A student who is tracking the population growth of a colony of water fleas (Daphnia magna) in a two gallon glass tank for his science fair project notices that after several weeks of rapid growth, the rate of increase begins to slow down. The tank is also inhabited by a species of hydra that feeds on the water fleas.
The student adds a specific amount of food to the tank each day and keeps an aerator set to a specific level to control oxygenation rates of the water. The temperature of the tank is kept constant by a tank heater, and a timer ensures a consistent amount of light each day.
Which of the following is the BEST explanation of the student's observation regarding the slowing growth rates of the D. magna population?