Water Fleas and Algae: How Food Limits a Pond Population

Water Fleas at Heron Pond

1Heron Pond Field Station sits beside a shallow pond in the hills outside the city. Each summer, seventh graders from Pinecrest Middle School spend a week there helping biologist Ingrid Solberg study water fleas, also called Daphnia. A water flea is not an insect. It is a tiny crustacean, usually 1 to 4 mm long, with a see-through body. It swims in jerky hops and filters single-celled green algae out of the water to eat. Water fleas are an important food for small fish, so the station tracks them closely. In summer, a female usually carries her eggs in a pouch on her back, where they can be counted under a microscope, and the young hatch and swim out within a few days.

2In the station's lab, the students set up three tanks. Each tank held 4 L of filtered pond water, started with 10 adult water fleas and was kept at 20 °C under the same lights. Every morning, each tank received a dose of algae: a low, a medium or a high amount. Once a week, the students counted the water fleas in each tank, moving each one with a dropper and returning it afterward (Table 1). In week 3, they also placed five adult females from each tank under a microscope, measured their body lengths and counted the eggs in each pouch. Table 2 shows the averages.

3Out on the pond, the station's long-term survey gives a bigger picture. Each month from April to September, the biologists measure how much algae floats in the open water, in thousands of algae cells per milliliter (Table 3). They also count the water fleas in 1 L samples of pond water (the bar graph in Section 2). In the pond, water fleas are eaten by small fish and insect larvae, but the biologists noted that the numbers of these predators changed little from month to month during the survey year.