Abstract

In aquatic food webs, zooplankton grazing is an important factor that affects the flow of energy and materials from lower trophic food chain levels. Underwater light climate may however affect this process of energy and material transfer to higher trophic levels and induce a system shifts, from phytoplankton dominated food webs at high light availability towards bacterial dominated food webs at low light availability. In addition, fish predation on zooplankton may reduce zooplankton density and zooplankton grazing rates and thus, hinder energy transfer to higher trophic levels. The aim of this study was to investigate the importance of light as potential driving force of zooplankton grazing rates in the lake. An outdoor mesocosm experiment with 4 replicated treatments (control, light reduction, fish predation and combined light reduction and fish predation) were performed to investigate the effects of light reduction, fish predation and combined light reduction and fish predation on zooplankton grazing rate in Lake Erken. The three hypothesis tested were: (1) Light reduction minimize phytoplankton growth leads to decreased zooplankton grazing rate. (2) Presence of fish leads to increased predation on zooplankton which decreases the biomass of the zooplankton community and thereby reduces zooplankton grazing rate on phytoplankton. (3) Light reduction and fish presence affect zooplankton grazing rate synergistically. Our results indicate that there were negative effect of light reduction on zooplankton grazing rate and there were no effects of fish predation and combined fish predation and light reduction. From this experiment, results showed that increased light intensity reduced phytoplankton biomass, particularly in the light reduction treatment. This study added to increasing evidence that effect of light can reduce zooplankton grazing rates and can limit nutrient transport to higher trophic levels.

Key words: phytoplankton, climate change, terrestrial runoff, cloudiness, incubation