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Cefarelli, AO, Mascioni M, Simonato J, Almandoz GO, Vernet M.  2018.  Haslea crucigeroides var. australis, var. nov. (Bacillariophyta), a new marine diatom from southern South America and Antarctica. Phytotaxa. 357:179-188.   10.11646/phytotaxa.357.3.2   AbstractWebsite

A new variety of the marine diatom Haslea crucigeroides, H. crucigeroides var. australis, var. nov., is described here based on phytoplankton samples collected in sub-Antarctic coastal waters of the southern South America and Antarctica. The new taxon was characterized by using light and electron microscopy, and the holotype of H. crucigeroides, the only existing material type, was analyzed with light microscopy. Haslea crucigeroides var. australis differs from the nominal variety by having acute valve apices instead of acute rostrate; furthermore, it has a different geographical distribution. The species is being recorded in the Southern Hemisphere for the first time. Cell abundance and a brief discussion on the species habitat are also presented.

Ziegler, AF, Smith CR, Edwards KF, Vernet M.  2017.  Glacial dropstones: islands enhancing seafloor species richness of benthic megafauna in West Antarctic Peninsula fjords. Marine Ecology Progress Series. 583:1-14.   10.3354/meps12363   AbstractWebsite

The West Antarctic Peninsula (WAP) margin is dominated by glaciomarine fjords and has experienced rapid climate warming in recent de cades. Glacial calving along the peninsula delivers icerafted debris (e.g. dropstones) to heavily sedimented fjord basins and the open continental shelf. Dropstones provide hard substrate, increase habitat heterogeneity, and may function as island habitats surrounded by mud. We used seafloor photographic transects to evaluate the distribution and community structure of Antarctic hard-substrate megafauna and the role of dropstones as island habitats in 3 WAP fjords and at 3 nearby shelf stations. Several lines of evidence indicate that dropstones function as island habitats; their communities adhere to principles of island biogeography theory with (1) a positive correlation between dropstone size and species richness, (2) an increase in the proportion of colonized dropstones with increasing dropstone size, and (3) a species-area scaling exponent consistent with island habitats measured globally. Previous work on the soft-sediment megafauna of this region found strong differences in community composition between fjord and shelf sites, whereas we found that dropstone communities differed within sites at small scales (1 km and smaller). We identified 73 megafaunal morphotypes associated with dropstones, 29 of which were not previously documented in the soft-sediment mega fauna. While dropstones constituted < 1% of the total seafloor area surveyed, they contributed 20% of the overall species richness of WAP megabenthos at depths of 437-724 m. WAP dropstone communities adhere to key principles of island biogeography theory, contribute environmental heterogeneity, and increase biodiversity in the WAP region.

Svensen, C, Vernet M.  2016.  Production of dissolved organic carbon by Oithona nana (Copepoda: Cyclopoida) grazing on two species of dinoflagellates. Marine Biology. 163   10.1007/s00227-016-3005-9   AbstractWebsite

Production of dissolved organic carbon (DOC) by sloppy feeding copepods may represent an important source of DOC in marine food webs. By using the C-14-labeling technique, we quantify for the first time the production of DOC by the small cyclopoid copepod Oithona nana on two species of dinoflagellates, Oxyrrhis marina and Karlodinium sp. We found significant production of DOC when O. nana grazed on O. marina, corresponding to 6-15 % of the carbon ingested. When grazing the smaller Karlodinium sp., no DOC was produced. In additional experiments, we compared O. nana feeding rates on the dinoflagellate species Prorocentrum micans, Akashiwo sanguinea, Karlodinium sp. and O. marina. Clearance rates varied with prey size, with highest and lowest clearance rates on O. marina and Karlodinium sp., respectively. Our study indicates that even though O. nana feed efficiently on dinoflagellates, some of the carbon cleared can be lost as DOC. However, the DOC production by O. nana was lower than rates reported for calanoid copepods. We hypothesize that this is a result of the ambush feeding behavior of O. nana, which is considered a more specialized feeding mode than, for instance, suspension feeding. Due to high abundances and global distribution, we suggest that Oithona can represent an important source of DOC in marine ecosystems. This would particularly be the case during autumn and winter, where they may contribute to maintaining the microbial loop activities during periods of low primary production.

Smith, SR, Glé C, Abbriano RM, Traller JC, Davis A, Trentacoste E, Vernet M, Allen AE, Hildebrand M.  2016.  Transcript level coordination of carbon pathways during silicon starvation-induced lipid accumulation in the diatom Thalassiosira pseudonana. New Phytologist.   10.1111/nph.13843   Abstract

* Diatoms are one of the most productive and successful photosynthetic taxa on Earth and possess attributes such as rapid growth rates and production of lipids, making them candidate sources of renewable fuels. Despite their significance, few details of the mechanisms used to regulate growth and carbon metabolism are currently known, hindering metabolic engineering approaches to enhance productivity. * To characterize the transcript level component of metabolic regulation, genome-wide changes in transcript abundance were documented in the model diatom Thalassiosira pseudonana on a time-course of silicon starvation. Growth, cell cycle progression, chloroplast replication, fatty acid composition, pigmentation, and photosynthetic parameters were characterized alongside lipid accumulation. * Extensive coordination of large suites of genes was observed, highlighting the existence of clusters of coregulated genes as a key feature of global gene regulation in T. pseudonana. The identity of key enzymes for carbon metabolic pathway inputs (photosynthesis) and outputs (growth and storage) reveals these clusters are organized to synchronize these processes. * Coordinated transcript level responses to silicon starvation are probably driven by signals linked to cell cycle progression and shifts in photophysiology. A mechanistic understanding of how this is accomplished will aid efforts to engineer metabolism for development of algal-derived biofuels.

Cape, MR, Vernet M, Skvarca P, Marinsek S, Scambos T, Domack E.  2015.  Foehn winds link climate-driven warming to ice shelf evolution in Antarctica. Journal of Geophysical Research-Atmospheres. 120(21):11037-11057.   10.1002/2015JD023465   Abstract

Rapid warming of the Antarctic Peninsula over the past several decades has led to extensive surface melting on its eastern side, and the disintegration of the Prince Gustav, Larsen A, and Larsen B ice shelves. The warming trend has been attributed to strengthening of circumpolar westerlies resulting from a positive trend in the Southern Annular Mode (SAM), which is thought to promote more frequent warm, dry, downsloping foehn winds along the lee, or eastern side, of the peninsula. We examined variability in foehn frequency and its relationship to temperature and patterns of synoptic-scale circulation using a multidecadal meteorological record from the Argentine station Matienzo, located between the Larsen A and B embayments. This record was further augmented with a network of six weather stations installed under the U.S. NSF LARsen Ice Shelf System, Antarctica, project. Significant warming was observed in all seasons at Matienzo, with the largest seasonal increase occurring in austral winter (+3.71 degrees C between 1962-1972 and 1999-2010). Frequency and duration of foehn events were found to strongly influence regional temperature variability over hourly to seasonal time scales. Surface temperature and foehn winds were also sensitive to climate variability, with both variables exhibiting strong, positive correlations with the SAM index. Concomitant positive trends in foehn frequency, temperature, and SAM are present during austral summer, with sustained foehn events consistently associated with surface melting across the ice sheet and ice shelves. These observations support the notion that increased foehn frequency played a critical role in precipitating the collapse of the Larsen B ice shelf.

Helly, JJ, Vernet M, Murray AE, Stephenson GR.  2015.  Characteristics of the meltwater field from a large Antarctic iceberg using delta O-18. Journal of Geophysical Research-Oceans. 120:2259-2269.   10.1002/2015jc010772   AbstractWebsite

Large tabular icebergs represent a disruptive influence on a stable water column when drifting in the open ocean. This is a study of one iceberg, C18A, encountered in the Powell Basin in the Weddell Sea in March 2009, formed from iceberg C18 ( 76x7km) originating from the Ross Ice Shelf in May 2002. C18A was lunate in shape with longest dimensions of 31kmx7kmx184m. The meltwater field from C18A was characterized using 18O from water samples collected near C18A (Near-field, 0.4-2 km) and contrasted with a Far-field comprised of samples from an Away site (19 km from C18A), a Control site (70 km away), and a region populated with small icebergs (Iceberg Alley, 175 km away). The in-sample fractions of meteoric water were calculated relative 18O in iceberg ice and Weddell Deep Water and converted to meteoric water height (m) and a percentage within 100 m depth bins. The Near-field and Far-field difference from surface to 200 m was 0.510.28%. The concentration of meteoric water dropped to approximately half that value below 200 m, approximate keel depth of the iceberg, although detectable to 600 m. From surface to 600 m, the overall difference was statistically significant ( P<0.0001). From this, we estimate the Near-field volume astern of the iceberg ( 0.16km3d-1) as a continuous source of meteoric water.

Diaz, SB, Paladini AA, Braile HG, Dieguez MC, Deferrari GA, Vernet M, Vrsalovic J.  2014.  Global and direct UV irradiance variation in the Nahuel Huapi National Park (Patagonia, Argentina) after the eruption of Puyehue-Cordon Caulle (Chile). Journal of Atmospheric and Solar-Terrestrial Physics. 112:47-56.   10.1016/j.jastp.2014.02.006   AbstractWebsite

On June 4th, 2011, the Puyehue-Cordon Caulle volcanic complex (40 degrees 35'25"S 72 degrees 07'02"W, Chile) started eruption, sending ash 45,000 feet into the atmosphere. After the initial period, the eruption continued for several months, with less intensity. Changes in global irradiance in the UV-B and UV-A, and direct irradiance and AOD in the UV-A, as consequence of the eruption, were studied. Global irradiance has been permanently measured at the Laboratory of Photobiology (LPh) (41.13S, 71.42W, 804 msl) since 1998. In addition, in the frame of a project to study altitude effect on direct and global irradiance, field campaigns were performed during September 17th to 23rd, 2010 and September 14th to 18th, 2011, in the region of the Nahuel Huapi National Park, near 100 km from the eruption. In those periods, simultaneous measurements of direct and global irradiance and aerosol optical depth (ACID) were carried out at three sites: Laboratory of Photobiology (LPh), Mt Otto (41.15S, 71.38W, 1386 msl) and Mt Catedral (41.17S, 71.48W, 1930 msl). The analysis of aerosols in 2011, three to four month after the eruption started, showed the presence of larger particles and more variability than in 2010, at all sites. Global irradiance, at LPh, also exhibited larger variability, compared to 1999, when no eruption or any other event that could have produced major changes in aerosols occurred. The mean decrease, as consequence of the volcano activity, at LPh, was around 20%, at 305 nm and closed to 10%, at 320 nm. At 380 nm, the decrease was very small and not statistically significant, although in particular days, with large aerosol load, a significant decrease was observed. Direct irradiance, in the UV-A, showed larger decrease than global irradiance. The effect of the eruption was more pronounced at the low altitude site. (c) 2014 Elsevier Ltd. All rights reserved.

Cape, MR, Vernet M, Kahru M, Spreen G.  2014.  Polynya dynamics drive primary production in the Larsen A and B embayments following ice shelf collapse. Journal of Geophysical Research-Oceans. 119:572-594.   10.1002/2013jc009441   AbstractWebsite

The climate-driven collapses of the Larsen A and B ice shelves have opened up new regions of the coastal Antarctic to the influence of sea ice resulting in increases in seasonal primary production. In this study, passive microwave remote sensing of sea ice concentration and satellite imagery of ocean color are employed to quantify the magnitude of and variability in open water area and net primary productivity (NPP) in the Larsen embayments between 1997 and 2011. Numerical model output provides context to analyze atmospheric forcing on the coastal ocean. Following ice shelf disintegration the embayments function as coastal, sensible heat polynyas. The Larsen A and B are as productive as other Antarctic shelf regions, with seasonally averaged daily NPP rates reaching 1232 and 1127 mg C m(-2) d(-1) and annual rates reaching 200 and 184 g C m(-2) yr(-1), respectively. A persistent cross-shelf gradient in NPP is present with higher productivity rates offshore, contrasting with patterns observed along the West Antarctic Peninsula. Embayment productivity is intimately tied to sea ice dynamics, with large interannual variability in NPP rates driven by open water area and the timing of embayment opening. Opening of the embayment is linked to periods of positive Southern Annular Mode and stronger westerlies, which lead to the vertical deflection of warm, maritime air over the peninsula and down the leeward side causing increases in surface air temperature and wind velocity. High productivity in these new polynyas is likely to have ramifications for organic matter export and marine ecosystem evolution. Key Points Primary production and sea ice dynamics after ice shelf disintegration Larsen embayments function as productive coastal sensible heat polynyas High sea ice interannual variability affects total production

Garcia-Munoz, C, Lubian LM, Garcia CM, Marrero-Diaz A, Sangra P, Vernet M.  2013.  A mesoscale study of phytoplankton assemblages around the South Shetland Islands (Antarctica). Polar Biology. 36:1107-1123.   10.1007/s00300-013-1333-5   AbstractWebsite

Phytoplankton assemblages around the South Shetland Islands (SSI) were closely related to mesoscale physical features, based on high spatial resolution sampling performed during the summer of 2010. Sampling was done in 8 transects with stations 9 km apart. Phytoplankton groups were described using flow cytometry, FlowCAM and HPLC/CHEMTAX pigment analysis. Nanophytoplankton (2-20 mu m) was predominant throughout the study area, which was dominated by small diatoms. They were distributed along the stratified waters of the SSI shelf and in the centre of the Bransfield Strait where an anticyclonic eddy was detected, established between two frontal structures [Bransfield Front and Peninsula Front (PF)]. The highest concentrations correlated with mid-to-high temperatures (1.07 A degrees C) and mid-salinities (34.03) corresponding with Transitional Bellinghausen Water stations. Haptophytes distribution co-varied with small diatoms but also appeared in those vertical mixed stations with Transitional Zonal Water with Weddell Sea influence. A shift from smaller to larger diatoms was detected at the ice edge in the Antarctic Sound. Cryptophytes were restricted to stratified stations of the SSI shelf and those associated with the PF, while small prasinophytes were the only group occupying deeper and colder waters of the Drake Passage, beneath the Antarctic Surface Water, north of a narrow frontal region described here for the first time (Shetland Front). Phytoplankton assemblages around the SSI were strongly connected with the Bransfield Current System, supporting a clockwise circulation around the archipelago. The Bransfield Current System components are permanent structures during the austral summer suggesting that the distribution of phytoplankton, which responds to these structures, must also be a quasi-permanent feature.

Peloquin, J, Swan C, Gruber N, Vogt M, Claustre H, Ras J, Uitz J, Barlow R, Behrenfeld M, Bidigare R, Dierssen H, Ditullio G, Fernandez E, Gallienne C, Gibb S, Goericke R, Harding L, Head E, Holligan P, Hooker S, Karl D, Landry M, Letelier R, Llewellyn CA, Lomas M, Lucas M, Mannino A, Marty J-C, Mitchell BG, Muller-Karger F, Nelson N, O'Brien C, Prezelin B, Repeta D, Jr. Smith WO, Smythe-Wright D, Stumpf R, Subramaniam A, Suzuki K, Trees C, Vernet M, Wasmund N, Wright S.  2013.  The MAREDAT global database of high performance liquid chromatography marine pigment measurements. 5(1):109-123.: Copernicus Publications AbstractWebsite
Ferrario, ME, Almandoz GO, Cefarelli AO, Fabro E, Vernet M.  2013.  Stephanopyxis species (Bacillariophyceae) from shelf and slope waters of the Argentinean Sea: Ultrastructure and distribution. Nova Hedwigia. 96:249-263.   10.1127/0029-5035/2012/0077   AbstractWebsite

Stephanopyxis is a cosmopolitan planktonic marine diatom genus. It comprises a large number of fossil species but only four living species: S. nipponica, S. orbicularis, S. palmeriana and S. turns. In this study, the morphology and occurrence of Stephanopyxis species were studied by means of light and scanning electron microscopy from the inner shelf to slope waters of the Argentinean Sea. Two species, S. nipponica and S. turns, were found. In both species the ultrastructure of the valve showed two different morphological forms. One form had true poroid aereolae, with an external foramen and an internal cribral velum while the second one had a network of hexagonal compartments with an external opening but lacking an internal cribral velum. The structure of the linking rimoportulae and the presence of acceptant process in the vegetative cells of S. nipponica distinguished this species from the other living Stephanopyxis species. We consider that the orientation of the rimoportulae located on the mantle margin as well as the presence of scattered rimoportulae on the valve surface in the resting spores are not useful taxonomic characters to differentiate between the vegetative and resistance cells of S. nipponica. Stephanopyxis turns' main morphometric and ultrastructural features coincided with the diagnosis of this species. However, some lightly silicified specimens presented a different type of areolae in the valvar mantle, not previously described. We give also new information on cingulum structure for S. turns, which is high with a narrow valvocopula and numerous segmented girdle bands, similar to other species of the genus. Finally, we provide data on the S. nipponica and S. turris occurrence, cell abundance and environmental data in the Argentinean Sea, including remarks on their distribution in southern South America.

Vernet, M, Smith KL, Cefarelli AO, Helly JJ, Kaufmann RS, Lin H, Long DG, Murray AE, Robison BH, Ruhl HA, Shaw TJ, Sherman AD, Sprintall J, Stephenson GR, Stuart KM, Twining BS.  2012.  Islands of ice: influence of free-drifting Antarctic icebergs on pelagic marine ecosystems. Oceanography. 25:38-39.   10.5670/oceanog.2012.72   AbstractWebsite

Regional warming around West Antarctica, including the Antarctic Peninsula, is related to the retreat of glaciers that has resulted in significant ice mass loss in recent decades (De Angelis and Skvarca, 2003). Large icebergs (> 18.5 km long) originating from ice shelves in the Ross and Weddell Seas (Scambos et al., 2000) are attributed primarily to major loss events in these regions. Once free, icebergs become entrained in the counterclockwise Antarctic Coastal Current (Figure 1), eventually entering a strong northward flow in the Northwest Weddell Sea. We examined free-drifting icebergs in the Atlantic sector of the Southern Ocean in December 2005, aboard ARSV Laurence M. Gould, and in June 2008 and March/April 2009, aboard RVIB Nathaniel B. Palmer. Prior to these studies, little information was available about the effects of icebergs on the pelagic realm. On these cruises, we investigated the "iceberg ecosystem" (Smith et al., 2007; Smith, 2011) to assess the degree to which icebergs are (1) hotspots of biological activity across multiple trophic levels, and (2) focal points for enhanced export of organic carbon to the deep sea. An important focus of this work was to examine the fundamental mechanisms by which icebergs affect the pelagic ecosystem, including physical disruption and effects on the availability of critical nutrients (e.g., iron, nitrate).

Ducklow, HW, Schofield O, Vernet M, Stammerjohn S, Erickson M.  2012.  Multiscale control of bacterial production by phytoplankton dynamics and sea ice along the western Antarctic Peninsula: A regional and decadal investigation. Journal of Marine Systems. 98-99:26-39.   10.1016/j.jmarsys.2012.03.003   AbstractWebsite

We present results on phytoplankton and bacterial production and related hydrographic properties collected on nine annual summer cruises along the western Antarctic Peninsula. This region is strongly influenced by inter-annual variations in the duration and extent of sea ice cover, necessitating a decade-scale study. Our study area transitions from a nearshore region influenced by summer runoff from glaciers to an offshore, slope region dominated by the Antarctic Circumpolar Current. The summer bacterial assemblage is the product of seasonal warming and freshening following spring sea ice retreat and the plankton succession occurring in that evolving water mass. Bacterial production rates averaged 20 mg C m(-2) d(-1) and were a low (5%) fraction of the primary production (PP). There was significant variation in BP between regions and years, reflecting the variability in sea ice, chlorophyll and PP. Leucine incorporation was significantly correlated (r(2) ranging 0.2-0.7, p<0.001) with both chlorophyll and PP across depths, regions and years indicating strong phytoplankton-bacteria coupling. Relationships with temperature were variable, including positive, negative and insignificant relationships (r(2)<0.2 for regressions with p<0.05). Bacterial production is regulated indirectly by variations in sea ice cover within regions and over years, setting the levels of phytoplankton biomass accumulation and PP rates; these in turn fuel BP, to which PP is coupled via direct release from phytoplankton or other less direct pathways. (C) 2012 Elsevier B.V. All rights reserved.

Herrmann, M, Najjar RG, Neeley AR, Vila-Costa M, Dacey JWH, DiTullio GR, Kieber DJ, Kiene RP, Matrai PA, Simo R, Vernet M.  2012.  Diagnostic modeling of dimethylsulfide production in coastal water west of the Antarctic Peninsula. Continental Shelf Research. 32:96-109.   10.1016/j.csr.2011.10.017   AbstractWebsite

The rate of gross biological dimethylsulfide (DMS) production at two coastal sites west of the Antarctic Peninsula, off Anvers Island, near Palmer Station, was estimated using a diagnostic approach that combined field measurements from 1 January 2006 through 1 March 2006 and a one-dimensional physical model of ocean mixing. The average DMS production rate in the upper water column (0-60 m) was estimated to be 3.1 +/- 0.6 nM d(-1) at station B (closer to shore) and 2.7 +/- 0.6 nM d(-1) at station E (further from shore). The estimated DMS replacement time was on the order of 1 d at both stations. DMS production was greater in the mixed layer than it was below the mixed layer. The average DMS production normalized to chlorophyll was 0.5 +/- 0.1 (nM d(-1))/(mg m(-3)) at station B and 0.7 +/- 0.2 (nM d(-1))/(mg m(-3)) at station E. When the diagnosed production rates were normalized to the observed concentrations of total dimethylsulfoniopropionate (DMSPt, the biogenic precursor of DMS), we found a remarkable similarity between our estimates at stations B and E (0.06 +/- 0.02 and 0.04 +/- 0.01 (nM DMS d(-1))/(nM DMSP), respectively) and the results obtained in a previous study from a contrasting biogeochemical environment in the North Atlantic subtropical gyre (0.047 +/- 0.006 and 0.087 +/- 0.014 (nM DMS d(-1))/(nM DMSP) in a cyclonic and anticyclonic eddy, respectively). We propose that gross biological DMS production normalized to DMSPt might be relatively independent of the biogeochemical environment, and place our average estimate at 0.06 +/- 0.01 (nM DMS d(-1))/(nM DMSPt). The significance of this finding is that it can provide a means to use DMSPt measurements to extrapolate gross biological DMS production, which is extremely difficult to measure experimentally under realistic in situ conditions. (C) 2011 Elsevier Ltd. All rights reserved.

Ferrario, ME, Cefarelli AO, Robison B, Vernet M.  2012.  Thalassioneis signyensis (Bacillariophyceae) from northwest Weddell Sea icebergs, an emendation of the generic description. Journal of Phycology. 48:222-230.   10.1111/j.1529-8817.2011.01097.x   AbstractWebsite

We offer an emended description of the genus Thalassioneis based on new observations of the type species, T. signyensis Round, from material sampled in the northwest Weddell Sea. Specimens from algal communities attached to submerged flanks of several icebergs were collected with a remote-operated vehicle (ROV-Phantom DS 2). The analyses were carried out by LM and SEM. Fresh material and frustules without organic matter allowed us to observe details not included in the original description such as type and structure of colonies and chloroplasts. The frustule shows an asymmetry with respect to the location of the apical pore fields, one of them situated on the valvar face and the other one displaced toward the mantle; the former is involved in joining contiguous cells to form long chains. Furthermore, we present details on the ultrastructure of the cingulum that consists of three to four open copulae with one or more rows of poroids. A brief discussion on the habit and ecology of this taxon, which may be endemic to the northwest Weddell Sea, is also presented. A comparison with similar genera, such as Brandinia, Creania, Fossula, Fragilaria, Rimoneis, Synedropsis, and Ulnaria, is included with an evaluation of morphological characteristics useful to differentiate them.

Huang, K, Ducklow H, Vernet M, Cassar N, Bender ML.  2012.  Export production and its regulating factors in the West Antarctica Peninsula region of the Southern Ocean. Global Biogeochemical Cycles. 26   10.1029/2010gb004028   AbstractWebsite

In connection with the Palmer LTER program, mixed layer water samples were collected during the cruise of the L. M. Gould in Jan., 2008 at 49 stations on a 20 x 100 km grid in the West Antarctica Peninsula (WAP) region of the Southern Ocean. In this study, [O-2]/[Ar] ratios and the triple isotope composition of dissolved O-2 were measured, and were used to estimate net community O-2 production (NCP) and gross primary O-2 production (GPP), respectively. These estimates are further converted to carbon export production, primary production and the f-ratio. Our measurements give NCP ranging from -3 to 76 mmol O-2 m(-2) day(-1) (-25 to 650 mg C m(-2) day(-1)), and GPP from 40 to 220 mmol O-2 m(-2) day(-1) (180 to 1010 mg C m(-2) day(-1)). The O-2 NCP/GPP ratios range from -0.04 to 0.43, corresponding to f-ratios of -0.08 to 0.83. NCP and the NCP/GPP ratio are highest in the northern coastal areas, and decrease to lower values toward the southern coastal area and the open ocean. The inshore-offshore gradient appears to be regulated primarily by iron availability, as supported by the positive correlation between NCP and F-v/F-m ratios (r(2) = 0.22, p < 0.05). Mixed layer depth (MLD) is inversely correlated with NCP (r(2) = 0.21, p < 0.002) and NCP/GPP (r(2) = 0.21, p < 0.02), and highest NCP occurred in the fresh water lenses probably formed from melted coastal glaciers. These results suggest that export production and the f-ratio increase where water stratification is intensified by input of fresh meltwater, and that mixed layer stratification is the major factor regulating NCP in the inner-shelf and coastal regions. Along-shelf variability of phytoplankton community composition is highly correlated with NCP, i.e., NCP increases when the diatom-dominated community in the south transitions to the cryptophyte-dominated one in the north. A high correlation is also observed between NCP and the logarithm of the surface chlorophyll concentration (r(2) = 0.72, p < 0.0001), which makes it possible to estimate carbon export as a function of Chl a concentration in this region.

Vernet, M, Kozlowski WA, Yarmey LR, Lowe AT, Ross RM, Quetin LB, Fritsen CH.  2012.  Primary production throughout austral fall, during a time of decreasing daylength in the western Antarctic Peninsula. Marine Ecology-Progress Series. 452:45-61.   10.3354/meps09704   AbstractWebsite

Antarctic phytoplankton is characterized by a pronounced seasonality in abundance, driven mainly by changes in sunlight. We combined measurements and modeling to describe the influence of changing daylength on fall and winter phytoplankton production in coastal waters of the western Antarctic Peninsula (wAP) in 2001 and 2002. The model was parameterized with field observations from the Palmer Long-Term Ecological program in the wAP during summer and early fall and from the Southern Ocean Global Ecosystems Dynamics program fall and winter cruises to Marguerite Bay and shelf waters. Shorter daylength and a deepening of the mixed layer account for most of the decrease in primary production during March, April, and May. At this time, biomass decreases by an order of magnitude and remains low and constant until the end of August. An additional loss rate was added to the primary production model to fit output to observations. This loss rate, estimated at similar to 0.1 to 0.15 d(-1), is due to physical, chemical, and biological processes such as scavenging by sea ice, zooplankton grazing, cell lysis, and cell sedimentation, which are expected to be high at this time of year. Growth and loss rates of phytoplankton populations are similar on 1 March, with growth decreasing rapidly over time while the loss rates remain constant. By the beginning of winter (1 June), growth is low, with minimum rates in July and increasing towards September. During a period of diminishing food supply, preliminary estimates of grazing indicate that fall biomass could support existing macrozooplankton populations, but the timing and concentration of food supply is variable and expected to affect health of zooplankton as they enter the winter.

Lowe, AT, Ross RM, Quetin LB, Vernet M, Fritsen CH.  2012.  Simulating larval Antarctic krill growth and condition factor during fall and winter in response to environmental variability. Marine Ecology-Progress Series. 452:27-43.   10.3354/meps09409   AbstractWebsite

The first winter in the life cycle of Antarctic krill Euphausia superba is a critical period in which larval survival and recruitment to the adult population are highly sensitive to environmental conditions, yet little is known about larval physiological dynamics during this period. An individual-based model was developed to investigate patterns of larval krill growth and condition factor in response to environmental variability during fall and winter, west of the Antarctic Peninsula. Field and experimental observations from Southern Ocean Global Ocean Ecosystems Dynamics cruises in 2001 and 2002 and the Palmer Long-Term Ecological Research program were used to parameterize the model. Growth was modeled by partitioning total body carbon between length and condition factor. Total body carbon was simulated with empirical temperature-dependent rates of ingestion of phytoplankton and respiration, and ingestion of algae grown on a surface to simulate sea ice algae. Light-driven diel vertical migration modulated ingestion of phytoplankton and sea ice algae as a function of latitude, season and sea ice cover. Simulations highlighted 3 environmental processes that controlled food availability, and consequently, physiological condition of krill: the fall phytoplankton decline, sea ice advance and development of sea ice microbial communities, and the late winter increase in sea ice microbial community biomass. Fall phytoplankton dynamics were identified as a major driver of the physiological condition of larval krill throughout this critical period. The model presents a mechanism that links larval krill survival and recruitment to fall and winter variability in phytoplankton and sea ice dynamics along the western Antarctic Peninsula.

Fritsen, CH, Memmott JC, Ross RM, Quetin LB, Vernet M, Wirthlin ED.  2011.  The timing of sea ice formation and exposure to photosynthetically active radiation along the Western Antarctic Peninsula. Polar Biology. 34:683-692.   10.1007/s00300-010-0924-7   AbstractWebsite

Understanding the flow of solar energy into ecosystems is fundamental to understanding ecosystem productivity and dynamics. To gain a better understanding of this fundamental process in the Antarctic winter sea ice, we produced a model that estimates the time-integrated exposure of seasonal Antarctic sea ice to PAR through the use of remotely sensed sea ice concentrations, sea ice movement and spatially distributed PAR calculations that account for cloud cover and have applied this model over the past three decades. The resulting spatially distributed estimates of sea ice exposure to PAR by mid-winter are evaluated in context of changes in the timing of sea ice formation that have been documented along the Western Antarctic Peninsula (WAP) region and its potential effects on the variation (seasonal and inter-annual) in the accumulation of sea ice algae in this region. The analysis shows the ice pack is likely to have large inter-annual variations (10-100 fold) in productivity throughout the autumn to winter transition in the sea ice along the WAP. Moreover, the pack ice is likely to have spatial structure in regards to biological processes that cannot be determined from analysis of sea ice concentration information alone. The resulting inter-annual variations in winter processes are likely to affect the dynamics of Antarctic krill (Euphausia superba).

Robison, BH, Vernet M, Smith KL.  2011.  Algal communities attached to free-drifting, Antarctic icebergs. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:1451-1456.   10.1016/j.dsr2.2010.11.024   AbstractWebsite

Disintegration of the Antarctic Peninsula's eastern ice shelves has increased the population of icebergs traversing the Weddell Sea, but until recently little was known about their ecological impact on the pelagic environment. Here we describe a class of algal communities that occur on the submerged flanks of large, free-drifting, glacially-derived tabular icebergs. We used remotely operated vehicles to examine these icebergs directly for the first time, to survey the algal communities and collect material for shipboard laboratory studies. The communities, principally diatoms, were associated with a characteristic cupped configuration of the ice surface, and they served as feeding sites for aggregations of Antarctic krill. Production rate measurements indicate that these communities are providing a substantial contribution to regional primary production in summer. As the number of icebergs grows, the number of algae communities may also be increasing, along with their cumulative contribution to organic carbon flux. (C) 2010 Elsevier Ltd. All rights reserved.

Smith, KL, Sherman AD, Shaw TJ, Murray AE, Vernet M, Cefarelli AO.  2011.  Carbon export associated with free-drifting icebergs in the Southern Ocean. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:1485-1496.   10.1016/j.dsr2.2010.11.027   AbstractWebsite

Enrichment of the pelagic ecosystem associated with the proliferation of free-drifting icebergs prompts questions about increased productivity and the export flux of organic carbon to the deep ocean with continued climate warming. Lagrangian Sediment Traps (LST) were deployed autonomously beneath a large tabular, free-drifting iceberg (C-18a) in the NW Weddell Sea during March and April 2009 to collect sinking particles at a depth of 600 m. Three LST deployments associated with C-18a, within a 30-km radius, collected sinking diatom frustules, dominated by Corethron pennatum and Fragilariopsis nana, euphausiid fragments, crustacean and fish fecal material, detrital aggregates and mineral grains. One LST deployment at a "control" site 74 km away in open water devoid of icebergs collected diatom frustules, euphausiid molts, crustacean fecal material and detrital aggregates. Phytoplankton abundance, microbial abundance and biomass were significantly higher in the LST samples than in open-water collections at 500 m depth. The mean mass flux and organic carbon flux associated with iceberg C-18a were twice as high, 124 mg m(-2) d(-1) and 5.6 mg C(org) m(2) d(-1), respectively, than at the control site. A similar trend was observed in C(org)/(234)Th activity, being highest near C-18a and lowest at the control site. Extrapolation of the area of enrichment to 30 km radius around C-18a, 2826 km(2), produces an estimated mass flux of 350 tons d(-1) and carbon flux of 15.8 tons C(org) d(-1). Five similar sized icebergs to C-18a were identified in satellite images in a surrounding 47,636 km(2) area at the same time of sampling. Assuming a 30-km radius as the area of influence around each of these five icebergs, 46% of the total area would be enhanced with an export flux at 600 m of 122.4 tons C(org) d(-1). The large numbers of smaller icebergs identified visually in this area would only increase this area of influence. Icebergs serve as areas of local enrichment and with increased proliferation, must be considered in the cycling of carbon in the Southern Ocean. (C) 2010 Elsevier Ltd. All rights reserved.

Helly, JJ, Kaufmann RS, Stephenson GR, Vernet M.  2011.  Cooling, dilution and mixing of ocean water by free-drifting icebergs in the Weddell Sea. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:1346-1363.   10.1016/j.dsr2.2010.11.010   AbstractWebsite

Iceberg C-18a (35 x 7 x 0.184 km) was studied repeatedly by five circumnavigational surveys in March-April 2009. During the period of the surveys, C-18a travelled 109 nautical miles in 23 days covering an area of 8.1 x 10(3) km(2). This iceberg was formed from iceberg C-18 (76 x 7 km) that originated from the Ross Ice Shelf in May, 2002. Ship-based measurements show that this iceberg produced fresh meltwater above the seasonal pycnocline that diluted and chilled the water it passed through from the surface to a depth of approximately 50 m (summer mixed layer). The surface meltwater effects were detectable as far away as 19 km and persisted for at least 10 days. We also found evidence that this iceberg was disrupting the Weddell Deep Water to depths up to 1500 m. If we include these deep effects through the water column, the estimate of ocean water altered by this single iceberg reaches 3 x 10(12) m(3) over 23 days. Chemical and biological effects were detected at the same space and time scales as the physical properties, with decreasing partial pressure of carbon dioxide (pCO(2)) close to the iceberg and lower particle and chlorophyll concentration. Ten days after the passage of C-18a, chlorophyll-a had increased by 15%. These results are consistent with alternative hypotheses regarding the role of icebergs as mediators of a localized geophysical disturbance (H(1)) as well as promoters of chlorophyll-a production (H(2)). (C) 2010 Elsevier Ltd. All rights reserved.

Vernet, M, Sines K, Chakos D, Cefarelli AO, Ekern L.  2011.  Impacts on phytoplankton dynamics by free-drifting icebergs in the NW Weddell Sea. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:1422-1435.   10.1016/j.dsr2.2010.11.022   AbstractWebsite

Glacier ice released to the oceans through iceberg formation has a complex effect on the surrounding ocean waters. We hypothesized that phytoplankton communities would differ in abundance, composition and production around or close to an iceberg. This paper tests the influence of individual icebergs on scales of meters to kilometers, observed through shipboard oceanographic sampling on March-April 2009. Surface waters (integrated 0-100 m depth, within the euphotic zone) sampled close to the iceberg C-18a ( <1 km) were characterized by lower temperatures, more dissolved nitrate, less total chlorophyll a (chla) concentration, less picoplankton ( <3 mu m) cell abundance, and higher transparency than surface conditions 18 km upstream. However, enrichment of large cells, identified as diatoms, was the basis of an active food chain. Upward velocity of meltwater and dissolved Fe concentrations in excess of 1-2 nM are expected to facilitate diatom specific growth. The presence of diatoms close to the iceberg C-18a and the higher variable fluorescence (Fv/Fm) indicated healthy cells, consistent with Antarctic waters rich in micronutrients. Furthermore, chla increased significantly 2 km around the iceberg and 10 days after the iceberg's passage. We hypothesize that the lower biomass next to the iceberg was due to high loss rates. Underwater melting is expected to dilute phytoplankton near the iceberg by entraining deep water or by introducing meltwater. In addition, high zooplankton biomass within 2 km of the iceberg, mainly Antarctic krill Euphausia superba and salps Salpa thompsonii, are expected to exert heavy grazing pressure on phytoplankton, the krill on large cells >10 mu m and the salps on smaller cells, 3-10 mu m. The iceberg's main influence in the austral fall is measured not so much by phytoplankton accumulation but by reactivation of the classic Antarctic food chain, facilitating diatom growth and sustaining high Antarctic krill populations. (C) 2011 Elsevier Ltd. All rights reserved.

Cefarelli, AO, Vernet M, Ferrario ME.  2011.  Phytoplankton composition and abundance in relation to free-floating Antarctic icebergs. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:1436-1450.   10.1016/j.dsr2.2010.11.023   AbstractWebsite

Free-drifting icebergs in the Weddell Sea are expected to affect the surrounding marine ecosystem. Sampling associated with iceberg C-18a, a large tabular, free-drifting iceberg in the NW Weddell Sea, carried out from 10 March to 7 April 2009, was designed to test the hypothesis that the iceberg's presence modified phytoplankton composition and abundance. Areas that define a gradient of possible iceberg influence were sampled for phytoplankton: stations close ( < 1 km) and far (18 km) from iceberg C-18a, an area with numerous small icebergs, Iceberg Alley, and a control site 74 km away. Quantitative samples were obtained from Niskin bottles and counted with an inverted microscope for species abundance. Qualitative samples were collected with nets from the ship's seawater intake. Taxonomic determinations were performed with light and electron microscopy. Overall, diatoms dominated in the mixed layer (surface-similar to 40 m) and unidentified small flagellated and coccid cells at depth (similar to 100 m). Fragilariopsis nana, a diatom 2.4-15.5 mu m in length, dominated numerically the phytoplankton and was most abundant at the control area. The iceberg's effect on phytoplankton composition was consistent with the hypothesis that they facilitate phytoplankton communities enriched in diatoms, as found in other productive areas of Antarctica. Near the iceberg, diatoms were most abundant, principally at depth, while small flagellate concentration diminished. However, total phytoplankton abundance was lowest at Iceberg Alley in the area of highest meltwater contribution, as indicated by low mean temperature in the mixed layer, and highest at the control site. These results suggest that during austral fall, low growth or high zooplankton grazing could be counteracting the positive effect by icebergs on phytoplankton biomass, otherwise observed in summer months. (C) 2010 Elsevier Ltd. All rights reserved.