Nitrogen fixation in a turbulent ocean: mesoscale forcing of

J. Mar. Res. 54, 311-342. [4] Zehr, J. P. et al., 2001, Unicellular cyanobacteria fix N2 in the sub- tropical North Pacific Ocean. Nature, 412, 635 - 638. 20-Bou-a.
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AICME II abstracts

Small and mesoscale processes in the ocean and their ...

Nitrogen fixation in a turbulent ocean: mesoscale forcing of phytoplankton-Trichodesmium-nutrient dynamics Khalid Boushaba1 , Mercedes Pascual2 and Glenn Flierl3 . A modeling study was conducted to examine the effect of mesoscale eddies on the dynamics of phytoplankton under nitrogen fixation. Two functional groups are included in the phytoplankton component to represent nitrogen fixers (Trichodesmium) and non-nitrogen fixers. The ecological feedbacks between these functional groups are both negative, through competition for light, and positive, through nitrogen fixation. To examine the effect of mesoscale spatio-temporal variability on phytoplankton productivity, the ecological model (PTN for Phytoplankton-Trichodesmium-Nutrients, cf. [1]) is coupled to an eddy model consisting of a quasigeostrophic 1 1/2 layer approximation. The coupling includes two main effects: the horizontal advection by the flows of the biota laterally and the vertical displacement of the base of the mixed layer, which can cause entrainment leading to the dilution of plankton and the input of nutrients. To establish the existence of a critical biological parameter, the ratio of normalized growth rates, we first analyze the ecological dynamics in the absence of eddy forcing. The resulting bifurcation diagram is used to select two different biological scenarios corresponding to nitrogen fixation by (1) Trichodesmium and (2) small cells with faster growth presumed to exist in the ocean but not yet identified (see [4]). The bifurcation analysis is shown to be relevant to the case with eddy forcing. It provides a basis to

Small and mesoscale processes in the ocean and their ...

understand the successional sequence observed in the simulations, which includes a secondary phytoplankton bloom in space and time. Comparisons of simulations of the PTN model with and without eddies and mean nutrient concentrations adjusted to equal levels, show that forcing can increase phytoplankton productivity but only for the small cells’ case. The role of spatial variability in this effect is discussed.

References [1] Boushaba, K. & M. Pascual, Dynamics of the ’echo’ effect in a phytoplankton system with nitrogen fixation. (submitted) [2] Flierl, G. R. & C. S. Davis, 1993, Biological Effects of Gulf Strean Meandering, J. MAR. RES., 51, 529-560. [3] Dadou, I. & V. Gar¸con & G. R. Flierl & C. S. Davis, 1996, Impact of the North Equatorial Current meandering on a pelagic ecosystem: A modeling approach. J. Mar. Res. 54, 311-342. [4] Zehr, J. P. et al., 2001, Unicellular cyanobacteria fix N2 in the subtropical North Pacific Ocean. Nature, 412, 635 - 638.

1

Department of Ecology and Evolutionary Biology, University of Michigan Ann Arbor, MI 48109-1048 USA (e-mail: [email protected]). 2 Department of Ecology and Evolutionary Biology, University of Michigan Ann Arbor, MI 48109-1048 USA (e-mail: [email protected]). 3 Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA 02139-4307 USA (e-mail: [email protected]).

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AICME II abstracts

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