van Dijken Gert L.

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van Dijken
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Gert L.
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  • Preprint
    The influence of winter water on phytoplankton blooms in the Chukchi Sea
    ( 2015-06) Lowry, Kate E. ; Pickart, Robert S. ; Mills, Matthew M. ; Brown, Zachary W. ; van Dijken, Gert L. ; Bates, Nicholas R. ; Arrigo, Kevin R.
    The flow of nutrient-rich winter water (WW) through the Chukchi Sea plays an important and previously uncharacterized role in sustaining summer phytoplankton blooms. Using hydrographic and biogeochemical data collected as part of the ICESCAPE program (June-July 2010-11), we examined phytoplankton bloom dynamics in relation to the distribution and circulation of WW (defined as water with potential temperature ≤ -1.6°C) across the Chukchi shelf. Characterized by high concentrations of nitrate (mean: 12.3 ± 5.13 μmol L-1) that typically limits primary production in this region, WW was correlated with extremely high phytoplankton biomass, with mean chlorophyll a concentrations that were three-fold higher in WW (8.64 ± 9.75 μg L-1) than in adjacent warmer water (2.79 ± 5.58 μg L-1). Maximum chlorophyll a concentrations (~30 μg L-1) were typically positioned at the interface between nutrient-rich WW and shallower, warmer water with more light availability. Comparing satellite-based calculations of open water duration to phytoplankton biomass, nutrient concentrations, and oxygen saturation revealed widespread evidence of under-ice blooms prior to our sampling, with biogeochemical properties indicating that blooms had already terminated in many places where WW was no longer present. Our results suggest that summer phytoplankton blooms are sustained for a longer duration along the pathways of nutrient-rich WW and that biological hotspots in this region (e.g. the mouth of Barrow Canyon) are largely driven by the flow and confluence of these extremely productive pathways of WW that flow across the Chukchi shelf.
  • Article
    ASPIRE : the Amundsen Sea Polynya International Research Expedition
    (The Oceanography Society, 2012-09) Yager, Patricia L. ; Sherrell, Robert M. ; Stammerjohn, Sharon E. ; Alderkamp, Anne-Carlijn ; Schofield, Oscar M. E. ; Abrahamsen, E. Povl ; Arrigo, Kevin R. ; Bertilsson, Stefan ; Garay, D. Lollie ; Guerrero, Raul ; Lowry, Kate E. ; Moksnes, Per-Olav ; Ndungu, Kuria ; Post, Anton F. ; Randall-Goodwin, Evan ; Riemann, Lasse ; Severmann, Silke ; Thatje, Sven ; van Dijken, Gert L. ; Wilson, Stephanie
    In search of an explanation for some of the greenest waters ever seen in coastal Antarctica and their possible link to some of the fastest melting glaciers and declining summer sea ice, the Amundsen Sea Polynya International Research Expedition (ASPIRE) challenged the capabilities of the US Antarctic Program and RVIB Nathaniel B. Palmer during Austral summer 2010–2011. We were well rewarded by both an extraordinary research platform and a truly remarkable oceanic setting. Here we provide further insights into the key questions that motivated our sampling approach during ASPIRE and present some preliminary findings, while highlighting the value of the Palmer for accomplishing complex, multifaceted oceanographic research in such a challenging environment.
  • Article
    Under-ice phytoplankton blooms inhibited by spring convective mixing in refreezing leads
    (John Wiley & Sons, 2018-01-07) Lowry, Kate E. ; Pickart, Robert S. ; Selz, Virginia ; Mills, Matthew M. ; Pacini, Astrid ; Lewis, Kate M. ; Joy-Warren, Hannah L. ; Nobre, Carolina ; van Dijken, Gert L. ; Grondin, Pierre-Luc ; Ferland, Joannie ; Arrigo, Kevin R.
    Spring phytoplankton growth in polar marine ecosystems is limited by light availability beneath ice-covered waters, particularly early in the season prior to snowmelt and melt pond formation. Leads of open water increase light transmission to the ice-covered ocean and are sites of air-sea exchange. We explore the role of leads in controlling phytoplankton bloom dynamics within the sea ice zone of the Arctic Ocean. Data are presented from spring measurements in the Chukchi Sea during the Study of Under-ice Blooms In the Chukchi Ecosystem (SUBICE) program in May and June 2014. We observed that fully consolidated sea ice supported modest under-ice blooms, while waters beneath sea ice with leads had significantly lower phytoplankton biomass, despite high nutrient availability. Through an analysis of hydrographic and biological properties, we attribute this counterintuitive finding to springtime convective mixing in refreezing leads of open water. Our results demonstrate that waters beneath loosely consolidated sea ice (84–95% ice concentration) had weak stratification and were frequently mixed below the critical depth (the depth at which depth-integrated production balances depth-integrated respiration). These findings are supported by theoretical model calculations of under-ice light, primary production, and critical depth at varied lead fractions. The model demonstrates that under-ice blooms can form even beneath snow-covered sea ice in the absence of mixing but not in more deeply mixed waters beneath sea ice with refreezing leads. Future estimates of primary production should account for these phytoplankton dynamics in ice-covered waters.
  • Article
    Nitrogen limitation of the summer phytoplankton and heterotrophic prokaryote communities in the Chukchi Sea
    (Frontiers Media, 2018-10-15) Mills, Matthew M. ; Brown, Zachary W. ; Laney, Samuel R. ; Ortega-Retuerta, Eva ; Lowry, Kate E. ; van Dijken, Gert L. ; Arrigo, Kevin R.
    Major changes to Arctic marine ecosystems have resulted in longer growing seasons with increased phytoplankton production over larger areas. In the Chukchi Sea, the high productivity fuels intense benthic denitrification creating a nitrogen (N) deficit that is transported through the Arctic to the Atlantic Ocean, where it likely fuels N fixation. Given the rapid pace of environmental change and the potentially globally significant N deficit, we conducted experiments aimed at understanding phytoplankton and microbial N utilization in the Chukchi Sea. Ship-board experiments tested the effect of nitrate (NO3-) additions on both phytoplankton and heterotrophic prokaryote abundance, community composition, photophysiology, carbon fixation and NO3- uptake rates. Results support the critical role of NO3- in limiting summer phytoplankton communities to small cells with low production rates. NO3- additions increased particulate concentrations, abundance of large diatoms, and rates of carbon fixation and NO3- uptake by cells >1 μm. Increases in the quantum yield and electron turnover rate of photosystem II in +NO3- treatments suggested that phytoplankton in the ambient dissolved N environment were N starved and unable to build new, or repair damaged, reaction centers. While some increases in heterotrophic prokaryote abundance and production were noted with NO3- amendments, phytoplankton competition or grazers likely dampened these responses. Trends toward a warmer more stratified Chukchi Sea will likely enhance summer oligotrophic conditions and further N starve Chukchi Sea phytoplankton communities.
  • Article
    Iron supply and demand in an Antarctic shelf ecosystem
    (John Wiley & Sons, 2015-10-08) McGillicuddy, Dennis J. ; Sedwick, Peter N. ; Dinniman, M. S. ; Arrigo, Kevin R. ; Bibby, Thomas S. ; Greenan, Blair J. W. ; Hofmann, Eileen E. ; Klinck, John M. ; Smith, Walker O. ; Mack, Stefanie L. ; Marsay, Christopher M. ; Sohst, Bettina M. ; van Dijken, Gert L.
    The Ross Sea sustains a rich ecosystem and is the most productive sector of the Southern Ocean. Most of this production occurs within a polynya during the November–February period, when the availability of dissolved iron (dFe) is thought to exert the major control on phytoplankton growth. Here we combine new data on the distribution of dFe, high-resolution model simulations of ice melt and regional circulation, and satellite-based estimates of primary production to quantify iron supply and demand over the Ross Sea continental shelf. Our analysis suggests that the largest sources of dFe to the euphotic zone are wintertime mixing and melting sea ice, with a lesser input from intrusions of Circumpolar Deep Water and a small amount from melting glacial ice. Together these sources are in approximate balance with the annual biological dFe demand inferred from satellite-based productivity algorithms, although both the supply and demand estimates have large uncertainties.