Toole Dierdre A.

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Dierdre A.

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Now showing 1 - 5 of 5
  • Article
    A light-driven, one-dimensional dimethylsulfide biogeochemical cycling model for the Sargasso Sea
    (American Geophysical Union, 2008-04-12) Toole, Dierdre A. ; Siegel, David A. ; Doney, Scott C.
    We evaluate the extent to which dimethylsulfide (DMS) cycling in an open-ocean environment can be constrained and parameterized utilizing emerging evidence for the significant impacts of solar ultraviolet radiation (UVR) on the marine organic sulfur cycle. Using the Dacey et al. (1998) 1992–1994 Sargasso Sea DMS data set, in conjunction with an offline turbulent mixing model, we develop and optimize a light driven, one-dimensional DMS model for the upper 140 m. The DMS numerical model is primarily diagnostic in that it incorporates observations of bacterial, phytoplankton, physical, and optical quantities concurrently measured as part of the Bermuda Atlantic Time-series Study (BATS) and Bermuda Bio-Optical Project (BBOP) programs. With the exception of sea-to-air ventilation, each of the sulfur cycling terms is explicitly parameterized or altered by the radiation field. Overall, the model shows considerable skill in capturing the salient features of the DMS distribution, specifically the observed DMS summer paradox whereby peak summer DMS concentrations occur coincident with annual minima in phytoplankton pigment biomass and primary production. The dominant processes controlling the upper-ocean DMS concentrations are phytoplankton UVR-induced DMS release superimposed upon more surface oriented processes such as photolysis and sea-to-air ventilation. The results also demonstrate that mixing alone is not enough to parameterize DMS distributions in this environment. It is critical to directly parameterize the seasonal changes in the flux and attenuation of solar radiation in the upper water column to describe the DMS distribution with depth and allow for experimentation under a variety of climate change scenarios.
  • Article
    Light-driven cycling of dimethylsulfide (DMS) in the Sargasso Sea : closing the loop
    (American Geophysical Union, 2004-05-06) Toole, Dierdre A. ; Siegel, David A.
    The factors driving dimethylsulfide (DMS) cycling in oligotrophic environments are isolated using a time-series of DMS sampled in the Sargasso Sea. The observed distribution of DMS is inconsistent with bottom-up processes related to phytoplankton production, biomass, or community structure changes. DMS concentrations and estimates of net biological community production are most highly correlated with physical and optical properties, with the dose of ultraviolet radiation (UVR) accounting for 77% of the variability in mixed layer DMS concentrations. Physiological stresses associated with shallow mixed layers and high UVR are the first order determinant of biological production of DMS, indicating that DMS cycling in open-ocean regions is fundamentally different than in eutrophic regions where phytoplankton blooms provide the conditions for elevated DMS concentrations. The stress regime presented here effectively closes the DMS-climate feedback loop for open-ocean environments. This response may also provide a climatic role for phytoplanktonic processes in response to anthropogenic forcing.
  • Preprint
    Environmental, biochemical and genetic drivers of DMSP degradation and DMS production in the Sargasso Sea
    ( 2011-10) Levine, Naomi M. ; Varaljay, Vanessa A. ; Toole, Dierdre A. ; Dacey, John W. H. ; Doney, Scott C. ; Moran, Mary Ann
    Dimethylsulfide (DMS) is a climatically relevant trace gas produced and cycled by the surface ocean food web. Mechanisms driving intraannual variability in DMS production and dimethylsulfoniopropionate (DMSP) degradation in open-ocean, oligotrophic regions were investigated during a 10 month time-series at the Bermuda Atlantic Time-series Study site in the Sargasso Sea. Abundance and transcription of bacterial DMSP degradation genes, DMSP lyase enzyme activity, and DMS and DMSP concentrations, consumption rates, and production rates were quantified over time and depth. This interdisciplinary dataset was used to test current hypotheses of the role of light and carbon supply in regulating upper-ocean sulfur cycling. Findings supported UV-A dependent phytoplankton DMS production. Bacterial DMSP degraders may also contribute significantly to DMS production when temperatures are elevated and UV-A dose is moderate, but may favor DMSP demethylation under low UV-A doses. Three groups of bacterial DMSP degraders with distinct intraannual variability were identified and niche differentiation was indicated. The combination of genetic and biochemical data suggest a modified ‘bacterial switch’ hypothesis where the prevalence of different bacterial DMSP degradation pathways is regulated by a complex set of factors including carbon supply, temperature, and UV-A dose.
  • Article
    Revising upper-ocean sulfur dynamics near Bermuda : new lessons from 3 years of concentration and rate measurements
    (CSIRO Publishing, 2015-11-10) Levine, Naomi M. ; Toole, Dierdre A. ; Neeley, Aimee ; Bates, Nicholas R. ; Doney, Scott C. ; Dacey, John W. H.
    Oceanic biogeochemical cycling of dimethylsulfide (DMS), and its precursor dimethylsulfoniopropionate (DMSP), has gained considerable attention over the past three decades because of the potential role of DMS in climate mediation. Here we report 3 years of monthly vertical profiles of organic sulfur cycle concentrations (DMS, particulate DMSP (DMSPp) and dissolved DMSP (DMSPd)) and rates (DMSPd consumption, biological DMS consumption and DMS photolysis) from the Bermuda Atlantic Time-series Study (BATS) site taken between 2005 and 2008. Concentrations confirm the summer paradox with mixed layer DMS peaking ~90 days after peak DMSPp and ~50 days after peak DMSPp : Chl. A small decline in mixed layer DMS was observed relative to those measured during a previous study at BATS (1992–1994), potentially driven by long-term climate shifts at the site. On average, DMS cycling occurred on longer timescales than DMSPd (0.43 ± 0.35 v. 1.39 ± 0.76 day–1) with DMSPd consumption rates remaining elevated throughout the year despite significant seasonal variability in the bacterial DMSP degrader community. DMSPp was estimated to account for 4–5 % of mixed layer primary production and turned over at a significantly slower rate (~0.2 day–1). Photolysis drove DMS loss in the mixed layer during the summer, whereas biological consumption of DMS was the dominant loss process in the winter and at depth. These findings offer new insight into the underlying mechanisms driving DMS(P) cycling in the oligotrophic ocean, provide an extended dataset for future model evaluation and hypothesis testing and highlight the need for a reexamination of past modelling results and conclusions drawn from data collected with old methodologies.
  • Article
    High dimethylsulfide photolysis rates in nitrate-rich Antarctic waters
    (American Geophysical Union, 2004-06-09) Toole, Dierdre A. ; Kieber, David J. ; Kiene, Ronald P. ; White, E. M. ; Bisgrove, J. ; del Valle, Daniela A. ; Slezak, D.
    The photochemistry of dimethylsulfide (DMS) was examined in the Southern Ocean to assess its impact on the biogeochemical dynamics of DMS in Antarctic waters. Very high DMS photolysis rate constants (0.16–0.23 h−1) were observed in surface waters exposed to full sunlight. DMS photolysis rates increased linearly with added nitrate concentrations, and 35% of the DMS loss in unamended samples was attributed to the photochemistry of ambient nitrate (29 μM). Experiments with optical filters showed that the UV-A band of sunlight (320–400 nm) accounted for ~65% of DMS photolysis suggesting that dissolved organic matter was the main photosensitizer for DMS photolysis. During the austral spring, DMS photolysis was the dominant loss mechanism under non-bloom and non-ice cover conditions owing to the high doses and deep penetration of UV radiation in the water column, low observed microbial consumption rates, and high in situ nitrate concentrations.