Food web dynamics in open and closed systems

Abstract: This thesis is a summary of enclosure and microcosm experiments that aimed to study the impact of allochtonous subsidies on food web dynamics in a heterogeneous food web. In the enclosure studies, a three trophic level littoral food web was used. The food web consisted of two growth forms of primary producers: phytoplankton and periphyton and their associated herbivores: scraping and filtering herbivores. The predator used, YOY perch, affects both pathways in the food web. Manipulation of the openness for the different trophic levels showed that the inflow of phytoplankton and cross-habitat foraging by the herbivore level reinforced the compensatory response between the two growth forms of primary producers In the microcosm experiment, the response of an herbivore food web and a microbial community to inflow of resources and food web configuration was studied, using a model food web. The model food web consisted of two forms of primary producers, i.e. phytoplankton and periphyton, and two herbivores, i.e. Daphnia pulex feeding on phytoplankton, and Chydorus sphaericus feeding on both periphyton and phytoplankton. Three different food web configurations, all having the phytoplankton and periphyton, but either one of the herbivores, or both, were set up. The flow regimes consisted of an open treatment receiving a constant supply of phytoplankton, and a closed treatment with an initial resource pool. The effect of the inflow of phytoplankton was affected by the food web configuration. In the presence of D. pulex, the inflow of phytoplankton was made accessible to periphyton, and indirectly to C. sphaericus, which increased to such high densities that D. pulex was negatively affected. The inflow of phytoplankton had an indirect negative effect on the microbial community, since the biomass of herbivores increased, which imposed a higher grazing pressure on all parts of the microbial community.