[Show abstract][Hide abstract] ABSTRACT: Spatial and temporal changes in sea ice microbial communities were investigated at 4 stations located along a south-north transect on the land-fast ice of Dumont d'Urville station area (Adélie Land, 66°40' S, 141°01' E) during the ice coverage period (April to December). A seasonal pattern was observed in microalgae, bacteria and protozoan abundance distribution. A maximum chlorophyll a concentration occurred during fall ice formation in the surface layer with the highest values at the near-shore station (100 to 215 μg l-1). A second maximum was observed before ice breaking in the bottom ice (50 to 90 μg l-1). Microalgal communities were dominated by diatoms (>86% of the total cells), mainly represented by Fragilariopsis, Nitzschia, Navicula and Pseudonitzschia species. Fragilariopsis curta was the dominant species during the first bloom whereas Fragilariopsis cylindrus, Nitzschia longissima and Tropidoneis sp. were the main contributors during the second bloom at the bottom ice core. Maximum protozoan abundance was recorded during the fall bloom in the surface layer with dominance of ciliates, which contributed more than 75 % of total cell numbers. During this period, the maximum ciliate abundance was associated with the maximum bacteria and diatom numbers and microalgal biomass. The dramatic decrease of the ice algal biomass from south to north paralleled that of the underlying water phytoplankton available for new ice incorporation. The spatial algal biomass decrease could explain the parallel decrease in the abundance of bacteria and heterotrophic ciliates through trophic interactions.
Full-text · Article · May 2006 · Aquatic Microbial Ecology
[Show abstract][Hide abstract] ABSTRACT: Size-fractionated chlorophyll a (Chl a) biomass and phytoplankton community distributions were investigated along two transects located in the Indian Ocean frontal region between 43–46°S and 62–65°E during late summer 1999. Chl a distribution was linked to the merged Subantarctic Front (SAF) and the Subtropical Front (STF) which marked the border between the cold and less saline subantarctic waters and the warm and more saline subtropical waters. Chl a concentrations increased from
No preview · Article · Oct 2004 · Journal of Marine Systems
[Show abstract][Hide abstract] ABSTRACT: A mesoscale study was conducted in January and February 1999 in the Crozet Basin frontal zones (43degrees50' to 45degrees20'S, 61degrees00' to 64degrees30'E) within the southernmost and easternmost convergence area of the Antarctic Circumpolar Current (ACC) and the Agulhas Return Current (ARC). Distribution of biogeochemical parameters was strongly linked to the merged Subtropical (STF) and Subantarctic (SAF) Fronts which mark the border between the cold and less saline subantarctic waters and the warm and more saline subtropical waters. This survey took place during a post-bloom period. Chlorophyll a concentrations were low throughout the study area ranging from 0.2 mug l(-1) in the Polar Frontal Zone (PFZ) to 0.4 mug l(-1) in the Subtropical Zone (STZ). Maximum chlorophyll a values (0.8 mug l(-1)) associated with an increase in biogenic silica concentration (from 0.03 to 0.34 muM) and a diatom peak (1.2 x 10(5) cells l(-1)) were encountered in the northeastern part of the STF edge. Despite northwardly decreasing concentrations of nitrates from 14 muM in the PFZ to 6 PM in the STZ, they were not the main factor limiting phytoplankton growth. Low silicic acid (mean = 0.6 muM) could have limited diatom development in the PFZ and the STZ where diatom numbers were low. In STZ waters, where average diatom numbers were highest, various species of Nitzschia and Thalassiothrix were common, but Pseudonitzschia spp. were dominant. Throughout the survey area, pico- and nano-sized cells dominated the phytoplankton assemblage, and their number was the highest in the STZ. Cyanobacteria, only present in subtropical waters >12.5degreesC, were the major component of the picoplankton size-fraction. While dinoflagellate numbers were low in the Subantarctic Zone (SAZ), their abundance and species numbers increased in the STZ, where Oxytoxum laticeps became dominant and several further large-size species of Prorocentrum, Ceratium and Gymnodinium appeared in addition to those at the STF. The distribution of different biogeochemical parameters suggests that the Crozet Basin frontal region is a non-exporting system at the end of summer. During this post-bloom period, biological activity is low and phytoplankton growth severely limited. This is evidenced by the weak dependence of the partial pressure of carbon dioxide (pCO(2)) on biological activity and the importance of the air-sea exchange in maintaining pCO(2) close to saturation.
Full-text · Article · Mar 2003 · Marine Ecology Progress Series
[Show abstract][Hide abstract] ABSTRACT: Surface phytoplankton assemblages were studied in January/February 1999 in the Crozet Basin (4350S–4520S; 61E–6430E) between the northern Polar Zone and the Agulhas Front. Cell concentrations increased several fold northwards from the SubAntarctic Zone (SAZ) and reached peak numbers (average 2106 cells l–l ) in the central and western Subtropical Zone (STZ). The most spectacular increase in cell numbers occurred at the Subtropical Front (STF) and was attributed to dinoflagellates and diatoms. Nanoflagellates and picoplankton were dominant in the entire area (average 2.8105–1.6106 cells l–l). In the SAZ they were followed by coccolithophorids, dinoflagellates and diatoms. In the STZ coccolithophorids were often outnumbered by dinoflagellates. Diatoms were dominated by Pseudonitzschia delicatissima and were generally the least abundant algae, but reached peak densities of 1.2–4105 cells l–l at, and north of the STF. Coccolithophorids contained mainly Emiliania huxleyi, but in the SAZ and STF Gephyrocapsa oceanica was a co-dominant species. Dinoflagellates were dominated by nano-sized species of Gymnodinium, Gyrodinium and Prorocentrum. The numbers of dinoflagellate and coccolithophorid species increased considerably in the convergence zone (STZ), which suggests their in-situ development. Heterotrophic dinoflagellates and ciliates were mainly present in the subtropics. Cell carbon biomass was attributed chiefly to auto- and heterotrophic dinoflagellates (av. 23–72g C l–l; 68–87%), showing their important contribution to the carbon flow. Variations in cell concentrations across the fronts and water masses, and the distribution of major species were most likely controlled by the combined effect of such factors as nutrient renewal in the convergence zone, availability of iron, increased water-column stability at fronts, and high horizontal gradients in surface-water temperature.
[Show abstract][Hide abstract] ABSTRACT: 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 (<0.7 μg l−1 chlorophyll a), low dissolved iron concentrations (<0.33 nM), and strong meridional gradients in temperature, salinity and macronutrient concentrations: (1) the Polar Frontal Zone (PFZ) near 46°S, 65°E (∼19 μM nitrate and 1.2 μM silicic acid); (2) the confluence of the Subantarctic and Subtropical Fronts (SAF/STF) near 44°12′S, 63°23′E (∼5.4 μM nitrate and 0.5 μM silicic acid); and (3) the southern Subtropical Zone (STZ) near 43°18′S, 62°31′E (<0.1 μM nitrate and ∼1.4 μM 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 mid to late summer.
Full-text · Article · Dec 2002 · Deep Sea Research Part II Topical Studies in Oceanography
[Show abstract][Hide abstract] ABSTRACT: During October 1995 the dynamics of phytoplankton were investigated in the Indian sector of the Southern Ocean along a 62°E meridian transect between 49°00′S and 58°50′S. During spring, chlorophyll a (chl a) concentrations did not exhibit a large spatial variability. The highest chl a concentrations (0.30–0.45mgm−3) were encountered in the surface water of the marginal ice zone (MIZ) and the polar frontal zone (PFZ), while in the permanent open-ocean zone (POOZ) maximum values were always 10μm were always dominant in the MIZ. Despite vertical water stratification due to ice melting and low grazing pressure the chl a concentrations remained low (
No preview · Article · Dec 2002 · Deep Sea Research Part II Topical Studies in Oceanography
[Show abstract][Hide abstract] ABSTRACT: The coastal sea ice in the vicinity of Dumont d'Urville station, Antarctica (66degrees 40' S, 140degrees01'E) supports a diverse microbial community. To investigate seasonal changes in bacterial, microalgal and protozoan biomass during the ice-coverage period, a survey was conducted on fast ice in the continental shelf of Terre Adelie. A reference station was sampled twice a month from April to December 1997. In April and May, during sea ice formation, the autotrophic biomass reached relatively high levels ranging from 400 to 800 mgC m(-3) in the subsurface ice. A second increase reaching 1500 mgC m(-3) in mid-December occurred in early summer in the bottom ice layer. Bacterial biomass ranged from < 10 mg C m(-3) in August and September to > 200 Mg C m(-3) in May. Maximal levels of bacterial biomass were detected during ice formation and just before the summer thaw. Total protozoan biomass increased from May to July. Maximum protozoan biomass (31.8 mgC Mm(3)) occurred in the surface ice layer in July. Except for during a short period (late May to early June), ciliates were dominant, accounting for 50 to 77% of the total protozoan biomass. During the ice-covered period, phototrophic biomass was always dominant. Microalgal biomass contributed on average 80.6% of total biomass whereas bacterial and protozoan biomass accounted for only 16.4 and 3%, respectively. Despite their low biomass, protozoa seem to play a major role in bacterial regulation.
[Show abstract][Hide abstract] ABSTRACT: The silicon and carbon cycles in the Subantarctic region have been investigated in the summer 1999 during the Antares 4 cruise in the Indian sector of the Southern Ocean. Both biogenic silica (BSi) biomass and production values were low and indicative of the end of the productive period. Biogeochemical parameters revealed that the siliceous components of the phytoplankton were scarce. The study area was characterized by nanoplanktonic communities. Depth-integrated values of BSi showed a north–south positive gradient: average values were 45.65 mmol m −2 in the Polar Frontal Zone (PFZ) and 19.82 mmol m −2 in the Subtropical Zone (STZ). Diatom frustules appeared slightly deteriorated, and BSi distribution suggested particulate matter sedimentation especially at the southern boundary of the study area, where the highest concentrations were found (750 nmol l −1 ). BSi tended to accumulate along the Subtropical Front, with values reaching up to 420 nmol l −1 . Depth-integrated BSi production rates showed an inverse pattern compared to BSi: the maximum and the minimum values were found, respectively, in the STZ (0.72 mmol Si m −2 d −1 ) and in the PFZ (0.25 mmol Si m −2 d −1 ). The Subantarctic region, located between the PFZ and the STZ, showed intermediate values for both BSi standing stock (31.58 mmol m −2 ) and production rates (0.50 mmol Si m −2 d −1 ). A production regime based on orthosilicic acid regeneration is hypothesized in the STZ where surface temperatures appear high enough to sustain higher BSi dissolution rates. During summer, BSi production in the frontal zone of the Indian sector is comparable to the lowest production rates of oligotrophic waters.
Full-text · Article · Mar 2002 · Deep Sea Research Part II Topical Studies in Oceanography
[Show abstract][Hide abstract] ABSTRACT: Shipboard iron-addition incubation experiments were carried out in the Indian sector of the Subantarctic Southern Ocean during the Antares-IV campaign in late January–February 1999. The aim of these experiments was to estimate the dissolved iron requirements of the native phytoplankton community in this oceanic region, in order to improve the parameterisation of iron as a limiting nutrient in a coupled 1D physical–biogeochemical ocean model. The experiments were conducted with plankton collected from the upper water column (∼20 m depth) at three sites in the Crozet Basin between 43–46°S and 61–65°E: (1) the Polar Front Zone (PFZ, dissolved Fe=0.33 nM), (2) the confluence of the Subantarctic and Subtropical Fronts (SAF/STF, dissolved Fe=0.29 nM), and (3) the southern Subtropical Zone (STZ, dissolved Fe=0.09 nM). Experimental results from each site indicate that algal community growth rates varied as a function of added iron concentration. Monod saturation functions fitted to the experimental data yield estimates for the community half-saturation constant for growth with respect to iron (Kμ) of 0.41–0.45 nM Fe (PFZ), 0.055–0.086 nM Fe (SAF/STF) and 0.092–0.093 nM Fe (STZ, with macronutrients added), each of which has an estimated uncertainty of ±20%. The Kμ estimate for the SAF/STF site reflects the mixed algal assemblage (diatoms+nanoplankton+dinoflagellates) that grew in the experimental incubations, whereas the Kμ estimates for the PFZ and STZ sites probably reflect the Fe requirements of the small pennate diatoms such as Pseudo-nitzschia spp., which dominated the algal biomass produced in these experiments. The fact that there are significant differences between the Kμ estimates for the PFZ and STZ sites suggests that similar diatom assemblages may have quite different iron requirements, perhaps due to differences in environmental conditions (e.g., light, macronutrient levels). We also examine the sensitivity of a one-dimensional coupled physical–biogeochemical model to the choice of Kμ for iron, using time-series observations from the KERFIX station close to the Polar Front. The model was best able to simulate the KERFIX observations using a diatom Kμ value of ∼0.1 nM Fe, which is considerably less than our experimental estimate of ∼0.4 nM Fe for the PFZ. This discrepancy probably reflects differences in the iron requirements of diatom populations immediately north and south of the Polar Front in the Kerguelen region, due to differences in diatom species composition, availability of light and silicic acid, or the environmental and physiological histories of the diatom populations.
Full-text · Article · Jan 2002 · Deep Sea Research Part II Topical Studies in Oceanography
[Show abstract][Hide abstract] ABSTRACT: As part of the Southern Ocean JGOFS program, the ANTARES 4 cruise took place in January-February 1999 in the Crozet Basin frontal zone aboard the R/V "Marion Dufresne 2". The close vicinity of the Agulhas Return Current (ARC), the Subtropical Front (STF) and the Subantarctic Front (SAF) resulted in a sharp horizontal gradient in temperature and salinity. The fronts constituted the boundaries between different zones of the Antarctic Circumpolar Current (ACC): the Subtropical Zone (STZ), north of STF, the Subantarctic Zone (SAZ) between the STF and SAF, and the Polar Frontal Zone (PFZ) between the SAF and the Polar Front (PF) (Park et al., 1993). Analyti- cal flow cytometry was used to determine abundance and size structure of the ultra- plankton (
[Show abstract][Hide abstract] ABSTRACT: JGOFS-KERFIX (KERguelen point FIXe) time-series station, located south of the polar front in the Indian sector of the Antarctic Ocean, was occupied monthly between January 1990 and March 1995. Annual cycles of dissolved inorganic carbon (DIC), total alkalinity (TALK), oxygen (O2) and nutrients (nitrate, silicate, phosphate and ammonia) in the upper ocean are presented for this site. From seasonal drawdown of nutrients and DIC, we estimate a spring–summer net community production of 3.2±0.5 mol m−2 and C/N/P ratios of 100/16/1. The Si/N ratio varies between 1.8 and 3, suggesting low iron concentrations. The spring–summer biogenic silicon export derived from silicate drawdown is 1.18 mol m−2, consistent with model estimates of silicate export at this site. Seasonal and interannual variations of oxygen, nitrate and DIC due to physical and biological processes are quantified using a simple month-to-month budget formulation. From these budgets, an annual net community production of 5.7±3.3 mol m−2 yr−1 is estimated, about twice the averaged spring–summer production, indicating that, at KERFIX, there is a positive net community production throughout the year. Air–sea CO2 fluxes show that KERFIX is a strong CO2 sink for the atmosphere of 2.4–5.1 mol m−2 yr−1 in 1993, depending on the gas exchange formulation used. A 2.1–3.3 mol m−2 yr−1 outgassing of O2 is observed at KERFIX except in 1993 and 1994 where a decreasing trend of temperature induces an increase of O2 solubility.
No preview · Article · Jul 2001 · Deep Sea Research Part I Oceanographic Research Papers
[Show abstract][Hide abstract] ABSTRACT: In the context of the iron hypothesis, the biogeochemical characteristics of the water masses located in the N–E wake of the Kerguelen archipelago were studied in austral spring 1995 during the ANTARES 3/France JGOFS cruise. In agreement with satellite observations (CZCS and SeaWifs data), this cruise showed the surface waters in the wake of the Kerguelen archipelago to be rich in both chlorophylla and dissolved iron, compared to other observations in the HNLC oceanic waters of the Southern Ocean. This gives support to the hypothesis that natural fertilization due to iron inputs occurs in the waters surrounding the Antarctic Islands. However, the mesoscale structure of the water masses in the study area was complex, and three contrasted zones were identified within the region of lesser or greater iron enrichment. The coastal zone was characterized by very high concentrations of dissolved iron (>10nM) and lithogenic material, but the phytoplankton biomass, dominated by small species, was low. An intrusion of cold Antarctic surface water, rich in silicic acid, was separated from the coastal zone by a shelf-break front. This water tongue contained chlorophylla concentrations as low (2nM), both the coastal zone and the Antarctic water tongue were chlorophylla poor. This was accounted for by either a heavy grazing pressure exerted by copepods (coastal zone) or an unfavourable light-mixing regime (Antarctic water tongue). In the offshore waters, by contrast, dissolved iron concentrations in the range 0.45–0.7nM were sufficient to support higher phytoplankton growth under a more favourable light-mixing regime.
Full-text · Article · Jan 2001 · Deep Sea Research Part I Oceanographic Research Papers
[Show abstract][Hide abstract] ABSTRACT: The partial pressure of carbon dioxide (pCO2), calculated from pH and total alkalinity measurements, was monitored together with chlorophyll a and bacterioplankton biomass in shallow coastal water located inside and outside a giant kelp bed (Macrocystis pyrifera) situated in the Kerguelen Archipelago, Southern Ocean. In spite of large changes over a short time-scale, pCO2 variations over the year are large and exhibit a seasonal pattern in which the different stages of the annual biological
turnover are well marked. The overall pattern of pCO2 variations is related to biological activity (development of both photosynthesis and respiration) during almost the whole
year. However, physical and thermodynamical constraints exert a strong influence on pCO2 at meso time-scale (10 days) and/or when biological activity is weak. Macrocystis acts to maintain pCO2 below saturation almost the whole year and large undersaturations (pCO2 as low as 20 μatm) were observed within the kelp bed. Furthermore, primary production of Macrocystis covers a period of 8 ∼ 9 months a year from winter to late summer and the kelp bed seems to favour the spring phytoplanktonic
bloom. The buffer factor β indicates that, outside the kelp bed, inorganic carbon dynamics are mainly influenced by air-sea
exchange and photosynthesis without calcification. Inside the kelp bed, β suggests calcification by the epiphytic community.
[Show abstract][Hide abstract] ABSTRACT: The partial pressure of carbon dioxide (pCO(2)), calculated from pH and total alkalinity measurements. was monitored together with chlorophyll a and bacterioplankton biomass in shallow coastal water located inside and outside a giant kelp bed (Macrocystis pyrifera) situated in the Kerguelen Archipelago, Southern Ocean. In spite of large changes over a short time-scale pCO(2) variations over the year are large and exhibit a seasonal pattern in which the different stages of the annual biological turnover are well marked. The overall pattern of pCO(2) variations is related to biological activity (development of both photosynthesis and respiration) during almost the whole year. However, physical and thermodynamical constraints exert a strong influence on pCO(2) at meso time-scale (10 days) and/or when biological activity is weak. Macrocystis acts to maintain pCO(2) below saturation almost the whole year and large undersaturations (pCO(2) as low as 20 mu atm) were observed within the kelp bed. Furthermore, primary production of Macrocystis covers a period of 8 similar to 9 months a year from winter to late summer and the kelp bed seems to favour the spring phytoplanktonic bloom. The buffer factor beta indicates that, outside the kelp bed, inorganic carbon dynamics are mainly influenced by air-sea exchange and photosynthesis without calcification. Inside the kelp bed. beta suggests calcification by the epiphytic community.
[Show abstract][Hide abstract] ABSTRACT: The influence of diesel fuel and “Arabian light” crude oil contamination were investigated on the land fast ice located in
the continental shelf of Terre Adélie, Antarctica, during the austral winter 1993. Autotrophic biomass exhibited a clear seasonal
pattern. In uncontaminated sea ice, chlorophyll a concentration showed two maxima, one in April (50 mg m−3) during sea-ice formation, and the second one in spring just before the ice thaw (20 mg m−3). The crude oil and diesel fuel contamination induced a negative effect on ice-microalgae biomass, which remained at a weak
level throughout the ice-covered period. However, the inhibitory effect of diesel contamination was immediate while the crude
oil effect occurred after the autumn phytoplankton bloom. Addition of fertilizer (Inipol EAP-22) to diesel and crude oil had
a clear favourable effect on ice-microalgae. Chlorophyll a biomass exhibited the same seasonal pattern in fertilized and uncontaminated areas.
[Show abstract][Hide abstract] ABSTRACT: Between January 1990 and March 1995, the research project KERFIX undertook the first regular noncoastal multiyear acquisition of parameters related to the carbon cycle in the Southern Ocean at a time series station located at 50°40' S-68°25' E, 60 miles southwest of the Kerguelen Islands. The objectives of KERFIX are (1) to monitor the ocean/atmosphere CO2 and O2 exchanges and to understand which processes govern these exchanges (2) to observe and interpret the seasonal and interannual variability of the production, flux, decomposition and dissolution of carbon and associated elements at this location. In addition, micropaleontological studies describe the present and past flux dynamics in this oceanic area, to improve the knowledge of the transfer functions of some oceanographic proxies. This paper presents a survey of the KERFIX program: scientific objectives, organization of the field operations and some main results obtained since the beginning of KERFIX program, as well as the results of the temporal evolution of hydrological, chemical and biological parameters.
Full-text · Article · Nov 1998 · Journal of Marine Systems
[Show abstract][Hide abstract] ABSTRACT: During the late austral summer of 1994, Antarctic waters were characterized by low phytoplankton biomass. Along the 62 degrees E meridian transect, between 49 degrees S and 67 degrees S, chlorophyll (Chl.) a concentration in the upper 150 m was on average 0.2 mg m(-3). However, in the Seasonal Ice Zone (SIZ) chlorophyll a concentrations were higher, with a characteristic deep chlorophyll maximum. The highest value (0.6 mg Chi. a m(-3)) was measured at the Antarctic Divergence, 64 degrees S, corresponding to the depth of the temperature minimum (similar to 100 m). This deep biomass maximum decreased from South to North, disappeared in the Permanently Open Ocean Zone (POOZ) and reappeared with less vigour in the vicinity of the Polar Front Zone (PFZ). In the SIZ, the upper mixed layer was shallow, biomass was higher and the > 10 mu m fraction was predominant. In this zone the > 10 mu m, 2-10 mu m and < 2 mu m size fractions represented on the average 46%, 25.1% and 28.9% of the total integrated Chi. a stock in the upper 100 m, respectively. The phytoplankton assemblage was diverse, mainly composed of large diatoms and dinoflagellate cells which contributed 42.7% and 33.1% of the autotrophic carbon biomass, respectively. Moving northwards, in parallel with the decrease in biomass, the biomass of autotrophic pico- and nanoflagellates (mainly Cryptophytes) increased steadily. In the POOZ, the picoplanktonic size fraction contributed 47.4% of the total integrated Chi. a stock. A phytoplankton community structure with low biomass and picoplankton-dominated assemblage in the POOZ contrasted with the relatively rich, diverse and diatom-dominated assemblage in the SIZ. These differences reflect the spatial and temporal variations prevailing in the Southern Ocean pelagic ecosystem.
No preview · Article · Nov 1998 · Journal of Marine Systems