Zooplankton are extremely vulnerable to environmental change, serving as indicators of environmental stress. Knowing this, we tested how two different species that differed in body size responded to changes in food quality and quantity. We exposed Daphnia magna (larger) and Daphnia pulex (smaller) to either green algae (Scenedesmus sp.) (Mycrocystis sp.). Overall, our results showed D. magna had better reproduction and survivorship when exposed to green algae, while D. pulex did better when fed cyanobacteria. These results imply that the two species differ in their competitive ability based on food quality, and their presence in a given body of water may be indicative of environmental change.