Seasonal, inter-annual and long term variation in top-down versus bottom-up regulation of primary production

Environmental change strongly affects primary productivity of ecosystems via modifYing bo([om-up and top-down regulation of primary producers. Here we present a novel approach to quantifY the relative importance of regulating factors in natural systems over various time scales: we calculated daily effect sizes of major factors affecting phytoplankron , growth during the spring bloom period during almost three decades oflake oligotrophication using numerical experiments with a data based simulation model. We show that with oligotrophication the regulation of spring phytoplankton shifts from primarily top-down to bo([om-up, and that the changes in regulation are non-linearly related to the nutrient (phosphorus) concentrations. Our findings indicate that long-term changes in rop-down regulation cannot be undersrood without considering multiple herbivore taxa, here, microzooplankton (ciliates) and mesozooplankton (daphnids). We further demonstrate that bottomup and topdown regulation are not independent from each other and that their interaction is time-scale dependent.