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2019
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.

1996
Vernet, M, Mitchell BG, Sakshaug E, Johnsen G, Iturriaga R, Wassmann P.  1996.  Evidence for a novel pigment with in vivo absorption maximum at 708 nm associated with Phaeocystis cf pouchetii blooms. Marine Ecology-Progress Series. 133:253-262.   10.3354/meps133253   AbstractWebsite

The presence of a chlorophyll a-like (chi a) pigment, with an in vivo absorption maximum in the near-infrared region at 708 to 712 nm, was observed mainly in sedimenting material and Calanus hyperboreus fecal pellets associated with Phaeocystis cf. pouchetii Harlot blooms. (In vivo absorption is attributed to natural absorption found in naturally occurring particulate matter and seston. In vitro absorption refers to extracts in organic solvents.) This absorption peak was observed in conjunction with the absorption peak at 674 to 676 nm, commonly attributed to chi a and its derivatives. The in vivo absorption maximum in the near infrared, centered around 708 nm, was observed only in particulate matter and not in methanolic and aqueous acetonic extracts. Absorption efficiency (Q(a)) of individual particles measured by microphotometry revealed particles 3 to 4 mu m in diameter with an in vivo absorption maximum at 708 to 713 nm and no in vivo absorption peak at 676 nm, as expected for chlorophylls and its phaeopigments, indicating a different type of particle in the sample. The visible spectrum also had a broad absorption peak in the blue region, between 420 and 450 nm, suggesting a chlorophyll-like spectrum. The main chi degradation product analyzed by high-performance liquid chromatography of sedimenting matter was identified as a phaeophorbide a-like pigment, with absorption maximum in the red at 665 to 666 mn in organic solvents, with no indication of a pigment with in vitro absorption properties in the near infrared. The accumulation of the 708 nm in vivo absorption peak in particles associated with P. cf. pouchetii seems to be a widespread feature as it was observed during 3 different cruises to the Barents Sea, Fram Strait and the Kattegat. We propose 3 different hypotheses on the origin of this novel. peak in in vivo absorption not previously observed in marine environments: (1) P. cf. pouchetii has a chi a breakdown pathway which promotes the accumulation of a known chi degradation product with an in vivo absorption at 708 mn; (2) there is a new chl degradation product produced by grazing of C. hyperboreus on P. cf. pouchetii, as yet to be isolated; and (3) there is a new pigment, synthesized by either P. cf. pouchetii or another organism associated with this alga, during mature blooms of Phaeocystis. These hypotheses are discussed in view of the available evidence.