Assmy Philipp

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  • Article
    Synthesis of iron fertilization experiments : from the Iron Age in the Age of Enlightenment
    (American Geophysical Union, 2005-09-28) Baar, Hein J. W. de ; Boyd, Philip W. ; Coale, Kenneth H. ; Landry, Michael R. ; Tsuda, Atsushi ; Assmy, Philipp ; Bakker, Dorothee C. E. ; Bozec, Yann ; Barber, Richard T. ; Brzezinski, Mark A. ; Buesseler, Ken O. ; Boye, Marie ; Croot, Peter L. ; Gervais, Frank ; Gorbunov, Maxim Y. ; Harrison, Paul J. ; Hiscock, William T. ; Laan, Patrick ; Lancelot, Christiane ; Law, Cliff S. ; Levasseur, Maurice ; Marchetti, Adrian ; Millero, Frank J. ; Nishioka, Jun ; Nojiri, Yukihiro ; van Oijen, Tim ; Riebesell, Ulf ; Rijkenberg, Micha J. A. ; Saito, Hiroaki ; Takeda, Shigenobu ; Timmermans, Klaas R. ; Veldhuis, Marcel J. W. ; Waite, Anya M. ; Wong, Chi-Shing
    Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.
  • Article
    The seeding of ice algal blooms in Arctic pack ice : the multiyear ice seed repository hypothesis
    (John Wiley & Sons, 2017-07-03) Olsen, Lasse M. ; Laney, Samuel R. ; Duarte, Pedro ; Kauko, Hanna Maria ; Fernández-Méndez, Mar ; Mundy, Christopher J. ; Rösel, Anja ; Meyer, Amelie ; Itkin, Polona ; Cohen, Lana ; Peeken, Ilka ; Tatarek, Agnieszka ; Róźańska-Pluta, Magdalena ; Wiktor, Jozef ; Taskjelle, Torbjørn ; Pavlov, Alexey K. ; Hudson, Stephen R. ; Granskog, Mats A. ; Hop, Haakon ; Assmy, Philipp
    During the Norwegian young sea ICE expedition (N-ICE2015) from January to June 2015 the pack ice in the Arctic Ocean north of Svalbard was studied during four drifts between 83° and 80°N. This pack ice consisted of a mix of second year, first year, and young ice. The physical properties and ice algal community composition was investigated in the three different ice types during the winter-spring-summer transition. Our results indicate that algae remaining in sea ice that survived the summer melt season are subsequently trapped in the upper layers of the ice column during winter and may function as an algal seed repository. Once the connectivity in the entire ice column is established, as a result of temperature-driven increase in ice porosity during spring, algae in the upper parts of the ice are able to migrate toward the bottom and initiate the ice algal spring bloom. Furthermore, this algal repository might seed the bloom in younger ice formed in adjacent leads. This mechanism was studied in detail for the dominant ice diatom Nitzschia frigida. The proposed seeding mechanism may be compromised due to the disappearance of older ice in the anticipated regime shift toward a seasonally ice-free Arctic Ocean.
  • Article
    Leads in Arctic pack ice enable early phytoplankton blooms below snow-covered sea ice
    (Nature Publishig Group, 2019-01-17) Assmy, Philipp ; Fernández-Méndez, Mar ; Duarte, Pedro ; Meyer, Amelie ; Randelhoff, Achim ; Mundy, Christopher J. ; Olsen, Lasse M. ; Kauko, Hanna Maria ; Bailey, Allison ; Chierici, Melissa ; Cohen, Lana ; Doulgeris, Anthony P. ; Ehn, Jens K. ; Fransson, Agneta ; Gerland, Sebastian ; Hop, Haakon ; Hudson, Stephen R. ; Hughes, Nick ; Itkin, Polona ; Johnsen, Geir ; King, Jennifer A. ; Koch, Boris P. ; Koenig, Zoe ; Kwasniewski, Slawomir ; Laney, Samuel R. ; Nicolaus, Marcel ; Pavlov, Alexey K. ; Polashenski, Christopher M. ; Provost, Christine ; Rösel, Anja ; Sandbu, Marthe ; Spreen, Gunnar ; Smedsrud, Lars H. ; Sundfjord, Arild ; Taskjelle, Torbjørn ; Tatarek, Agnieszka ; Wiktor, Jozef ; Wagner, Penelope M. ; Wold, Anette ; Steen, Harald ; Granskog, Mats A.
    The Arctic icescape is rapidly transforming from a thicker multiyear ice cover to a thinner and largely seasonal first-year ice cover with significant consequences for Arctic primary production. One critical challenge is to understand how productivity will change within the next decades. Recent studies have reported extensive phytoplankton blooms beneath ponded sea ice during summer, indicating that satellite-based Arctic annual primary production estimates may be significantly underestimated. Here we present a unique time-series of a phytoplankton spring bloom observed beneath snow-covered Arctic pack ice. The bloom, dominated by the haptophyte algae Phaeocystis pouchetii, caused near depletion of the surface nitrate inventory and a decline in dissolved inorganic carbon by 16 ± 6 g C m−2. Ocean circulation characteristics in the area indicated that the bloom developed in situ despite the snow-covered sea ice. Leads in the dynamic ice cover provided added sunlight necessary to initiate and sustain the bloom. Phytoplankton blooms beneath snow-covered ice might become more common and widespread in the future Arctic Ocean with frequent lead formation due to thinner and more dynamic sea ice despite projected increases in high-Arctic snowfall. This could alter productivity, marine food webs and carbon sequestration in the Arctic Ocean.