Publications

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2018
Paulsen, ML, Seuthe L, Reigstad M, Larsen A, Cape MR, Vernet M.  2018.  Asynchronous accumulation of organic carbon and nitrogen in the Atlantic gateway to the Arctic Ocean. Frontiers in Marine Science. 5   10.3389/fmars.2018.00416   AbstractWebsite

Nitrogen (N) is the main limiting nutrient for biological production in the Arctic Ocean. While dissolved inorganic N (DIN) is well studied, the substantial pool of N bound in organic matter (OM) and its bioavailability in the system is rarely considered. Covering a full annual cycle, we here follow N and carbon (C) content in particulate (P) and dissolved (D) OM within the Atlantic water inflow to the Arctic Ocean. While particulate organic carbon (POC), particulate organic nitrogen (PON), and dissolved organic carbon (DOC) accumulated in the surface waters from January to May, the dissolved organic nitrogen (DON)-pool decreased substantially (Delta - 50 mu g N L-1). The DON reduction was greater than the simultaneous reduction in DIN (Delta - 30 mu g N L-1), demonstrating that DON is a valuable N-source supporting the growing biomass. While the accumulating POM had a C/N ratio close to Redfield, the asynchronous accumulation of C and N in the dissolved pool resulted in a drastic increase in the C/N ratio of dissolved organic molecules (DOM) during the spring bloom. This is likely due to a combination of the reduction in DON, and a high release of carbon-rich sugars from phytoplankton, as 32% of the spring primary production (PP) was dissolved. Our findings thus caution calculations of particulate PP from DIN drawdown. During post-bloom the DON pool increased threefold due to an enhanced microbial processing of OM and reduced phytoplankton production. The light absorption spectra of DOM revealed high absorption within the UV range during spring bloom indicating DOM with low molecular weight in this period. The absorption of DOM was generally lower in the winter months than in spring and summer. Our results demonstrate that the change in ecosystem function (i.e., phytoplankton species and activity, bacterial activity and grazing) in different seasons is associated with strong changes in the C/N ratios and optical character of DOM and underpin the essential role of DON for the production cycle in the Arctic.

2012
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.

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.

2008
Montes-Hugo, MA, Vernet M, Martinson D, Smith R, Iannuzzi R.  2008.  Variability on phytoplankton size structure in the western Antarctic Peninsula (1997-2006). Deep-Sea Research Part Ii-Topical Studies in Oceanography. 55:2106-2117.   10.1016/j.dsr2.2008.04.036   AbstractWebsite

The temporal and spatial variability of phytoplankton size structure of waters west of the Antarctica Peninsula (WAP) was investigated between 1997 and 2006. Time series of satellite-derived (phytoplankton size structure index or gamma b(bp), chlorophyll a concentration or chl(T), and sea-ice extent) and shipboard (temperature, salinity, nutrients, and mixed-layer depth) variables were generated during spring-summer in slope, middle shelf and inshore waters and analyzed in relation to atmospheric anomalies (El Nino Southern Oscillation, ENSO, and Southern Annular Mode, SAM). The sampling design included stations north (northern, 62 degrees S) and within (central and southern, 64-68 degrees S) the Pal-LTER (Palmer Long Term Ecological Research) study site. It is hypothesized that contribution of 'small' phytoplankton (< 20 mu m) has increased in the last decade in WAP waters due the ongoing regional climate change. Relationships between gamma b(bp) the spectral slope of particle backscattering, and environmental parameters were explored based on non-parametric trends (Mann-Kendall test) and cross-correlation coefficients (Spearman matrix). Three types of temporal patterns were detected in satellite-derived phytoplankton size distributions: (1) inter-annual variations of spring-summer gamma b(bp) related to monthly sea-ice extent, (2) abrupt transitions toward dominance of 'small' (<20gm) phytoplankton cells (high gamma b(bp)) and low chI(T) values ( < 1 mg m(-3)) during 1998 and 2003 summer seasons, and (3) positive or negative trends (decrease vs increase of mean cell size) in specific domains of central and northern stations. Temporal transitions in cell size coincided with a switch on ENSO and SAM anomalies as well as increase of heat content of shelf waters over the WAP region. The lack of offshore spring bloom and summer shelf bloom most likely explains the dominance of relative small phytoplankton cells during the 1998 and 2003 summer seasons. A greater frequency of southerly winds during spring and autumn is expected to favor the dominance of 'small' (< 20 mu m) phytoplankton cells over WAP waters. Conversely, the greater intensification of the Antarctic Circumpolar Current interaction with the WAP shelf-break during SAM+ years is expected to intensify topographically induced upwelling and favor the dominance of 'large' ( > 20 mu m) phytoplankton cells on slope waters of central stations. The well-described 50-year warming trend in the Antarctic Peninsula has not resulted in a consistent trend in phytoplankton size structure, as originally hypothesized, but a mosaic of trends attributed to anomalous mesoscale changes of sea-ice extent and circulation patterns. (C) 2008 Published by Elsevier Ltd.

2007
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.

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.