Stéphane Blain

Effect of natural iron fertilization on carbon sequestration in the Southern Ocean

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial–interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization—an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below—as invoked in some palaeoclimatic and future climate change scenarios—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

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.

IRON (II) AND IRON (III) DETERMINATION IN SEA WATER AT THE NANOMOLAR LEVEL WITH SELECTIVE ON-LINE PRECONCENTRATION AND SPECTROPHOTOMETRIC DETERMINATION

Abstract A method is presented for the shipboard determination of iron(II) and iron(III) at the sub-nanomolar level. A preconcentration step using a C 18 phase column is required to remove the major ions as well as to concentrate iron. This column is impregnated with ferrozine, a selective ligand for Fe(II). After passing the sample through the chelating resin, the complex is eluted with methanol and detected with a spectrophotometer. The determination of Fe(III) is realised in the same manner after reduction by an ascorbic acid solution. To reduce the risks of contamination, the manifold developed includes on-line filtration, acidification, reduction, preconcentration and detection. The main parameters influencing the different phases studied were for the preconcentration step: flow-rate, concentration of ferrozine and pH; the optimum values of these parameters were 2 ml min −1 , 10 −3 M and pH 4–5, respectively. For the detection step these parameters were reduction of the Schlieren effect and interfering ions; for the reduction step they were flow-rate and concentration of reducing agent (0.1 ml min −1 and 100 μM, respectively) at pH 4–5, temperature 60 °C and reaction time 2 min. The method is characterised by its precision. It varied from 3% for 2 min of preconcentration to 15% near the limit of detection. The analysis of the certified sea water NASS-4 demonstrated the accuracy of the method, the blank was below the limit of detection for Fe(II) and 0.3 nM for Fe(III). The limits of detection were 0.1 nM for Fe(II) and 0.3 nM for Fe(III)

Biomass, growth rates and limitation of Equatorial Pacific diatoms

Biomass, growth and species composition of siliceous phytoplankton were studied in the Equatorial Pacific during October 1994. Experiments were carried out in different nutrient conditions along the Equator. An oligotrophic area, with nitrate concentrations as low as 10 nM in the upper layer, was encountered in the western part of the transect (166°E–170°W). The concentration of biogenic silica varied from 10 nmol 1−1 in the surface layer up to 40 nmol 1−1 in the deep chlorophyll maximum located near the nutracline. Biogenic silica production, measured by the 32Si method, showed a similar vertical pattern in the nitrate-depleted water, and the mean assimilation rate for Si was 0.4±0.2 nmol l−1 h−1 (integrated mean value: 63 μmol m−2 h−1). In contrast, nitrate concentration ranged from 2 to 4 μM in the surface layer in the high-nutrient low-chlorophyll (HNLC) area located from 170 to 150°W and biogenic silica increased to 200 nmol 1−1. The Si assimilation rate was 1.7±1.0 nmol 1−1 h−1 (integrated mean value: 162 μmol m-Z h−1). In both areas, 80% of Si biomass was concentrated in larger cells ( > 10 μm). Scanning electron microscopy was used to estimate diatom numbers and cell surface areas. This latter parameter correlates well with biogenic silica and warrants a discussion of the contribution of different species to the total biogenic silica. The measured values for specific uptake rate of Si never reached the optimum uptake or growth rate deduced from environmental parameters and kinetic constants reported in the literature. In addition the mean growth rate (0.9±0.3 doubling per day) for the nitrate-depleted water does not differ from the mean value (0.8±0.2 doubling per day) in the HNLC area. Therefore it can be concluded that diatom growth is severely limited in both regions. This agrees well with the nutrient balance study. In the oligotrophic area, N supply appears to be the limiting factor in the upper layer. In the HNLC region, the results of this study are consistent with the hypothesis that diatom growth might be limited by a micro nutrient such as iron. Carbon production by diatoms was estimated to be 31 ± 4 mmol Cm−2 d−1, which is one-third of the total carbon production of the nutrient-enriched area.

Resource limitation of phytoplankton growth in the Crozet Basin, Subantarctic Southern Ocean

In January–February 1999, we performed shipboard iron- and macronutrient-addition experiments in the Crozet Basin, Indian sector of the Subantarctic Southern Ocean, to evaluate the sufficiency of ambient iron and macronutrient concentrations for algal growth. Experiments were conducted with near-surface seawater collected from three locations in a narrow latitudinal band characterized by relatively low algal biomass (o0.7m gl � 1 chlorophyll a), low dissolved iron concentrations (o0.33 nM), and strongmeridional g radients in temperature, salinity and macronutrient concentrations: (1) the Polar Frontal Zone (PFZ) near 461S, 651 E( B19mM nitrate and 1.2mM silicic acid); (2) the confluence of the Subantarctic and Subtropical Fronts (SAF/STF) near 44112 0 S, 63123 0 E( B5.4mM nitrate and 0.5mM silicic acid); and (3) the southern Subtropical Zone (STZ) near 43118 0 S, 62131 0 E( o0.1mM nitrate and B1.4mM silicic acid). Our experimental results reveal three distinct regimes of resource limitation of phytoplankton growth. In the PFZ, iron availability exerted the primary limitation on nitrate drawdown and biomass accumulation, thus community growth, with silicic acid availability exerting a secondary limitation on diatom growth and biogenic silica production. Within the SAF/STF, iron deficiency was also the primary limitation on algal community growth; however, here we observed evidence of secondary limitation of nitrate drawdown and biomass accumulation by silicic acid deficiency, via control of algal community structure—such that iron addition preferentially stimulated the growth of non-diatom nanoplankton—suggesting that the algal community was poised close to co-limitation by iron and silicic acid. As expected, our experimental results indicate that macronutrients (nitrate/phosphate) were the primary limitation on community growth in the STZ waters; however, our results also suggest that iron deficiency imposed a significant secondary limitation on community growth, particularly diatom growth, such that the algal community was poised close to co-limitation by macronutrients and iron. We conclude that these same regimes of resource limitation are likely to regulate phytoplankton growth and export production over much of the open-ocean Subantarctic region during the

Phylogenetic and structural response of heterotrophic bacteria to dissolved organic matter of different chemical composition in a continuous culture study.

Dissolved organic matter (DOM) and heterotrophic bacteria are highly diverse components of the ocean system, and their interactions are key in regulating the biogeochemical cycles of major elements. How chemical and phylogenetic diversity are linked remains largely unexplored to date. To investigate interactions between bacterial diversity and DOM, we followed the response of natural bacterial communities to two sources of phytoplankton-derived DOM over six bacterial generation times in continuous cultures. Analyses of total hydrolysable neutral sugars and amino acids, and ultrahigh resolution mass spectrometry revealed large differences in the chemical composition of the two DOM sources. According to 454 pyrosequences of 16S ribosomal ribonucleic acid genes, diatom-derived DOM sustained higher levels of bacterial richness, evenness and phylogenetic diversity than cyanobacteria-derived DOM. These distinct community structures were, however, not associated with specific taxa. Grazing pressure affected bacterial community composition without changing the overall pattern of bacterial diversity levels set by DOM. Our results demonstrate that resource composition can shape several facets of bacterial diversity without influencing the phylogenetic composition of bacterial communities, suggesting functional redundancy at different taxonomic levels for the degradation of phytoplankton-derived DOM.

Silicon-nitrogen coupling in the equatorial Pacific upwelling zone

We describe the role of diatoms on nitrogen and silicon cycling in the equatorial Pacific upwelling zone (EUZ) using water column nutrient data from 19 equatorial cruises and particle concentration, new production, and sediment trap data from the U.S. Joint Global Ocean Flux Study (JGOFS) equatorial Pacific (EqPac), France JGOFS fluxes in the Pacific (FLUPAC), and U.S. Zonal Flux cruises. Our results suggest that production and sinking of diatoms dominate particulate nitrogen export at silicate concentrations above 4 μM. Below this level, silicate is preferentially retained; while inorganic nitrogen is completely utilized, silicate remains at concentrations of 1–2 μM and is completely exhausted only under nonsteady state conditions. This lower nutrient condition accounts for a majority of particulate nitrogen export in the EUZ with minor loss of particulate silicon. Retention of silicon relative to nitrogen appears due to a combination of new production by nondiatoms, dissolution of silica frustules after grazing, iron limitation, and steady state upwelling. This synthesis supports the argument that diatom production was tightly coupled to new production during the U.S. JGOFS EqPac survey II cruise [Dugdale and Wilkerson, 1998]. However, this compilation suggests EqPac survey II cruise took place during a period of atypically high subsurface nutrients. We conclude that silicon and nitrogen are tightly coupled only at periods of very high nutrient concentration and nonsteady state. In addition, nutrient cycling in the EUZ is consistent at all times with a mechanism of combined iron and grazing control of phytoplankton size classes [Landry et al., 1997].

Shipboard analytical intercomparison of dissolved iron in surface waters along a north-south transect of the Atlantic Ocean

A shipboard analytical intercomparison of dissolved (<0.2 μm) iron in the surface waters of the Atlantic Ocean was undertaken during October 2000. A single underway surface (1-2 m) seawater sampling and filtration protocol was used, in order to minimise differences from possible sample contamination. Over 200 samples (1/h) were collected over 12 days and analysed immediately using four different analytical methods, based on three variants of flow injection with luminol chemiluminescence (FI-CL) and cathodic stripping voltammetry (CSV). Dissolved iron concentrations varied between 0.02 and 1.61 nM during the intercomparison. On average, CSV Electroanalysis 12 (2000) 565 measured 0.08 nM higher iron concentrations than one FI-CL method Anal. Chim. Acta 361 (1998) 189, which measured 0.13 nM higher iron values than the other two Anal. Chem. 65 (1993) 1524; Anal. Chim. Acta 377 (1998) 113, Statistical analyses (paired two-tailed t-test) showed that each analytical method gave significantly different dissolved iron concentrations at the 95% confidence interval. These data however, represent a significant improvement over earlier intercomparison exercises for iron. The data have been evaluated with respect to accuracy and overall inter-laboratory replicate precision, which was generally better than the 95% confidence intervals reported for the NASS Certified Reference Materials. Systematic differences between analytical methods were probably due to the extraction of different physico-chemical forms of iron during preconcentration, either on the micro-column resin (in the FI methods) or with competing ligand equilibration (in the CSV method). Small systematic concentration differences may also have resulted from protocols used for quantification of the analytical blank and instrument calibration.

Preconcentration of trace metals from sea water with the chelating resin Chelamine

Abstract The complexing properties (capacity, pH effect, breakthrough curve) of the chelating resin Chelamine, containing a pentamine ligand, were investigated. The resin was used in a column procedure for the preconcentration of Cd(II), Cu(II), Mn(II), Ni(II), Pb(II) and Zn(II) from deionized water and sea water and the recoveries were 93–105 and 91–102%, respectively. The absolute blanks varied from less than 0.6 ng for Cd to 11 ng for Cu, permitting the determination of the above six metals in oceanic water. The accuracy of the method was demonstrated by replicate analyses of the marine reference material CASS-2. The high selectivity of the resin leads to low concentration of alkali and alkaline earth metal ions in the acidic eluate.

A submersible flow-injection analyser for the in-situ determination of nitrite and nitrate in coastal waters

An flow-injection system is described for the in-situ determination of nutrient concentrations in rivers and ocean waters. In estuarine and coastal waters, significant interferences may be caused by temperature and salinity variations. Therefore, we used a dual wavelength detector to measure simultaneously the reference and sample signals. The device has been used to measure spatial and temporal variations of nitrite and nitrate concentrations in coastal waters exhibiting strong salinity variations (Bay of Brest and Iroise Sea, France). Our original detection system coupled with flow-injection analysis (FIA) allows high-frequency measurements (40 samples per hour), very good precision (1%) and a low detection limit (0.45 μM NO3). The device can work up to a depth of 300 m, within a temperature range of 2 to 35 °C, and with salinities varying from 0 to 35. The dynamic range (0–150 μM NO3) can be adapted to the expected concentrations of the study area by using flow cells with various path lengths. Intercalibration with samples collected by conventional means and analysed with a spectrophotometric reference method at the laboratory showed a good agreement between both methods (1.3%).