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2019
Cape, MR, Vernet M, Pettit EC, Wellner J, Truffer M, Akie G, Domack E, Leventer A, Smith CR, Huber BA.  2019.  Circumpolar deep water impacts glacial meltwater export and coastal biogeochemical cycling along the West Antarctic Peninsula. Frontiers in Marine Science. 6   10.3389/fmars.2019.00144   AbstractWebsite

Warming along the Antarctic Peninsula has led to an increase in the export of glacial meltwater to the coastal ocean. While observations to date suggest that this freshwater export acts as an important forcing on the marine ecosystem, the processes linking ice-ocean interactions to lower trophic-level growth, particularly in coastal bays and fjords, are poorly understood. Here, we identify salient hydrographic features in Barilari Bay, a west Antarctic Peninsula fjord influenced by warm modified Upper Circumpolar Deep Water. In this fjord, interactions between the glaciers and ocean act as a control on coastal circulation, contributing to the redistribution of water masses in an upwelling plume and a vertical flux of nutrients toward the euphotic zone. This nutrient-rich plume, containing glacial meltwater but primarily composed of ambient ocean waters including modified Upper Circumpolar Deep Water, spreads through the fjord as a 150-m thick layer in the upper water column. The combination of meltwater-driven stratification, long residence time of the surface plume owing to weak circulation, and nutrient enrichment promotes phytoplankton growth within the fjord, as evidenced by shallow phytoplankton blooms and concomitant nutrient drawdown at the fjord mouth in late February. Gradients in meltwater distributions are further paralleled by gradients in phytoplankton and benthic community composition. While glacial meltwater export and upwelling of ambient waters in this way contribute to elevated primary and secondary productivity, subsurface nutrient enhancement of glacially modified ocean waters suggests that a portion of these macronutrients, as well any iron upwelled or input in meltwater, are exported to the continental shelf. Sustained atmospheric warming in the coming decades, contributing to greater runoff, would invigorate the marine circulation with consequences for glacier dynamics and biogeochemical cycling within the fjord. We conclude that ice-ocean interactions along the Antarctic Peninsula margins act as an important control on coastal marine ecosystems, with repercussions for carbon cycling along the west Antarctic Peninsula shelf as a whole.

2016
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.

2014
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

2013
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.

2012
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.

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.

2011
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.

Shaw, TJ, Smith KL, Hexel CR, Dudgeon R, Sherman AD, Vernet M, Kaufmann RS.  2011.  Th-234-Based Carbon Export around Free-Drifting Icebergs in the Southern Ocean. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:1384-1391.   10.1016/j.dsr2.2010.11.019   AbstractWebsite

The impact of free-drifting icebergs on the efficiency of carbon export from the upper water column was measured using the disequilibrium of Th-234 and its parent U-238. The study addressed the null hypothesis that free-drifting icebergs do not alter Th-234 deficiency and carbon export compared to surrounding waters. Upper-water-column inventories of Th-234 were measured at six stations in the Weddell Sea concurrently with four deployments of Lagrangian Sediment Traps (LSTs) during a cruise in March/April 2009. Four stations were sampled ranging from 0.3 km to <20 km of the edge of a large free-drifting iceberg (C-18a) and two were sampled at distances > 60 km from C-18a. Temperature and salinity anomalies indicated enhanced upwelling and turbulent mixing extending downstream of the iceberg to a minimum of similar to 20 km from the iceberg edge. Separate studies of the impact of C-18a on water column physical properties were used to define the extent of the iceberg's influence on surrounding waters. The largest upper-water-column deficiencies in the inventories of Th-234 were measured in close proximity and downstream of the iceberg and extending to below 100 m depth. A steady-state model was used to estimate the export of Th-234 from the upper water column. Organic carbon export was calculated using C/Th from the concurrent LST collections. Comparison of stations within the iceberg's influence (close proximity and downstream to within 20 km of the iceberg) and far-field (greater than 60 km) measurements showed a factor of 3 increase in organic carbon export near the iceberg. The factor correlated well with the results from the near- and far-field LST measurements. Differences in the magnitude of carbon export at 100 and 600 m indicate that similar to 90 percent of the exported material is regenerated by 600 m depth. This study confirms that the increased abundance of large free-drifting icebergs in the Southern Ocean can contribute to the drawdown of atmospheric CO2 through increased organic carbon export. (C) 2011 Elsevier Ltd. All rights reserved.

Kozlowski, WA, Deutschman D, Garibotti I, Trees C, Vernet M.  2011.  An evaluation of the application of CHEMTAX to Antarctic coastal pigment data. Deep-Sea Research Part I-Oceanographic Research Papers. 58:350-364.   10.1016/j.dsr.2011.01.008   AbstractWebsite

Presented is an evaluation of the application of CHEMTAX (CHEMical TAXonomy) to Antarctic coastal pigments collected along the western Antarctic Peninsula (wAP). Overall analytical error is < 20% for all pigments involved in the analysis. CHEMTAX was stable within a range of input pigment ratios; data were analyzed in three bins based on light depths, with each year's data run independently. Results were validated by comparison to those from CHEMTAX methods that included randomized error, feedback loops and additional diagnostic pigments. Blooms during mid-summer (chlorophyll a concentrations > 5 mu g L(-1)) were dominated primarily by either diatoms or cryptomonads. Mixed flagellates can also be abundant and Pheaocystis spp. and prasinophytes are frequently present in low concentrations. Comparison with microscopy shows CHEMTAX to give superior results in identifying Pheaocystis spp. with favorable results for other groups. This analysis shows CHEMTAX to be a reliable and stable tool for providing estimations of the main phytoplankton taxa in wAP waters based on long-term data collected during a 12-year time series. (C) 2011 Elsevier Ltd. All rights reserved.

2010
Cefarelli, AO, Ferrario ME, Almandoz GO, Atencio AG, Akselman R, Vernet M.  2010.  Diversity of the diatom genus Fragilariopsis in the Argentine Sea and Antarctic waters: morphology, distribution and abundance. Polar Biology. 33:1463-1484.   10.1007/s00300-010-0794-z   AbstractWebsite

Fragilariopsis species composition and abundance from the Argentine Sea and Antarctic waters were analyzed using light and electron microscopy. Twelve species (F. curta, F. cylindrus, F. kerguelensis, F. nana, F. obliquecostata, F. peragallii, F. pseudonana, F. rhombica, F. ritscheri, F. separanda, F. sublinearis and F. vanheurckii) are described and compared with samples from the Frenguelli Collection, Museo de La Plata, Argentina. F. peragallii was examined for the first time using electron microscopy, and F. pseudonana was recorded for the first time in Argentinean shelf waters. New information on the girdle view is included, except for the species F. curta, F. cylindrus and F. nana, for which information already existed. In the Argentine Sea, F. pseudonana was the most abundant Fragilariopsis species, and in Antarctic waters, F. curta was most abundant. Of the twelve species of Fragilariopsis documented, four occurred in the Argentine Sea, nine in the Drake Passage and twelve in the Weddell Sea. F. curta, F. kerguelensis, F. pseudonana and F. rhombica were present everywhere.

2009
Vernet, M, Diaz SB, Fuenzalida HA, Camilion C, Booth CR, Cabrera S, Casiccia C, Deferrari G, Lovengreen C, Paladini A, Pedroni J, Rosales A, Zagarese HE.  2009.  Quality of UVR exposure for different biological systems along a latitudinal gradient. Photochemical & Photobiological Sciences. 8:1329-1345.   10.1039/b904540f   AbstractWebsite

The exposure of organisms to ultraviolet radiation (UVR) is characterized by the climatology (annual cycle) and the variance (anomalies) of biologically-weighted irradiances at eight geographical locations in austral South America, from 1995-2002. The net effect of UVR on biological systems is a result of the balance of damage and repair which depends on intensity and duration of irradiance and is modulated by its variability. The emphasis in this study is on day-to-day variability, a time scale of importance to adaptive strategies that counteract UVR damage. The irradiances were weighted with DNA-and phytoplankton photosynthesis-action spectra. Low latitude sites show high average UVR. For all sites, the frequency of days with above average irradiances is higher than below average irradiances. Persistence in anomalies is generally low (<= 0.36 autocorrelation coefficient), but higher for DNA-than phytoplankton photosynthesis-weighted irradiances due to their higher correspondence to stratospheric ozone. Cloudiness and other factors with small wavelength dependence (e. g. aerosols and albedo) are highly correlated with UVR anomalies at low latitudes (24-33 degrees S); ozone correlates higher at high latitudes (42-54.5 degrees S). Our results show that organisms in this region deal with several days of excess radiation and fewer, shorter and more intense periods of lower than average radiation. Relief from UVR stress (or higher frequency of days below the climatology) is more prevalent at high latitudes (54.5 degrees S). Thus, lower latitudes are more stressful to organisms not only because of higher average UVR irradiance but also for the higher frequency of days above the climatology.

2008
Vernet, M, Martinson D, Iannuzzi R, Stammerjohn S, Kozlowski W, Sines K, Smith R, Garibotti I.  2008.  Primary production within the sea-ice zone west of the Antarctic Peninsula: I-Sea ice, summer mixed layer, and irradiance. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 55:2068-2085.   10.1016/j.dsr2.2008.05.021   AbstractWebsite

in shelf waters of the western Antarctic Peninsula (wAP), with abundant macro- and micronutrients, water-column stability has been suggested as the main factor controlling primary production; freshwater input from sea-ice melting stabilizes the upper water column by forming a shallow summer mixed layer. Retreating sea ice in the spring and summer thus defines the area of influence, the sea-ice zone (SIZ) and the marginal ice zone (MIZ). A 12-year time series (1995-2006) was analyzed to address two main questions: (1) what are the spatial and temporal patterns in primary production; and (2) to what extent and in what ways is primary production related to sea-ice dynamics. Data were collected on cruises performed during January of each year, at the height of the growth season, within the region bounded by 64 degrees S and 64 degrees W to the north and 68 degrees S and 66 degrees W to the south. Average daily integrated primary production varied by an order of magnitude, from similar to 250 to similar to 1100mg cm(-2) d(-1), with an average cruise primary production of 745 mg C m(-2) d(-1). A strong onshore-offshore gradient was evident along the shelf with higher production observed inshore. Inter-annual regional production varied by a factor of 7: maximum rates were measured in 2006 (1788 mg cm(-2) d-(1)) and minimum in 1999 (248 Mg C m(-2) d(-1)). The results support the hypothesis that primary production in the wAP shelf is related to sea-ice dynamics. To first order, shallower summer mixed-layer depths in the shelf correlated with late sea retreat and primary production. Principal component analysis showed that high primary production in January was associated with enhanced shelf production toward the coast and in the south, explaining 63% of the variability in space and time. This first mode captured the inter-annual variability in regional production. Temporal variability in primary production (time series of anomalies defined for each location) showed spatial dependence: higher primary production correlated with shallow mixed-layer depths only at mid-shelf; in coastal and offshore waters, primary production correlated with deeper mixed layers. Thus, coastal primary production can show a non-linear relationship with summer mixed layers. Under conditions of large biomass (> 20 mg ch1 a m(-3)) and shallow mixed-layer depth (e.g., 5 m) phytoplankton production becomes light limited. This limitation is reduced with a deepening of the summer mixed layer (e.g., 20m). Dominance of diatoms and the ability to adapt and photosynthesize at higher light levels characterized the large phytoplankton blooms. No significant regional trend in primary production was detected within the 12-year series. We conclude that the regional average primary production on the wAP shelf is associated with shallow summer mixed layers in conjunction with late sea-ice retreat. An opposite relationship is observed for the highest production rates in coastal waters, associated with large biomass, where a deepening of the summer mixed layer relieves light limitation. (C) 2008 Elsevier Ltd. All rights reserved.

2007
Smith, KL, Robison BH, Helly JJ, Kaufmann RS, Ruhl HA, Shaw TJ, Twining BS, Vernet M.  2007.  Free-drifting icebergs: Hot spots of chemical and biological enrichment in the Weddell Sea. Science. 317:478-482.   10.1126/science.1142834   AbstractWebsite

The proliferation of icebergs from Antarctica over the past decade has raised questions about their potential impact on the surrounding pelagic ecosystem. Two free-drifting icebergs, 0.1 and 30.8 square kilometers in aerial surface area, and the surrounding waters were sampled in the northwest Weddell Sea during austral spring 2005. There was substantial enrichment of terrigenous material, and there were high concentrations of chlorophyll, krill, and seabirds surrounding each iceberg, extending out to a radial distance of similar to 3.7 kilometers. Extrapolating these results to all icebergs in the same size range, with the use of iceberg population estimates from satellite surveys, indicates that they similarly affect 39% of the surface ocean in this region. These results suggest that free-drifting icebergs can substantially affect the pelagic ecosystem of the Southern Ocean and can serve as areas of enhanced production and sequestration of organic carbon to the deep sea.

Matrai, P, Vernet M, Wassmann P.  2007.  Relating temporal and spatial patterns of DMSP in the Barents Sea to phytoplankton biomass and productivity. Journal of Marine Systems. 67:83-101.   10.1016/j.jmarsys.2006.10.001   AbstractWebsite

Dimethylsulfoniopropionate (DMSP), produced by many marine phytoplankton, is the main precursor of the climate relevant gas dimethylsulfide (DMS). Currently, it is generally accepted that the relationship between DMSP and phytoplankton biomass (as chlorophyll a), while not representative of the absolute magnitude of the DMSP pool, is a good indicator of ecosystem structure. In this study we test the strength of the relationships between DMSP and various phytoplankton parameters in Arctic shelf waters of the Barents Sea. Our objective is to assess the predictive value that traditional phytoplankton carbon parameters have on DMSP. We discuss C:DMSP-S variability as a function of seasonality, water masses, grazing and nutrient limitation. For this purpose we analyze data from 5 cruises including winter, spring and summer conditions and across the seasonal ice zone at the time of the study. Highest phytoplankton DMSP concentration was usually measured at the ice edge. Marked seasonal variability was observed in phytoplankton carbon biomass and production but not necessarily in the particulate fraction of DMSP (DMSPp), resulting in seasonally varying C:DMSP-S. High winter DMSPp concentrations, when chlorophyll a and primary production were lowest and flagellates dominant, suggest a heterotrophic source. The production of extracellular carbon and the pool of dissolved DMSP (DMSPd) followed similar seasonal trends, with enhanced concentrations in spring, and we suggest that high dissolved primary production induced by nutrient limiting conditions resulted in high DMSPd concentrations. Mesoscale changes in total DMSP (particulate + dissolved) may be modeled from basin-wide total phytoplankton primary production (rather than from phytoplankton biomass) at seasonal and interannual scales. We conclude there is predictive power of DMSP concentrations in the Barents Sea based on seasonality, the position of the ice edge and the distribution of phytoplankton variables. (c) 2006 Elsevier B.V. All rights reserved.

2006
Diaz, S, Camilion C, Escobar J, Deferrari G, Roy S, Lacoste K, Demers S, Belzile C, Ferreyra G, Gianesella S, Gosselin M, Nozais C, Pelletier E, Schloss I, Vernet M.  2006.  Simulation of ozone depletion using ambient irradiance supplemented with UV lamps. Photochemistry and Photobiology. 82:857-864.   10.1562/2005-09-28-ra-700   AbstractWebsite

In studies of the biological effects of UV radiation, ozone depletion can be mimicked by performing the study under ambient conditions and adding radiation with UV-B lamps. We evaluated this methodology at three different locations along a latitudinal gradient: Rimouski (Canada), Ubatuba (Brazil) and Ushuaia (Argentina). Experiments of the effect of potential ozone depletion on marine ecosystems were carried out in large outdoor enclosures (mesocosms). In all locations we simulated irradiances corresponding to 60% ozone depletion, which may produce a 130-1900% increase in 305 nm irradiance at noon, depending on site and season. Supplementation with a fixed percentage of ambient irradiance provides a better simulation of irradiance increase due to ozone depletion than supplementation with a fixed irradiance value, particularly near sunrise and sunset or under cloudy skies. Calculations performed for Ushuaia showed that, on very cloudy days, supplementation by the square-wave method may produce unrealistic irradiances. Differences between the spectra of the calculated supplementing irradiance and the lamp for a given site and date will be a function of the time of day and may become more or less pronounced according to the biological weighting function of the effect under study.

Ferrero, E, Eory M, Ferreyra G, Schloss I, Zagarese H, Vernet M, Momo F.  2006.  Vertical mixing and ecological effects of ultraviolet radiation in planktonic communities. Photochemistry and Photobiology. 82:898-902.   10.1562/2005-11-23-ra-736   AbstractWebsite

We present a mathematical model for a phytoplankton-zooplankton system, based on a predator-prey scheme. The model considers the effects of sinking in the phytoplankton, vertical mixing and attenuation of photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) in the water column. In a first approach, the model was studied under conditions of average PAR irradiance and shows fluctuations and stable equilibrium points. Secondly, we introduced the effects of photoperiod and photoinhibition by UVR and vertical mixing. Under these conditions, the phytoplankton biomass oscillates depending on the combined effects of UVR and mixing. Higher inhibition by UVR and longer mixing periods can induce strong fluctuations in the system but can also produce higher plankton peaks.

2005
Garibotti, IA, Vernet M, Ferrario ME.  2005.  Annually recurrent phytoplanktonic assemblages during summer in the seasonal ice zone west of the Antarctic Peninsula (Southern Ocean). Deep-Sea Research Part I-Oceanographic Research Papers. 52:1823-1841.   10.1016/j.dsr.2005.05.003   AbstractWebsite

The distribution of phytoplankton composition, cell abundance and biomass from an area along the Western Antarctic Peninsula was studied during three summers, with the aim of understanding its dynamics over spatial and interannual scales. The studied area is characterized by seasonal sea-ice retreat and advance. Algae composition and concentration were found to be highly variable through the area as well as from year to year. Small unidentified phytoflagellates, diatoms and cryptophytes were the main phytoplankton groups, contributing the major proportion of total phytoplankton cell abundance and biomass concentration. Three annually recurrent phytoplankton assemblages were recognized in the area according to the algae composition and abundance: a diatom bloom associated with the seaice edge, an assemblage dominated by small unidentified phytoflagellates and cryptophytes, and a diatom-enriched assemblage in open waters. The distribution of these assemblages varied from year-to-year. During the summers preceded by early sea-ice retreat, the diatom bloom was spatially restricted and the other two assemblages occupied extended regions, whereas during the late sea-ice retreat year, the diatom bloom extended over a larger region and the other assemblages occupied smaller regions or were just absent. It was detected that these assemblages resemble different stages of the phytoplankton seasonal cycle, and that their distribution through the area can be related to a latitudinal and longitudinal gradient in the phytoplankton growth onset timing, associated with the progressive sea-ice retreat during spring. The local environmental conditions associated with each assemblage were also analyzed, but further study is needed for understanding the causes of the replacement of one assemblage by another through the area. On the other hand, the interannual variability in the distribution of the assemblages can be related to year-to-year differences in the timing of phytoplankton growth onset, associated with variations in the timing of the sea-ice retreat. (c) 2005 Elsevier Ltd. All rights reserved.

2004
Moline, MA, Claustre H, Frazer TK, Schofield O, Vernet M.  2004.  Alteration of the food web along the Antarctic Peninsula in response to a regional warming trend. Global Change Biology. 10:1973-1980.   10.1111/j.1365-2486.2004.00825.x   AbstractWebsite

In the nearshore coastal waters along the Antarctic Peninsula, a recurrent shift in phytoplankton community structure, from diatoms to cryptophytes, has been documented. The shift was observed in consecutive years (1991-1996) during the austral summer and was correlated in time and space with glacial melt-water runoff and reduced surface water salinities. Elevated temperatures along the Peninsula will increase the extent of coastal melt-water zones and the seasonal prevalence of cryptophytes. This is significant because a change from diatoms to cryptophytes represents a marked shift in the size distribution of the phytoplankton community, which will, in turn, impact the zooplankton assemblage. Cryptophytes, because of their small size, are not grazed efficiently by Antarctic krill, a keystone species in the food web. An increase in the abundance and relative proportion of cryptophytes in coastal waters along the Peninsula will likely cause a shift in the spatial distribution of krill and may allow also for the rapid asexual proliferation of carbon poor gelatinous zooplankton, salps in particular. This scenario may account for the reported increase in the frequency of occurrence and abundance of large swarms of salps within the region. Salps are not a preferred food source for organisms that occupy higher trophic levels in the food web, specifically penguins and seals, and thus negative feedbacks to the ecology of these consumers can be anticipated as a consequence of shifts in phytoplankton community composition.

2002
Haberman, KL, Ross RM, Quetin LB, Vernet M, Nevitt GA, Kozlowski W.  2002.  Grazing by Antarctic krill Euphausia superba on Phaeocystis antarctica: an immunochemical approach. Marine Ecology-Progress Series. 241:139-149.   10.3354/meps241139   AbstractWebsite

An enzyme-linked immunosorbent assay (ELISA) for Phaeocystis antarctica was developed and used to determine the extent of grazing by Euphausia superba on this prymnesiophyte. First, a specific antiserum to P. antarctica was produced and tested on phytoplankton cells and extracts. Then, the antiserum was tested on extracts of stomachs from krill fed diets of either Thalassiosira antarctica or P. antarctica, and also on mixtures of these extracts. The antiserum response was a logarithmic function of the percentage of the extract mixture that came from P. antarctica-fed krill. In the field, the antiserum was tested on krill collected during December 1996 to February 1997 in the Palmer Long-Term Ecological Research study region west of the Antarctic Peninsula. The antiserum. response suggested that krill ingested P. antarctica at 20% of the stations tested, most of them in nearshore areas. Krill grazed P. antarctica only at stations where concentrations of the marker pigment for P. antarctica, 19'-hexanoyloxyfucoxanthin, were above 0.150 mug l(-1). At these stations, the percentage of P. antarctica in the krill stomach extracts was positively correlated with chlorophyll a concentrations, but negatively correlated with the proportion of P. antarctica within the phytoplankton community. Overall, P. antarctica appeared to comprise a relatively small proportion of the phytoplankton utilized by krill, both spatially and temporally.

2001
Sastre, MP, Vernet M, Steinert S.  2001.  Single-cell gel/comet assay applied to the analysis of UV radiation-induced DNA damage in Rhodomonas sp. (Cryptophyta). Photochemistry and Photobiology. 74:55-60.   10.1562/0031-8655(2001)074<0055:scgcaa>2.0.co;2   AbstractWebsite

The single-cell gel/comet assay is an electrophoretic technique used to detect single-strand breaks in DNA, Damage is assessed examining individual cells under an epifluorescent microscope. UV-induced DNA damage consists mostly of the formation of pyrimidine dimers; therefore, most of the damage cannot be detected using a standard comet assay. The enzyme T4 endonuclease V breaks DNA strands at sites of pyrimidine dimers, The main objective of this work is to evaluate the comet assay to detect UV-induced damage in DNA after an initial treatment of cells with T4 endonuclease V, This work was conducted on Rhodomonas sp, (Cryptophyta), a marine unicellular flagellate, Cells of Rhodomonas sp, were exposed to 12 h visible + ultraviolet-A + ultraviolet-B (VIS + UVA + UVB) and VIS (control), with and without T4 endonuclease V. Cells exposed to VIS + UVA + UVB showed approximately 200% more damage than control if these were treated with T4 endonuclease V, Rhodomonas sp, were exposed to 3, 6, 9 and 12 h of VIS, VIS + UVA and VIS + UVA + UVB, Damage induced by VIS + UVA + UVB as detected by the comet assay increased along with exposure time. However, damage caused by VIS and VIS + UVA remained relatively constant at all times. Results of this study indicate that the comet assay is more sensitive to UV radiation damage when used in conjunction with T4 endonuclease V. This modification of the comet assay can be used as an alternative technique to detect DNA damage in single cells caused by UV radiation.

2000
Dierssen, HM, Vernet M, Smith RC.  2000.  Optimizing models for remotely estimating primary production in Antarctic coastal waters. Antarctic Science. 12:20-32. AbstractWebsite

Primary productivity and associated biogeochemical fluxes within the Southern Ocean are globally significant, sensitive to change and poorly known compared to temperate marine ecosystems. We present seasonal time series data of chlorophyll a, primary productivity and in-water irradiance measured in the coastal waters of the Western Antarctica Peninsula and build upon existing models to provide a more optimum parameterization for the estimation of primary productivity in Antarctic coastal waters. These and other data provide strong evidence that bio-optical characteristics and phytoplankton productivity in Antarctic waters an different from temperate waters. For these waters we show that over 60% of the variability in primary production can be explained by the surface chlorophyll a concentration alone, a characteristic, which lends itself to remote sensing models. if chlorophyll a concentrations are accurately determined, then the largest source of error 13-18) results from estimates of the photoadaptive variable (P-opt(B)). Further, the overall magnitude of P-opt(B) is low (median 1.09 mg C mg chl(-1) h(-1)) for these data compared to other regions and generally fits that expected for a cold water system. However, the variability of P-opt(B) over the course of a season (0.4 to 3 mg C mg chl(-1) h(-1)) is not consistently correlated with other possible environmental parameters, such as chlorophyll, sea surface temperature, incident irradiance, day length, salinity, or taxonomic composition. Nonetheless, by tuning a standard depth- integrated primary productivity model to fit representative P-opt(B) values and the relatively uniform chlorophyll-normalized production profile found in these waters, we can improve the model to account for approximately 72-73% variability in primary production both for our data as well as for independent historic Antarctic data.

1999
Gabric, AJ, Matrai PA, Vernet M.  1999.  Modelling the production and cycling of dimethylsulphide during the vernal bloom in the Barents Sea. Tellus Series B-Chemical and Physical Meteorology. 51:919-937.   10.1034/j.1600-0889.1999.t01-4-00005.x   AbstractWebsite

Recent field work suggests an important role for the Arctic Ocean in the global budget of dimethylsulphide (DMS), a climatically active volatile sulphur compound. Here, we have used an existing DMS production model and local field data to examine the temporal dynamics of the DMS cycle during the spring bloom in the Arctic shelf of the Barents Sea. The timing and duration of the spring phytoplankton bloom has been shown to be a key determinant of the flux of DMS to the atmosphere. Particular oceanic conditions due to the retreating ice-edge (e.g., a shallow mixed layer) can have an important effect on the timing of the phytoplankton bloom and thus the efflux of DMS in this region. Model simulations support the view that algal taxonomy is not the most important factor determining DMS production in these waters. The mean vernal DMS flux is predicted to be 0.063 mg S m(-2) d(-1) which is in general agreement with previous summer season averages in the Arctic.

Wassmann, P, Ratkova T, Andreassen I, Vernet M, Pedersen C, Rey F.  1999.  Spring bloom development in the marginal ice zone and the central Barents Sea. Marine Ecology-Pubblicazioni Della Stazione Zoologica Di Napoli I. 20:321-346.   10.1046/j.1439-0485.1999.2034081.x   AbstractWebsite

The knowledge of the relative contribution of algal groups and the environmental factors that control their abundance in the marginal ice zone of the Parents Sea is rather limited. Therefore, a field investigation to study the hydrography and the phytoplankton composition of the marginal ice zone (M1Z) in the central Parents Sea was carried out along a south-north transect in May 1993. The weakly stratified Atlantic sector of the transect appeared to be in a prebloom state and had intermittent intrusions from the meandering Polar Front and the Norwegian Coastal Current, introducing water of a more advanced bloom state. Pico- and nanoplankton flagellates and monads dominated, with a few diatoms and Phaeocystis pouchetii colonies. The average new production rate of 26 g C.m(-2) as reflected by NO, depletion in the euphotic zone, however, indicated that the vernal bloom had been in progress for some time in the Atlantic sector without leaving specific signals in the suspended fraction. The ice-edge and Polar Front area was characterized by a dominance of centric colonial diatom genera Chaetoceros and Thalassiosira with some development of P. pouchetii. In the densely ice-covered and stratified Arctic zone the vernal bloom was at its maximum and dominated by the diatom genera Fragilariopsis and Chaetoceros. Diatoms were limited by silicate concentrations <2 mu M in 32% of all samples. New production, as revealed by the C equivalent of nitrate depletion in the upper layer, ranged between 12 g C m(-2) in the north to 45 g C.m(-2) in the meandering Polar Front, with an average of about 27 +/- 28% g C.m(-2). The time development of the vernal bloom in the marginal ice zone and the central Parents Sea in late May 1993, with its complicated zonal structure, was not from south to north, but intermittently from north to south. Later during the year the general development of the vernal bloom was, as expected, from south to north.

1998
Smith, RC, Baker KS, Vernet M.  1998.  Seasonal and interannual variability of phytoplankton biomass west of the Antarctic Peninsula. Journal of Marine Systems. 17:229-243.   10.1016/s0924-7963(98)00040-2   AbstractWebsite

The spatial and temporal variability of phytoplankton biomass, estimated as chlorophyll-a (chl-a) concentration, is examined in the continental shelf-slope region west of the Antarctic Peninsula. Relationships between temporal observations in the nearshore Palmer Station grid (64 degrees 46.77'S, 64 degrees 04.36'W) and spatial observations in a larger regional grid 200 km on/off-shore and 900 km alongshore are presented. Average chi-a concentrations in the upper layers of the water column in the immediate vicinity of Palmer Station show strong seasonal and interannual variability. Biomass accumulation typically starts during mid-November, while strong blooms develop from December through January. The 1991/1992 and 1994/1995 seasons developed higher overall chi-a concentrations (average maximum water column values reaching 8 and 16 mg chi-a m(-3), respectively) than the 1992/1993 and 1993/1994 seasons (average maximum water column values of less than 3 mg chi-a m(-3)). The 1994/1995 season of extremely high chi-a concentrations also showed a prolonged bloom period into February, while the 1991/1992 season did not. Similar interannual variability was observed in the regional grid. Average chi-a concentration in the top 30 m was 0.91, 1.24 and 1.66 mg chi-a m(-3) for January of 1993, 1994 and 1995, respectively. The regional grid contains an on/off-shore gradient in bottom topography, measured physical and optical characteristics, as well as chi-a concentrations. Regional inshore grid stations in January had, on average, almost four times more chi-a biomass than off-shore stations (2.18 vs. 0.59 mg chi-a m(-3), respectively). There is evidence that this on/off-shore gradient is modulated alongshore by latitudinal variability which follows the annual advance and retreat of sea ice.