Time-series observations of geochemical tracers and diazotroph abundances in the northern subtropical gyres suggest variability in nitrogen fixation on interannual and longer timescales. Using a highly idealized model of the biogeochemistry and ecology of a subtropical gyre, we explore the previously proposed hypothesis that such variability is regulated by an internal "biogeochemical oscillator." We find, in certain parameter regimes, self-sustained oscillations in nitrogen fixation, community structure and biogeochemical cycles even with perfectly steady physical forcing. During the oscillations of nitrogen fixation, "blooms" of diazotrophs occur at intervals between a year and several decades, consistent with the observed variability. The period of the oscillations is strongly regulated by the exchange rate between the thermocline and mixed-layer waters. The oscillatory solutions occur in a relatively small region of parameter space, but one in which the relative fitness of diazotrophs and non-diazotrophs are closely matched and the time-averaged biomass of each class of phytoplankton is maximized.
Monteiro, Fanny M., and Michael J. Follows. 2009. "On the interannual variability of nitrogen fixation in the subtropical gyres." Journal of Marine Research 67, (1). https://elischolar.library.yale.edu/journal_of_marine_research/223