WEAK DENSITY-DEPENDENCE AND SHORT-TERM PERTURBATIONS AS DETERMINANTS OF PHYTOPLANKTON TEMPORAL DYNAMICS

We examined twelve temporal series of phytoplankton abundance in a div erse set of marine and freshwater habitats using semi-variogram analys is 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 estimat ed density-dependence parameters were remarkably similar in the variou s series and implied very weak regulation in phytoplankton abundance. Our results suggest that the magnitude of the daily variation in growt h rates induced by density-independent factors increases with nutrient availability. The model also describes the general form of the mean-v ariance (temporal) relationship and is nearly identical to the empiric al equation.