Small ponds, and the communities they contain, can be subjected to a variety of potential modifications by humans. Many such ecosystems are altered by the introduction of fish and by the run-off of agricultural fertilizers, such as ammonium nitrate. In a mesocosm experiment, we examined the effects of an invasive fish (mosquitofish, Gambusia affinis) and the addition of ammonium nitrate on a simple pond community. We measured the effects of the factorial combination of these factors on Green Frog (Rana clamitans) tadpoles, zooplankton, phytoplankton, and periphyton.

Results/Conclusions

Ammonium nitrate did not affect tadpole survival, but did increase mean tadpole mass at the end of the experiment. Mosquitofish had a strong negative effect on tadpole survivorship, but a positive effect on the mean mass of tadpoles at the end of the experiment. In general, zooplankton were unaffected by ammonium nitrate and there was no interaction between ammonium nitrate and mosquitofish. In contrast, there was a significant effect of mosquitofish on zooplankton that differed between taxa (e.g., positive for the rotifer Brachionus, negative for Daphnia). Ammonium nitrate and mosquitofish both had positive effects on chlorophyll a levels (an index of phytoplankton abundance), and there was a significant interaction between ammonium nitrate and mosquitofish such that mesocosms with both mosquitofish and ammonium nitrate had substantially more chlorophyll a than other mesocosms. There was more periphyton in mesocosms with mosquitofish than mesocosms without mosquitofish. Ammonium nitrate and the interaction between mosquitofish and ammonium nitrate had no effect on periphyton. Our results suggest that mosquitofish had a greater effect on this simple aquatic community than did ammonium nitrate. The introduction of these small fish to the communities of small ponds can clearly have big effects.