Aquatic food webs can be altered by humans through intentional or accidental changes to the top or base of the web, for example by the introduction of an exotic predator or a change in nutrient loading.  However, the relative importance of such alterations remains unclear, especially for food webs in the limnetic zone of small ponds.  We examined the relative strength of top-down and bottom-up regulation in a simple food web that mimicked the open water community found in local small ponds.  We manipulated the presence or absence of top-down and bottom-up factors in a replicated mesocosm experiment by changing the density of an invasive, zooplanktivorous fish (mosquitofish, Gambusia affinis) and by adding nutrients (nitrate and phosphate). 

Results/Conclusions

Zooplankton abundance in general was lower in mesocosms with mosquitofish, but was unaffected by nutrient additions.  The patterns were different for specific elements of the zooplankton community. Adult copepod abundance and total cladoceran abundance (Daphnia  and Ceriodaphnia pooled) were negatively affected by mosquitofish presence, but not by nutrient addition or the interaction of mosquitofish and nutrient addition.  There was a significant reduction of Bosmina abundance in the presence of higher densities of mosquitofish, as well as a significant interaction between mosquitofish presence and nutrient addition. Thus some zooplankton responded to the density of the mosquitofish while other zooplankton responded simply to the presence or absence of mosquitofish.  Chlorophyll a levels, an index of phytoplankton abundance, increased with mosquitofish presence, but were unaffected by the addition of nutrients.  There was a negative relationship between cladoceran abundance and chlorophyll a levels. Taken together our results suggest that in this simple experimental food web, the top-down effects of an invasive fish were greater than the bottom-up effects of nutrient addition, thus creating a trophic cascade.  The invasion of previously fishless ponds by mosquitofish could therefore have wide ranging impacts on pond ecosystems.