Emergence of Broadband Variability in a Marine Plankton Model Under External Forcing

Temporal variability in plankton community structure and biomass is often driven by environmental fluctuations: nutrient supplies, light, stratification and temperature. But plankton time series also exhibit variability that is not strongly correlated with key physical variables and is distinctly nonlinear in nature. There is evidence, from both laboratory and modeling studies, that oscillations can arise from ecological interactions alone. In the open ocean, it is challenging to establish the roles and relative importance of environmental versus intrinsic processes in generating the observed ecological variability. To explore this competition, we employ a marine plankton model that supports two mechanisms of intrinsic ecological variability operating at distinct frequencies: predator-prey interactions between zooplankton and phytoplankton, with timescales of weeks, and resource competition that occurs with multiple nutrients phytoplankton species, with timescales of years. The model is forced by imposing variable nutrient input rates. Representing typical open ocean situations, with periods ranging from subseasonal to multi-annual. We find that intrinsically-driven variability generally persists in the presence of extrinsic forcing, and that the interaction between the two can produce variability at frequencies that are not characteristic of either source. The intrinsic frequencies are found to be even more energetic when the extrinsic variability is augmented with stochastic noise. We conclude that interactions between intrinsic and extrinsic sources of variability may contribute to the wide range of observed frequencies in phytoplankton time series, and may explain why it is often difficult to relate planktonic variation to environmental variation alone.