Omega-3 good for aquatic health, study shows

With the help of a tiny animal called daphnia, a type of zooplankton commonly known as a water flea, researchers have made another discovery in aquatic health.

January 18, 2000
Web posted at: 11:58 a.m. EST (1658 GMT)

By Environmental News Network staff

Phytoplankton are microscopic plants at the bottom of the food chain that are eaten by zooplankton, tiny animals that provide the basic diet of healthy fish populations. The eating habits of zooplankton also work to control the level of algal biomass in lakes, a key factor in water quality.

A recent study conducted by scientists from the University of California at Davis and the University of Washington suggests that phytoplankton with high levels of eicosapentaneoic acid, an omega-3 fatty acid, can sustain a much higher population of zooplankton even if the overall number of phytoplankton is relatively low.

The link between phytoplankton and zooplankton is often the most critical in both freshwater and marine food chains, the study notes, and high growth rates of zooplankton have positive effects on both ends of the chain. Since zooplankton graze on algae, these minuscule animals keep algal growth in check and ensure better water quality. At the same time, they are important prey for small fish which, in turn, are eaten by larger fish.

"Phytoplankton that are more nutritious can have a major impact on the overall food web," said Michael Brett, co-author of the study and an assistant professor of civil and environmental engineering at the University of Washington.

In their research, the scientists used a tiny animal called daphnia, a type of zooplankton and one of the main consumers of algae in lakes. Using a controlled laboratory system, they fed the daphnia phytoplankton taken from Stonegate Pond in Davis, California, at different times of the year and measured the rate of energy conversion from plant to animal as a way of gauging the effects of a varying food supply on population growth.

In the summer, a phytoplankton called cyanobacteria, low in omega-3 fatty acids, dominated the pond. With this diet, the daphnia only experienced an energy conversion rate of 5 percent to 26 percent.

Diatoms, phytoplankton that reside in marine and freshwater environments, have high concentrations of omega-3 fatty acids.

During the winter and spring, however, a kind of phytoplankton called diatoms ruled the waters. Diatoms have high concentrations of omega-3 fatty acids, and the daphnia flourished on the new diet with an energy conversion rate of 50 to 65 percent.

"What the study shows is that the rate at which zooplankton convert phytoplankton biomass to zooplankton biomass depends on the supply of this class of essential fatty acids. This gives us important insights into what may determine how energy moves through aquatic food webs," said Brett.

With this information, scientists may be able to predict biomass and energy flow rates in aquatic ecosystems, which could provide helpful tools for fisheries managers. The findings may also be useful in lake restoration efforts.

The scientists speculate that the role omega-3 fatty acids play in maintaining healthy cell membranes and helping form hormones in animals could be the reason for the contrast. "Interestingly, eicosapentaneoic acid is the same dietary fatty acid that is thought to be the healthy component of fat-rich fish like salmon and tuna in human diets," Brett said.

The research team also included Charles Goldman of the department of environmental science and policy at UC Davis, and Anne Liston, a research technician at UC Davis. Their findings were published in the Dec. 6 issue of Nature.

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