Modeling carbohydrate ballasting by Trichodesmium spp.

We report the development of a numerical model simulating vertical movement of the cyanobacterium Trichodesmium spp. Given a range of physiological parameters derived from the literature, resultant model solutions allow us to explore the ecological significance of vertical migration by Trichodesmium colonies in a stratified oligotrophic oceanic system such as the North Pacific subtropical gyre (NPSG). Whereas the dominant type of model solution results in trapping of the migrating colony at the water surface, the next most frequent type of model solution is consistent with colony migration to the average depth of the phosphocline, followed by return to the upper euphotic zone, a scenario termed P-mining. Given the temporal phasing of these migrations, our results indicate that while Trichodesmium colonies could fulfill their entire P-quota in a single phosphocline-scale migration, the minimal colony size required for such a migration would be approximately 1000 mu m in radius. Colonies of this size and larger have been observed in nature. However, neither the frequency of occurrence nor the relative size distributions of natural populations is known. Using published estimates of colony abundance at Station ALOHA (22 degrees 45'N, 158 degrees 00'W, in the NPSG), we estimate that upward fluxes of P associated with Trichodesmium migration represent as much as 10% of the P-based export flux measured at this site. Inclusion of this simple model into coupled biophysical models should allow for better simulations of the vertical distribution of Trichodesmium biomass and thus improved representations of Trichodesmium productivity.