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2019
Sanz-Martin, M, Vernet M, Cape MR, Mesa E, Delgado-Huertas A, Reigstad M, Wassmann P, Duarte CM.  2019.  Relationship between carbon- and oxygen-based primary productivity in the Arctic Ocean, Svalbard Archipelago. Frontiers in Marine Science. 6   10.3389/fmars.2019.00468   AbstractWebsite

Phytoplankton contribute half of the primary production (PP) in the biosphere and are the major source of energy for the Arctic Ocean ecosystem. While PP measurements are therefore fundamental to our understanding of marine biogeochemical cycling, the extent to which current methods provide a definitive estimate of this process remains uncertain given differences in their underlying approaches, and assumptions. This is especially the case in the Arctic Ocean, a region of the planet undergoing rapid evolution as a result of climate change, yet where PP measurements are sparse. In this study, we compared three common methods for estimating PP in the European Arctic Ocean: (1) production of O-18-labeled oxygen (GPP-O-18), (2) changes in dissolved oxygen (GPP-DO), and (3) incorporation rates of C-14-labeled carbon into particulate organic carbon (C-14-POC) and into total organic carbon (C-14-TOC, the sum of dissolved and particulate organic carbon). Results show that PP rates derived using oxygen methods showed good agreement across season and were strongly positively correlated. While also strongly correlated, higher scatter associated with seasonal changes was observed between C-14-POC and C-14-TOC. The C-14-TOC-derived rates were, on average, approximately 50% of the oxygen-based estimates. However, the relationship between these estimates changed seasonally. In May, during a spring bloom of Phaeocystis sp., C-14-TOC was 52% and 50% of GPP-DO, and GPP-O-18, respectively, while in August, during post-bloom conditions dominated by flagellates, C-14-TOC was 125% of GPP-DO, and C-14-TOC was 175% of GPP-O-18. Varying relationship between C and O rates may be the result of varying importance of respiration, where C-based rates estimate net primary production (NPP) and O-based rates estimate gross primary production (GPP). However, uncertainty remains in this comparison, given differing assumptions of the methods and the photosynthetic quotients. The median O:C ratio of 4.75 in May is within the range of that observed for other regions of the world's ocean. However, the median O:C ratio for August is <1, lower than in any other reported region. Our results suggest further research is needed to estimate O:C in Arctic waters, and at different times of the seasonal cycle.

Olli, K, Halvorsen E, Vernet M, Lavrentyev PJ, Franze G, Sanz-Martin M, Paulsen ML, Reigstad M.  2019.  Food web functions and interactions during spring and summer in the Arctic Water inflow region: Investigated through inverse modeling. Frontiers in Marine Science. 6   10.3389/fmars.2019.00244   AbstractWebsite

We used inverse modeling to reconstruct major planktonic food web carbon flows in the Atlantic Water inflow, east and north of Svalbard during spring (18-25 May) and summer (9-13 August), 2014. The model was based on three intensively sampled stations during both periods, corresponding to early, peak, and decline phases of a Phaeocystis and diatom dominated bloom (May), and flagellates dominated post bloom stages (August). The food web carbon flows were driven by primary production (290-2,850 mg C m(-2) d(-1)), which was channeled through a network of planktonic compartments, and ultimately respired (180-1200 mg C m(2) d(-1)), settled out of the euphotic zone as organic particles (145-530 mg C m(-2) d(-1)), or accumulated in the water column in various organic pools. The accumulation of dissolved organic carbon was intense (1070 mg C m(-2) d(-1)) during the early bloom stage, slowed down during the bloom peak (400 mg C m(-2) d(-1)), and remained low during the rest of the season. The heterotrophic bacteria responded swiftly to the massive release of new DOC by high but decreasing carbon assimilation rates (from 534 to 330 mg C m(-2) d(-1)) in May. The net bacterial production was low during the early and peak bloom (26-31 mg C m(-2) d(-1)) but increased in the late and post bloom phases (>50 mg C m(-2) d(-1)). The heterotrophic nanoflagellates did not respond predictably to the different bloom phases, with relatively modest carbon uptake, 30-170 mg C m(2) d(-1). In contrast, microzooplankton increased food intake from 160 to 380 mg C m(2) d(-1) during the buildup and decline phases, and highly variable carbon intake 46-624 mg C m(2) d(-1), during post bloom phases. Mesozooplankton had an initially high but decreasing carbon uptake in May (220-48 mg C m(-2) d(-1)), followed by highly variable carbon consumption during the post bloom stages (40-190 mg C m(-2) d(-1)). Both, micro- and mesozooplankton shifted from almost pure herbivory (92-97% of total food intake) during the early bloom phase to an herbivorous, detritovorous and carnivorous mixed diet as the season progressed. Our results indicate a temporal decoupling between the microbial and zooplankton dominated heterotrophic carbon flows during the course of the bloom in a highly productive Atlantic gateway to the Arctic Ocean.

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.

2012
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.

2011
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.

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.

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.

1998
Vernet, M, Matrai PA, Andreassen I.  1998.  Synthesis of particulate and extracellular carbon by phytoplankton at the marginal ice zone in the Barents Sea. Journal of Geophysical Research-Oceans. 103:1023-1037.   10.1029/97jc02288   AbstractWebsite

Large phytoplankton biomass accumulates during ice-edge blooms in Arctic waters, where taxa such as diatoms or the prymnesiophyte Phaeocystis pouchetii usually dominate. Based on characteristics from temperate phytoplankton, we hypothesized that in Barents Sea waters, a larger fraction of primary production would be extracellular (for synthesis of colonial mucilage) during periods of dominance by P. pouchetii as opposed to periods when diatoms dominated. This alternation of P. pouchetii and diatoms would affect the relationship between the particulate and dissolved carbon pools in the upper water column of the marginal ice zone (MIZ). Results presented in this paper do not support this hypothesis. Although P. pouchetii contributed strongly to the extracellular carbon pool (mucilage and dissolved organic carbon, DOC) during an ice-edge bloom in May 1993, arctic diatoms contributed an equal amount of exuded carbon. Three process stations visited along a north-south transect in the MIZ in the Barents Sea, presented between 36% and 55% of the primary production as extracellular carbon, defined as labeled organic matter which passes through a Whatman GF/C filter. No difference in the carbon allocation between diatom-and P. pouchetii-rich phytoplankton was observed in these stations. In contrast, the station located ire ice-free waters had 18% of primary production in the extracellular fraction. These results (1) highlight similar carbon allocation for diatom-and P. pouchetii-dominated phytoplankton in surface waters of the Barents Sea during the spring and/or ice-edge bloom at the MIZ and (2) suggest that polar phytoplankton may be stronger producers of extracellular carbon, and possibly DOG, than previously thought.