Catherine Riaux-Gobin

Nutrients, algal biomass and communities in land-fast ice and seawater off Adélie Land (Antarctica)

Land-fast ice in the vicinity of Adélie Land was sampled during spring 1995. The ice was annual, thin, with no consistent snow cover, and exposed to oceanic conditions. Temporal and spatial variations of the vertical pigment distribution were studied in relation to environmental factors, during the break up of the ice. Different levels were sampled in the congelation ice and the platelet ice-like layer (PLI). Under-ice water and open water masses were also sampled. The algal biomass was greater in the PLI (24 ±14 μg chl a l−1 offshore and up to 9 mg chl a l−1 near-shore), than in the under-ice water, and fell to 0.9 ± 0.64 μg chl al−1 in open water masses. Homogenous low pigment concentrations were detected in the upper levels of congelation ice. A gradient was identified along a 7 km seaward transect, sampled in November, with the lowest biomass offshore. The integrated pigment concentrations in fast ice reached very high levels 500 mg chl a m−2 near the coast and 0.8 mg m−2 offshore), with apparently no relationship with either the ice thickness or snow cover. In the congelation ice nutrient concentrations were low and their distribution homogenous, whereas in the PLI high concentrations of nitrate (up to 100–300 μM NO3) and silicic acid [30–100 μM Si(OH)4] were detected, often related to high pigment concentrations and proximity to islands. The sea ice algae communities were diverse, but mostly composed of chain-forming and tube-dwelling pennate diatoms (Amphiprora, Berkeleya, Nitzschia and Navicula). Cell densities in PLI reached up to 1010 cells l−1. At very low biomass and cell densities 2 104 cells l−1) the phytoplankton also had a low diversity; some species were similar to those of the PLI, such as Navicula glaciei, but other were typically planktonic (Chaetoceros). At sea ice break-up it is estimated that a significant proportion of particulate matter (up to 0.5 g chl a m−2 near-shore) was transferred to the underlying water masses (on an average 15 t POC km−1 shoreline).

Microphyte pigments and resting spores at the water-sediment interface in the Subantarctic deep sea (Indian sector of the Southern Ocean)

During the ANTARES I cruise (April–May 1993) in the Indian Sector of the Southern Ocean, deep-sea sediments were collected using a multiple gravity corer in the Polar Frontal Zone (PFZ) and “frontal zone” (between Crozet and Kerguelen Islands) and in the Permanently Open Ocean Zone (POOZ, south-west of Kerguelen and Heard Islands). Microphyte biomass indicators (Chl a, b, c and related phaeopigments measured by spectrofluorometry, tests on natural fluorescence, and detection of encysted diatoms observed by scanning electron microscopy SEM) were carried out on both sediment and overlying “fluff”, whenever present. Evaluation of the phytoplankton biomass in the overlying surface water masses also was performed at each sampling station. The phytoplankton biomass was, on the whole, very low and decreased from north to south (0.25− 0.10 μg 1−1 Chl a, with a maximum on the “frontal zone”), whereas the sediment-water interface of the southern sampling stations (POOZ) was pigment-enriched. A consistent bioclastic “fluffy” layer was only present in the POOZ stations (up to 13 μg 1−1 Chl a, 157 μg 1−1 Phaeo a). Biodeposition of this type was very thin or absent north of the PF (Polar Front), illustrating stronger advection in the northern part of the study area. Hydrodynamic studies of the Crozet Basin and the presence of benthic pennate coastal diatoms at the sediment interface of these northern stations further suggest increased advective activity in the north. Surficial sediments (first 5 mm) also were enriched with pigments in the POOZ (up to 0.15 μg g−1 DW Chl a, 3.5 μg g−1 DW Phaeo a), whereas in the PFZ the Chl a concentrations were insignificant. Pigment gradients in the sediments, particularly well marked in the POOZ, indicate the absence of a secondary intense reworking such as bioturbation or resuspension. Nevertheless, also in the POOZ, an unusual and well preserved megafaunal faecal cast was sampled and analysed. Sediments at southern stations (50–55°S, 56–75°E; 3600–4700 m depth) were the most enriched with encysted microphytic cells (mainly diatoms belonging to the genus Chaetoceros). This was related primarily to the presence of a “fluffy” layer, composed mainly of well preserved detritic frustules of various diatom genera, including robust Nitzschia and Thalassiosira as well as chains of delicate Chaetoceros, Corethron and Rhizosolenia. Well preserved coccoliths and silicoflagellates also were noticeable in the fluffs. Revival tests (cultures on F2 medium, initiated with samples from the water-sediment interface) were positive for the southern stations, leading to low diversity diatom assemblages dominated by the genera Chaetoceros and Nitzschia.

Lipids and pigments in deep-sea surface sediments and interfacial particles from the Western Crozet Basin

Abstract Deep-sea sediment samples were collected in the Western Crozet Basin (Indian sector of the Southern Ocean) through Permanently Open Ocean Zone (POOZ), Polar Frontal Zone (PFZ) and Sub-Antarctic Zone (SAZ). Lipid class and fatty acid compositions were investigated to determine the sources and fate of organic matter in the first centimeter of sediment and, above this layer, in the fluff (when present) and particles in the overlying water. The total lipid content varied from 74 to 1033 μg l −1 in the overlying particles and fluffs, and from 24 to 97 μg g −1 dry mass (DM) in surficial sediments. Lipid composition was always dominated by phospholipids in the first centimeter of sediment and often in the overlying particles. The amount of phospholipids (labile compounds representative of fresh material) was compared to the amount of chlorophyll a (Chl a ), another compound that is susceptible to rapid degradation. A strong N–S gradient was observed in the distribution of these two compounds, which was attributed to the contrasting hydrodynamic of the study area. The high sedimentation rate in POOZ resulted in better preservation of Chl a in this zone than in other zones of the Crozet Basin (PFZ and SAZ). Phospholipid fatty acids suggested the presence of viable as well as morphologically intact organisms, and these organisms consisted essentially of bacteria with some diatom cysts in the fluff of POOZ. These spores were able to grow in the culture, indicating that they were still viable. Despite the strong hydrodynamic variability, phospholipid fatty acids analysed from the deep-sea surficial sediments were never representative of plankton. This pointed to the extremely labile nature of the phospholipids originally present in planktonic material compared with Chl a , which was always found in overlying particles and surficial sediments.

Land-fast ice microalgal and phytoplanktonic communities (Adélie Land, Antarctica) in relation to environmental factors during ice break-up

Annual land-fast ice, particularly an unconsolidated layer or “platelet ice-like” layer (PLI), was sampled in spring 1995 to study the spatial and short-term variations of ice-associated diatoms. Under-ice water, a lead and small polynyas were also sampled. Along a 7 km seaward transect a geographical gradient was evident, with some rare diatom species present only in the offshore PLI, whereas others (mainly pennate diatoms) were ubiquitous. The dense microphytic PLI community as well as the phytoplankton was diatom-dominated, but, within these two communities, marked differences appeared. First, the sea-ice communities (PLI and solid bottom ice) were moderately diverse (36 species), mostly composed of pennate diatoms, of which many were chain forming or tube-dwelling. Dominant taxa were Navicula glaciei, Berkeleya adeliensis, Nitzschia stellata, Amphiprora kufferathii and Nitzschia lecointei. Some differences in the distribution of the most dominant species appeared within the bottom ice and the PLI, attesting to differences in the origin or/and growing capability of these diatoms in these two ice compartments. Under-ice water species composition was mixed with sea-ice communities only on the most coastal sites and during ice melt. Maximum cell numbers were mostly noticed in the PLI, reaching up to 1010 cells l−1 and very high Chl a concentrations (exceptionally up to 9.8 mg Chl a l−1 or 1.9 g Chl a m−2, from a 10 to 20 cm thick PLI layer, close to the continent). Secondly, the phytoplankton in the lead and small polynyas had a low diversity, very low standing stocks (on an average 0.69 μg Chl a l−1) and cell densities (2 × 104 cells l−1). Some species from the polynyas were similar to those of the PLI, such as Navicula glaciei, but others were typically planktonic, such as Chaetoceros cf. neglectus. The presence of encysted cells (Chaetoceros and Chrysophytes) was also noticeable in the polynya water. In early spring no seeding process was obvious from the PLI to polynya water. A comparison with similar fast-ice diatom communities in other parts of coastal Antarctica, is presented.

Algal pigment diversity in coastal sediments from Kerguelen (sub-Antarctic Islands) reflecting local dominance of green algae, euglenoids and diatoms

SummaryHigh Performance Liquid Chromatography analysis of algal pigments from inter- and subtidal (deep and shallow) sediments from the Kerguelen Islands showed clear differences in the pigment composition at the different stations. High concentrations of chlorophyll c and fucoxanthin were present at all locations, indicating significant diatom densities, chlorophyll b was detected at all sites. At one station the other green algal pigments were also present; here green algae contributed more to chlorophyll a concentrations than diatoms, as estimated by using pigment ratios and microscopic observations. At another location chlorophyll b was associated with a high concentration of diadinoxanthin, indicating an abundance of euglenoids. This indicates that chemotaxonomy can be powerful tool in microphytobenthos studies since enumeration of living cells are difficult as many algae are attached to sediment particles (epipsammic algae). Ways of diagenesis, carotenoid degradation and the role of grazing are briefly mentioned. Phaeophorbide a-like pigments were the most significant chlorophyll a degradation products, with concentrations up to 110 μg · g−1 dry weight sediment, i.e. 10 times the chlorophyll a concentration. Some taxonomic estimations, based on pigments ratios, and their limits, are discussed.

Spring phytoplankton onset after the ice break-up and sea-ice signature (Adélie Land, East Antarctica)

The phytoplankton onset following the spring ice break-up in Adélie Land, East Antarctica, was studied along a short transect, from 400 m off the continent to 5 km offshore, during the austral summer of 2002. Eight days after the ice break-up, some large colonial and solitary diatom cells, known to be associated with land-fast ice and present in downward fluxes, were unable to adapt in ice-free waters, while some other solitary and short-colony forming taxa (e.g., Fragilariopsis curta, F. cylindrus) did develop. Pelagic species were becoming more abundant offshore, replacing the typical sympagic (ice-associated) taxa. Archaeomonad cysts, usually associated with sea ice, were recorded in the surface waters nearshore. Rough weather restricted the data set, but we were able to confirm that some microalgae may be reliable sea-ice indicators and that seeding by sea ice only concerns a few taxa in this coastal area of East Antarctica.

Microphytobenthos biomass at Kerguelen's Land (Subantarctic Indian Ocean): repartition and variability during austral summers

Abstract Microphytobenthos biomass has been measured at several coastal sites on the SE of the main island of the Kerguelen Archipelago (Indian Ocean), during several austral summers (1985–1992), using a conventional fluorometric method. Heterogeneity tests, conducted on two different intertidal sites (Port-Aux-Francais, PAF; and Port-Raymond, PRA), showed low standard deviations, whereas the mean concentrations were highly different. Pigment concentrations showed a high variability related to the characteristics of the sediments: from low biomass in coarse intertidal sand, submitted to intense scouring (0.32±0.31 μg Chl a g −1 dw, 0.29±0.14 μg Phaeo g −1 dw) to high biomass in intertidal muddy sand in sheltered areas, particularly along estuaries (54 μg Chl a , 15 μg Phaeo g −1 dw at Korrigan). The subtidal muddy sediments under a Macrocystis pyrifera (Linne) and Durvillaea antarctica (Chamisso in Choris) belt exhibited high concentrations in phaeopigment (Phaeo) (up to 136±83 μg g −1 dw; PRA), while the concentration of chlorophyll a (Chl a ) was relatively low. The dense macroalgal canopy supports an important epiphytic diatom biomass (mainly the genera Cocconeis Ehrenberg and Grammatophora Ehrenberg), which is sedimenting after degradation and is in part responsible of the high levels of Phaeo in all sediments. Macroalgal debris were observed, but diatom frustules were dominant in most surficial subtidal sediments. A circatidal mud, in the Morbihan Bay, made of a sponge spicule mat (50 m deep; 4.96 μg Chl a g −1 dw), showed a very low Chl a /Phaeo ratio (0.1), while it reached up to 6 in intertidal sand. Surprisingly, a penguin rookery beach, at the east side of Courbet Peninsula, was characterized by a very low biomass (0.07±0.04 μg Chl a g −1 dw), while it was nutrient enriched, particularly with nitrates. In comparison with the data at the similar latitude, but in temperate regions from the Northern Hemisphere, the microphytobenthos biomass, recorded at Kerguelens Land, exhibited relatively high pigment concentrations, particularly the Phaeo, and supported a dense and diversified subtidal macrofauna composed of polychaetes (particularly Thelepus extensus Hutchings and Glasby), sea urchins, mytillids and gammarids. The exuberant macroalgal canopy, coastal indentations and low tidal amplitude must be in part responsible of these large benthic primary and secondary biomasses.

COCCONEIS POTTERCOVEI SP. NOV. AND COCCONEIS PINNATA VAR. MATSII VAR. NOV., TWO NEW MARINE DIATOM TAXA FROM KING GEORGE ISLAND, ANTARCTICA

Details of two new small marine taxa of the genus Cocconeis Ehrenberg are described; C. pottercovei sp. nov. and C. pinnata var. matsii var. nov., which were observed as epipelic and epiphytic in Potter Cove, King George Island, Antarctica. Descriptions are based on both light and scanning electron microscope observations. C. pottercovei belongs to the group of Cocconeis taxa that bear very short and marginal striae on the rapheless valve while C. pinnata var. matsii belongs to the C. costata Gregory complex. They are mainly differentiated from the other similar taxa by stria number, stria arrangement and structure on both raphe and rapheless valves. A comparison with closely resembling taxa is given.

Olifantiella mascarenica gen. & sp. nov., a new genus of pennate diatom from Réunion Island, exhibiting a remarkable internal process

A new diatom genus, Olifantiella gen. nov., has been discovered from the coral reefs of Réunion Island (Western Indian Ocean). This monospecific genus belonging to the Biraphidineae, contains a small, marine taxon, Olifantiella mascarenica sp. nov., which shows close structural similarity with the recently described genus Labellicula Van de Vijver & Lange‐Bertalot, but with significant differences: (i) the presence of a remarkable internal tubular process – the buciniportula; (ii) the easily identifiable external funnel‐shaped opening of this process; (iii) the design of the external raphe endings; (iv) the general shape of the frustules; and (v) a lateral void and elevated crest. The buciniportula, associated with the other characters, is apparently new, and allows the establishment of a new genus. The morphology of this genus is compared with that of related genera, such as Luticola, Diadesmis, Labellicula, Brachysira, Neidium and Nupela, with some comments on the contribution of scanning electron microscopy towards distinguishing new genera, and the possible superabundance of these new entities, particularly of small diatoms.

Two new marine species of the diatom genus Cocconeis Ehrenberg : C. melchioroides sp. nov. and C. dallmannii sp. nov., from King George Island, Antarctica

Two new marine diatom species, Cocconeis melchioroides and C. dallmannii are described from Potter Cove, King George Island, Antarctica. Detailed descriptions are given based on LM and SEM images for both species and compared with the only known morphologically related taxon, Cocconeis melchiori Frenguelli and Orlando. Both C. melchioroides and C. dallmannii belong to the Cocconeis group, in which the sternum valve has very short and marginal striae. They differ from each other in the number of striae, number of areolae, central raphe endings, crista marginalis and valvocopula. A comparison with the Cocconeis costata group is provided. Both species are common inhabitants of the marine surface sediment in the Potter Cove area.