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2018
Randelhoff, A, Reigstad M, Chierici M, Sundfjord A, Ivanov V, Cape M, Vernet M, Tremblay JE, Bratbak G, Kristiansen S.  2018.  Seasonality of the physical and biogeochemical hydrography in the inflow to the Arctic Ocean through Fram Strait. Frontiers in Marine Science. 5   10.3389/fmars.2018.00224   AbstractWebsite

Eastern Fram Strait and the shelf slope region north of Svalbard is dominated by the advection of warm, salty and nutrient-rich Atlantic Water (AW). This oceanic heat contributes to keeping the area relatively free of ice. The last years have seen a dramatic decrease in regional sea ice extent, which is expected to drive large increases in pelagic primary production and thereby changes in marine ecology and nutrient cycling. In a concerted effort, we conducted five cruises to the area in winter, spring, summer and fall of 2014, in order to understand the physical and biogeochemical controls of carbon cycling, for the first time from a year-round point of view. We document (1) the offshore location of the wintertime front between salty AW and fresher Surface Water in the ocean surface, (2) thermal convection of Atlantic Water over the shelf slope, likely enhancing vertical nutrient fluxes, and (3) the importance of ice melt derived upper ocean stratification for the spring bloom timing. Our findings strongly confirm the hypothesis that this "Atlantification," as it has been called, of the shelf slope area north of Svalbard resulting from the advection of AW alleviates both nutrient and light limitations at the same time, leading to increased pelagic primary productivity in this region.

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.

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.

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.

Svensen, C, Halvorsen E, Vernet M, Franze G, Dmoch K, Lavrentyev PJ, Kwasniewski S.  2019.  Zooplankton communities associated with new and regenerated primary production in the Atlantic inflow north of Svalbard. Frontiers in Marine Science. 6   10.3389/fmars.2019.00293   AbstractWebsite

The Arctic Ocean is changing rapidly with respect to ice cover extent and volume, growth season duration and biological production. Zooplankton are important components in the arctic marine food web, and tightly coupled to the strong seasonality in primary production. In this study, we investigate zooplankton composition, including microzooplankton, copepod nauplii, as well as small and large copepod taxa, and primary productivity in the dynamic Atlantic water inflow area north of Svalbard in May and August 2014. We focus on seasonal differences in the zooplankton community and in primary productivity regimes. More specifically, we examine how a shift from "new" (nitrate based) spring bloom to a "regenerated" (ammonium based) post bloom primary production is reflected in the diversity, life history adaptations and productivity of the dominant zooplankton. North of Svalbard, the seasonal differences in planktonic communities were significant. In spring, the large copepod Calanus finmarchicus dominated, but the estimated production and ingestion rates were low compared to the total primary production. In summer, the zooplankton community was composed of microzooplankton and the small copepod Oithona similis. The zooplankton production and ingestion rates were high in summer, and probably depended heavily on the regenerated primary production associated with the microbial loop. There was clear alteration from dominance of calanoid copepod nauplii in spring to Oithona spp. nauplii in summer, which indicates different reproductive strategies of the dominating large and small copepod species. Our study confirms the dependence and tight coupling between the new (spring bloom) primary production and reproductive adaptations of C. glacialis and C. hyperboreus. In contrast, C. finmarchicus appears able to take advantage of the regenerated summer primary production, which allows it to reach the overwintering stage within one growth season in this region north of Svalbard. This suggests that C. finmarchicus will be able to profit from the predicted increased primary production in the Arctic, a strategy also recognized in small copepod species such as O. similis. We speculate that the ability of the copepod species to utilize the regenerated summer primary production and microbial food web may determine the winners and losers in the future Arctic Ocean.

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.