Relative strength of top-down effects of an invasive fish and bottom-up effects of nutrient addition in a simple aquatic food web.

Introduction of exotic predators or runoff of fertilizers can alter aquatic food webs, in particular zooplankton communities, through top-down and bottom-up effects. In a mesocosm experiment, we manipulated the density of Western Mosquitofish (Gambusia affinis) and nutrient levels (nitrate and phosphate independently) and observed effects on zooplankton and phytoplankton in a fall, temperate zone system. If top-down regulation were important, we expected mosquitofish predation to reduce zooplankton abundance, which would indirectly benefit phytoplankton. If bottom-up regulation were important, we expected nutrient addition to increase both primary producers and zooplankton. Western Mosquitofish predation significantly decreased the abundance of several zooplankton taxa, resulting in a trophic cascade with increased chlorophyll a (i.e., primary productivity). This effect did not differ between mesocosms with 5 or 10 fish. Nutrient addition had no significant effects on zooplankton; however, chlorophyll a was positively affected by both nitrogen addition and phosphorus addition. Our results suggest weak bottom-up regulation in our experimental community, but strong top-down regulation, emphasizing the potential consequences of introducing non-native Western Mosquitofish to native aquatic ecosystems.