In this article
Water fleas are Daphnia: tiny freshwater crustaceans, about 0.2–5 mm long, that graze on algae, bacteria and protozoa. They are normally clear, but they turn red when they make extra hemoglobin to survive low dissolved oxygen (DO). In a wastewater lagoon, red or pink streaks of Daphnia are a sign the water is short on oxygen.
Key takeaways
- Water fleas eat algae, bacteria and protozoa, which clears the water and lowers effluent TSS.
- Red or pink streaks in a lagoon are usually Daphnia stressed by low DO or a lack of food.
- Fix the low DO, not the Daphnia: clear the surface, cut the load and improve aeration and mixing.
What are water fleas?
Water fleas are Daphnia, small freshwater crustaceans in the group Cladocera, related to shrimp. They get their nickname from the way they hop through the water. They are multicellular animals, not single-celled organisms, and are just visible to the naked eye, especially when they swarm.
Females produce young without mating while conditions are good. When food runs short, water gets crowded or temperatures drop, males appear and the females lay thick-shelled resting eggs that hatch the next spring.
What do water fleas eat?
Daphnia are filter feeders. They eat single-celled algae, bacteria and protozoa, and in winter they also feed on sludge. Do water fleas eat algae? Yes, but they are picky: blue-green and filamentous algae are too large for them. Heavy grazing on green algae can give blue-green algae an opening. See blue-green algae in lagoons.
In a lagoon’s food web, bacteria break down the organic load, protozoa keep the bacteria in check, and metazoa such as Daphnia, rotifers and tardigrades graze on both. As Daphnia multiply after a spring algae bloom, they help clear cloudiness and remove TSS.
Why are there red or pink streaks in my lagoon?
Red or pink streaks or clouds are usually swarms of stressed Daphnia. Two things cause it:
- Low dissolved oxygen. Daphnia make more hemoglobin when oxygen runs low, which turns them red or pink.
- Running out of food. After a spring bloom, Daphnia multiply fast. Once they have grazed the algae down, the crowded population becomes stressed.
The streaks are not harmful on their own. They are an early warning that DO is falling and treatment may suffer next. See What color is your wastewater lagoon? for what other colors mean.
What do water fleas tell you about your lagoon?
| What you see | What it likely means | What to check |
|---|---|---|
| Clear water with some Daphnia | Healthy grazing; algae under control | Nothing; keep monitoring |
| Red or pink streaks or clouds | Low DO and/or food running out | DO profile, aeration run time, BOD load |
| Daphnia suddenly disappear | Possible ammonia spike or toxic shock | Ammonia, pH, recent industrial discharges |
| Green water returns after a Daphnia crash | Algae rebound; TSS likely to rise | Effluent TSS, mixing |
Daphnia are sensitive to ammonia. They prefer concentrations below about 0.15 mg/L and disappear above about 0.7 mg/L. They also do better at lower pH, likely because high pH makes ammonia more toxic.
How do I fix the cause?
Treat the low DO, not the Daphnia:
- Measure DO at several depths and locations, ideally early in the morning when DO is lowest.
- Clear the surface. Remove algae mats, duckweed or floating sludge that block oxygen transfer.
- Restore wind action. Trim trees, brush or fences that block wind across the water.
- Cut the BOD load. Add an influent screen, or run cells in parallel instead of in series.
- Improve aeration and mixing. Make sure blowers and diffusers are in service and sized for the load.
For a step-by-step guide, read Troubleshooting low dissolved oxygen in wastewater lagoons. Can you have too many water fleas? Yes: heavy grazing can crash the algae and then the Daphnia themselves. Michael Gerardi, author of The Biology and Troubleshooting of Facultative Lagoons, suggests water hyacinth as a natural control.
Frequently asked questions
What do water fleas eat?
Single-celled algae, bacteria and protozoa, and sludge in winter. They cannot eat blue-green or filamentous algae, which are too large.
Are water fleas unicellular or multicellular?
Multicellular. Water fleas are small crustaceans with a heart, gut and eyes, even though they are best seen under a microscope.
Do water fleas bite?
No. Despite the name, Daphnia are not insects or parasites and do not bite people or animals.
Are water fleas good or bad for a wastewater lagoon?
Mostly good. They graze algae and bacteria, improving clarity and lowering TSS. Red swarms are a warning sign of low DO, not a problem in themselves.
Why is my lagoon turning pink?
The most common cause is red Daphnia responding to low dissolved oxygen. Purple sulfur bacteria can also turn an anaerobic lagoon pink or purple; check DO and odor to tell them apart.
Should I kill water fleas in my lagoon?
No. Fix the low DO or overloading that is stressing them.
Red streaks keep coming back? Ares Aeration® combines fine-bubble oxygen transfer with coarse-bubble mixing to keep DO up across the whole lagoon.
