Bibby Thomas S.

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Thomas S.
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  • Preprint
    Temporal progression of photosynthetic-strategy in phytoplankton in the Ross Sea, Antarctica
    ( 2015-12) Ryan-Keogh, Thomas J. ; DeLizo, Liza M. ; Smith, Walker O. ; Sedwick, Peter N. ; McGillicuddy, Dennis J. ; Moore, C. Mark ; Bibby, Thomas S.
    The bioavailability of iron influences the distribution, biomass and productivity of phytoplankton in the Ross Sea, one of the most productive regions in the Southern Ocean. We mapped the spatial and temporal extent and severity of iron-limitation of the native phytoplankton assemblage using long- (>24 h) and short-term (24 h) iron- addition experiments along with physiological and molecular characterisations during a cruise to the Ross Sea in December-February 2012. Phytoplankton increased their photosynthetic efficiency in response to iron addition, suggesting proximal iron limitation throughout most of the Ross Sea during summer. Molecular and physiological data further indicate that as nitrate is removed from the surface ocean the phytoplankton community transitions to one displaying an iron-efficient photosynthetic strategy characterised by an increase in the size of photosystem II (PSII) photochemical cross section (σPSII) and a decrease in the chlorophyll-normalised PSII abundance. These results suggest that phytoplankton with the ability to reduce their photosynthetic iron requirements are selected as the growing season progresses, which may drive the well-documented progression from Phaeocystis antarctica- assemblages to diatom-dominated phytoplankton. Such a shift in the assemblage-level photosynthetic strategy potentially mediates further drawdown of nitrate following the development of iron deficient conditions in the Ross Sea.
  • Article
    Controls on dissolved cobalt in surface waters of the Sargasso Sea : comparisons with iron and aluminum
    (American Geophysical Union, 2012-05-19) Shelley, Rachel U. ; Sedwick, Peter N. ; Bibby, Thomas S. ; Cabedo-Sanz, P. ; Church, Thomas M. ; Johnson, Rodney J. ; Macey, A. I. ; Marsay, Christopher M. ; Sholkovitz, Edward R. ; Ussher, Simon J. ; Worsfold, Paul J. ; Lohan, Maeve C.
    Dissolved cobalt (dCo), iron (dFe) and aluminum (dAl) were determined in water column samples along a meridional transect (~31°N to 24°N) south of Bermuda in June 2008. A general north-to-south increase in surface concentrations of dFe (0.3–1.6 nM) and dAl (14–42 nM) was observed, suggesting that aerosol deposition is a significant source of dFe and dAl, whereas no clear trend was observed for near-surface dCo concentrations. Shipboard aerosol samples indicate fractional solubility values of 8–100% for aerosol Co, which are significantly higher than corresponding estimates of the solubility of aerosol Fe (0.44–45%). Hydrographic observations and analysis of time series rain samples from Bermuda indicate that wet deposition accounts for most (>80%) of the total aeolian flux of Co, and hence a significant proportion of the atmospheric input of dCo to our study region. Our aerosol data imply that the atmospheric input of dCo to the Sargasso Sea is modest, although this flux may be more significant in late summer. The water column dCo profiles reveal a vertical distribution that predominantly reflects ‘nutrient-type’ behavior, versus scavenged-type behavior for dAl, and a hybrid of nutrient- and scavenged-type behavior for dFe. Mesoscale eddies also appear to impact on the vertical distribution of dCo. The effects of biological removal of dCo from the upper water column were apparent as pronounced sub-surface minima (21 ± 4 pM dCo), coincident with maxima in Prochlorococcus abundance. These observations imply that Prochlorococcus plays a major role in removing dCo from the euphotic zone, and that the availability of dCo may regulate Prochlorococcus growth in the Sargasso Sea.
  • 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.