Daphnia are a genus of crustaceans, and while they are rare in activated sludge processes, they are commonly found in wastewater lagoon treatment systems. Often times, daphnia may be referred to as “water fleas,” a label that generally covers a broad range of crustaceans. The most common species found in nature are D. pulex (small) and D. magna (larger size), while over 100 species are currently recognized. From a microscopy perspective, recognizing them as a crustacean or a daphnia are adequate from a practical standpoint.  

There are high amounts of research publications available for daphnia. The main area of focus has been on daphnia’s role in the aquatic ecosystem and their evolution. To obtain food, daphnia are filter feeders, consuming small particles using a water current that they produce with their legs. Their large antenna is believed to be the predominant mechanism for swimming. Reproduction may occur sexually and asexually. When reproducing asexually, the newly created daphnia is a genetic clone of its mother, with its gender to be determined eventually by the environment.

In lagoon wastewater treatment processes, daphnia are most recognized to be algal predators that proliferate when uni-cellular algae are available at high abundance. Daphnia are also believed to prey on small bacteria, other debris and small sludge particles. It is common to witness red or pink streaks in the water if daphnia are high in abundance. Daphnia are recognized to be sensitive to ammonia and other stresses. In whole effluent toxicity testing, daphnia mortality rates are involved with passing/failing of the test due to their sensitivity to stress.

Many wastewater lagoon systems experience trends when algae proliferate, followed by periods of high abundance of algal predators (such as daphnia). When the predators eventually die off, nutrients are re-released, which can then help to promote renewed algal growth. Ultimately, controlling daphnia in lagoon systems is highly dependent on the ability to control algae growth. Common methods for controlling algae include limiting phosphorous concentrations, ensuring adequate mixing, limiting hydraulic retention time when possible, shade/sun-blocking methods, barley straw, ultrasonic devices, and other methods.

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About the author: Ryan Hennessy is the principal scientist at Ryan Hennessy Wastewater Microbiology. He was trained and mentored by Dr. Michael Richard for over 10 years in wastewater microbiology, and serves as a microbiology services consultant. Hennessy is a licensed wastewater treatment and municipal waterworks operator in the state of Wisconsin and fills in as needed for operations at several facilities. He can be reached at ryan@rhwastewatermicrobiology.com. Hennessy's new book "Wastewater Microbiology: Filamentous Bacteria Morphotype Identification Techniques, and Process Control Troubleshooting Strategies" is now available on Amazon.