined. Three sections were carried out between the PFr and the ice edge along the 6°W meridian from early October to late November 1992. This paper summarises the major findings of the cruise and discusses their implications for our understanding of Southern Ocean ecology and biogeochemistry. A major finding was the negligible build-up of plankton biomass and concomitant absence of CO2 drawdown associated with seasonal retreat of the ice cover. In striking contrast to this unexpected poverty of both the MIZ and the frontal region of the AWB, distinct phytoplankton blooms, dominated by different diatom species, accumulated in the PFr. Chlorophyll stocks in the sACC remained monotonously low throughout the study. Our findings confirm those of other studies that frontal regions are the major productive sites in the Southern Ocean and that input of meltwater and associated ice algae to the surface layer from a retreating ice edge is by itself an insufficient condition for induction of phytoplankton blooms. The blooms in the PFr developed under conditions of shallow mixing layers, high iron concentrations and relatively low grazing pressure. However, in all three blooms, high biomass extended to deeper than 70 m, which cannot be explained by either in situ growth or sinking out of a part of the population from the upper euphotic zone. Subduction of adjoining, shallower layers to explain depth distribution is invoked. Despite a clear CO2 drawdown in the Polar Frontal region, the highly variable conditions encountered render reliable estimation of annual CO2 fluxes in the Southern Ocean difficult.

Changes of carbon dioxide in surface waters during spring in the Southern Ocean

The fugacity of C02 (fCO2) and the content of chlorophyll a in surface-water were determined during consecutive sections between 47° and 60°S along 6°W in austral spring, October- November 1992. In the Polar Frontal region, the fCO2 of surface-water decreased from slightly below the atmospheric value to 50 μatm below it. This was accompanied by the development of diatom blooms. Seasonal warming of 1.2°C and air-sea exchange partly compensated the decrease of fCO2 by biological activity. Meanders of the Polar Frontal jet and a mesoscale eddy were reflected in spatial variability of fCO2 and chlorophyll a. Systematic observations indicated relationships between fCO2 and chlorophyll a, albeit changing with time. The combination of biological CO2- uptake with formation of Antarctic Intermediate Water (AAIW) makes the Polar Front a site of combined biological/physical CO2-drawdown from the atmosphere. In the southern part of the Antarctic Circumpolar Current (sACC) and the Southern Frontal region, fCO2 increased 7–8 μatm due to surface-water warming of 0.5°C. A sharp rise of surface water fCO2 of 13 μatm occurred south of the southern Frontal jet. As the ice-cover disappeared, the Boundary between the ACC and the Weddell Gyre released significant amounts of CO2. The Weddell Gyre would become a strong CO2-source after the imminent retreat of the ice. Clearly mechanisms behind changes of fCO2 in surface waters differ for the hydrographic regions. Interstitial brines of sea-ice had fCO2 as low as 100 μatm and had been depleted in nutrients. The summation of significant sources and sinks in the different regions indicates an overall minor oceanic CO2-sink of 0.3 mmol m−2 day−1 throughout the cruise, on the basis of the Wanninkhof relationship at in situ wind speed without skin effect. Uptake of C02 increased to 1.0 mmol m−2 day−1, when a uniform cold skin temperature difference of 0.2°C was assumed. The skin temperature difference derived from the physical model by Soloviev and Schl\ussel (1994a,b) had an average value of 0.2°C, leading to an uptake of CO2 of 1.2 mmol m−2 day−1. The measured skin temperature difference exceeded the calculated value. These assessments underline the uncertainty in the estimated air-sea exchange of C02 due to the thermal skin effect, the chosen parametrization of the gas transfer velocity, and the selected length of the wind speed interval. Limited understanding of the mechanistics of gas exchange, as well as large seasonal and spatial variability of the air-sea flux, still preclude a reliable estimate of the basin-wide annual flux for the Southern Ocean.

Cadmium, copper and iron in the Scotia Sea, Weddell Sea and Weddell/Scotia Confluence (Antarctica)

Until recently, little was known about trace metals in the Southern Ocean. Vertical profiles and surface water sections along 49°W exhibit Cd concentrations of 0.2–0.8 nM, increasing with depth, as for phosphate. A linear relationship between Cd and phosphate exists as in other oceans; however, the Cd/P slope at about 0.63–0.65 nM μM−1 is much higher than the generally assumed global deep water ratio of about 0.35–0.4 nM μM−1. Dissolved Cu levels range from 1 to 4 nM, increasing with depth, as for silicate. The linear relationship between Cu and silicate shows the same linear trend as in the North Atlantic Ocean, except for the shallow (less than 100 m) Antarctic waters. The South Orkneys shelf appears to be a source of dissolved Cu. Dissolved Fe levels range from 2 to 8 nM in the surface waters. Deep water Fe levels are similar. Over the South Orkneys shelf dissolved Fe is an order of magnitude higher (about 60 nM). Shelf sediments appear to be a major source for Fe; transport of weathered material by ice(bergs) may also contribute Fe to seawater.

Nutrient anomalies in Fragilariopsis kerguelensis blooms, iron deficiency and the nitrate/phosphate ratio (A. C. Redfield) of the Antarctic Ocean

Abstract During seasonal development of blooms in the Polar Frontal region, concentrations of nitrate and phosphate decreased in surface waters. In blooms of Fragilariopsis kerguelensis at the southern rim (49–50°S) of the Polar Frontal region the dissolved ratio NO 3 PO 4 increased from the winter value of ∼14 to 15.8 (18 October 1992) to as high as 25 (23 November 1992). Ambient dissolved Fe in these blooms was subnanomolar compared to ∼1.1–1.9 nM in the overall Polar Frontal region. Blooms more northerly in the Polar Frontal region were dominated by other diatoms and higher dissolved Fe (> 1 nM), and showed only very modest NO 3 PO 4 anomalies. From nutrient inventories the biogenic pools (PON and DON) and export of settling biogenic debris would have N P ratios as low as 4.4–6.1 compared to ∼14 in deep Antarctic waters. Such shifts are consistent with decreasing availability of Fe for nitrate reduction, but also may be due to intrinsically low N P in Fragilariopsis kerguelensis cells. Moreover, a low ratio DON DOP in dissolved organic matter and enhanced recycling of N versus P cannot be excluded either. Triplicate mesocosm (20 l) experiments were performed with a diatom-dominated community in ambient seawater (initial Fe = ∼0.9 nM) collected at the Polar Front during early spring. Three other triplicates were enriched with 2 nM Fe to total Fe ∼ 2.9 nM. During the incubations, the Fe-enriched experiments showed assimilation at near-perfect Redfield N P ratios of ∼15 and a virtually near-zero intercept. The untreated incubations showed significantly lower uptake ratios at ∼13 and non-zero intercepts, suggesting leftover nitrate after all phosphate was utilised. At initial Fe = ∼0.9 nM, the Fe-containing algal enzymes for reduction of nitrate appeared to be impaired, hence nitrate assimilation was less efficient. The observed N P fractionation in Fragilariopsis kerguelensis blooms at the Polar Frontal region, in combination with the local formation of AAIW flowing northward, might help maintain the lower N P ratio at ∼14 in Antarctic waters, as compared to a ∼15 as an average value for the other oceans. The functionality of Fe in C-fixation, nitrate assimilation as well as N2 fixation may partly explain the large variability of the NO 3 PO 4 ratio in this and other ocean basins (Fanning, 1992; Journal of Geophysical Research, 97, 5693–5712), as well as recently reported variations in the extended C/N/P ratio.

Fe (III) speciation in the High Nutrient , Low Chlorophyll Pacific region of the Southern Ocean

Fe speciation was measured with competitive ligand equilibration adsorptive cathodic stripping voltammetry [Gledhill, M., Van den Berg, C.M.G., 1994. Determination of complexation of iron (III) with natural organic complexing ligands in sea water using cathodic stripping voltammetry. Mar. Chem., 47, 41–54.] in the Pacific part of the Southern Ocean between 58° and 68°30′S along the 90°W meridian. The conditional stability constant (K′ with respect to [Fe3+]) was between 1020.6 and 1021.6 when one organic ligand was detected. The ligand concentration ([Lt]) varied between 2.2 and 12.3 equivalents of nM Fe (nEq of Fe). The ligand concentration was at least 6 times, and generally more than 10 times, that of the total dissolvable Fe concentration. At one station a depth profile was sampled where below 200 m depth, two organic ligands were measured with K1′=1021 and K2′=1022.4. Organic complexation of Fe was similar to results found elsewhere [(Gledhill, M., Van den Berg, C.M.G., 1994. Determination of complexation of iron (III) with natural organic complexing ligands in sea water using cathodic stripping voltammetry. Mar. Chem., 47, 41–54.); (Van den Berg, C.M.G., 1995. Evidence for organic complexation in seawater. Mar. Chem., 50, 139–159.); (Rue, E.L., Bruland, K.W., 1995. 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.); (Rue, E.L., Bruland, K.W., 1997. The role of organic complexation on ambient iron chemistry in the equatorial Pacific Ocean and the response of a mesoscale iron addition experiment. Limnol. Oceanogr., 42, 901–910.)] judging from the overall organic alpha value (K′*[Lt]) 1012.4–1013.9. The lower values of organic alpha were within one order of magnitude of our choice of the inorganic alpha (1011.9, [Millero, F.J., Yao, W., Aicher, J., 1995. The speciation of Fe (II) and Fe (III) in natural waters. Mar. Chem., 50, 21–39.]) in which case the organic and inorganic ligands could compete effectively for Fe. Different values of organic alpha and the occurrence of two organic ligand classes were consistent with differences in hydrography. South of the Polar Front, the least organic complexation occurred (organic alpha=1012.4, organic complexation around 80%), where the highest chlorophyll a concentrations were measured.

Iron-mediated effects on nitrate reductase in marine phytoplankton

The potential activity of nitrate reductase was determined in uni-algal cultures in the laboratory and in natural marine phytoplankton assemblages. In the laboratory bioassays, distinct differences in nitrate reductase activity were observed in iron replete versus depleted cultures for Emiliania huxleyi, Isochrysis galbana and Tetraselmis sp. Cells from iron-depleted cultures had 15 to 50 percent lower enzyme activity than those from iron-replete cultures. Upon addition of iron, nitrate reductase activity was enhanced in depleted cells up to levels comparable to those of the replete cells. Bioassays in the northern North Sea conducted in 1993, under low iron conditions, demonstrated similar results. Upon addition of 2.5 nM iron, a distinct enhancement, to a maximum of three times, of nitrate reductase activity was observed within 32 h after addition. Therefore, iron can stimulate nitrate reductase activity. In spite of the clean techniques used, some nitrate reductase activity was always observed. Iron deficiency was shown to impair nitrate reductase activity, but it is unlikely that nitrate reduction would cease completely.

A comparison of iron limitation of phytoplankton in natural oceanic waters and laboratory media conditioned with EDTA

The solubility of iron in oxic waters is so low that iron can be a limiting nutrient for phytoplankton growth in the open ocean. In order to mimic low iron concentrations in algal cultures, Ethylenediaminetetraacetate (EDTA) is commonly used. The presence of EDTA enables culture experiments to be performed at a low free metal concentration, while the total metal concentrations are high. Using EDTA provides for a more reproducible medium. In this study Fe speciation, as defined by EDTA in culture media, is compared with complexation by natural organic complexes in ocean water where Fe is thought to be limited. To grow oceanic species into iron limitation, a concentration of at least 10−4 M EDTA is necessary. Only then does the calculated [Fe3+] concentrations resemble those found in natural sea water, where the speciation is governed by natural dissolved organic ligands at nanomolar concentrations. Moreover, EDTA influences the redox speciation of iron, and thus frustrates research on the preferred source of Fe-uptake, Fe(III) or Fe(II), by algae. Nowadays, one can measure the extent of natural organic complexation in sea water, as well as the dissolved Fe(II) state, and can use ultra clean techniques in order to prevent contamination. Therefore, it is advisable to work with more natural conditions and not use EDTA to create iron limitation. This is especially important when the biological availability of the different chemical fractions of iron are the subject of research. Typically, many oceanic algae in the smallest size classes can still grow at very low ambient Fe and are not easily cultivated into limitation under ambient sea water conditions. However, the important class of large oceanic algae responsible for the major blooms and the large scale cycling of carbon, silicon and other elements, commonly has a high Fe requirement and can be grown into Fe limitation in ambient seawater.

Iron enrichment experiments in the Southern Ocean: physiological responses of plankton communities

The physiological responses of plankton to iron enrichment were investigated in experiments performed in 20-1 culture vessels. Natural phytoplankton communities in sea water, with mean ambient Fe concentrations ranging from 0.3-0.4 nM in the Antarctic Circumpolar Current to 1.2-1.9 nM in the Polar Frontal region, were incubated for several days. Upon addition of 2 nM of iron, synthesis of chlorophyll a and nutrient uptake was stimulated. The specific nitrate-uptake rates as determined by 15N-uptake experiments consistently increased, as well as the ratios of chlorophyll a to particulate carbon. Growth rates in iron-enriched bottles were consequently enhanced relative to control bottles. The biochemical composition of the plankton community, indicated by carbon to nitrogen ratios and fatty acid composition, remained unaffected by iron addition. On the basis of 14C incorporation into the major biochemical pools, no changes were observed in the allocation of carbon into proteins, polysaccharides, lipids and low-molecular-weight metabolites in the particulate fraction. Antarctic phytoplankton endures the low ambient iron concentrations by maintaining physiological processes at lower activity rates, whereas the biochemical composition of the plankton remains virtually unaffected.