Weak density-dependence and short-term perturbations as determinants of phytoplankton temporal dynamics

We examined twelve temporal series of phytoplankton abundance in a diverse set of marine and freshwater habitats using semi-variogram analysis coupled with a stochastic discrete-time (daily) Gompertz model. The analysis revealed a very good correspondance between the theoretical semi-variogram function and the empirical semi-variograms. The estimated density-dependence parameters were remarkably similar in the various series and implied very weak regulation in phytoplankton abundance. Our results suggest that the magnitude of the daily variation in growth rates induced by density-independent factors increases with nutrient availability. The model also describes the general form of the mean-variance (temporal) relationship and is nearly identical to the empirical equation.