January 4, 2000
Algal food quality, not quantity, critical factor in healthy lake ecosystems
Phytoplankton high in a certain essential fatty acid may be a major factor in supporting thriving fish populations and clear water in lake ecosystems, according to researchers in California and Washington.
A new study points to the quality, rather than the quantity, of the phytoplankton, which are tiny plants at the bottom of the food chain, in nourishing robust communities of zooplankton, the tiny animals that provide a food base for greater fish production and keep algae biomass in lakes under control.
The finding, which could have important implications for water quality and fisheries managers, is being published in the Jan. 6 issue of the journal Nature.
The critical link in freshwater and marine food chains is often the one between tiny plants and animals – phytoplankton and zooplankton – according to research associate D?e M?Navarra and professor Charles Goldman, both with the department of environmental science and policy at the University of California, Davis, and Michael Brett, assistant professor of civil and environmental engineering at the University of Washington.
Their research indicates that phytoplankton with high concentrations of eicosapentaneoic acid, commonly known as an omega-3 fatty acid, support much higher zooplankton growth rates, even if the overall amount of phytoplankton is relatively low. High zooplankton growth rates have positive effects on both ends of the food chain: the zooplankton graze on algae, keeping it under control and enhancing water quality, and they are important prey for small fish, which are in turn eaten by sport and food fish.
“Phytoplankton that are more nutritious can have a major impact on the overall food web,” Brett said. “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.”
Those insights could help scientists predict biomass and energy flow rates in aquatic ecosystems, providing possible tools for fisheries managers, M?Navarra added. “It may even be useful to enhance the success of lake restoration efforts.”
In the study, the researchers used tiny animals called Daphnia, a type of zooplankton and one of the main consumers of algae in lakes. Using a controlled laboratory system, they fed Daphnia algae taken from Stonegate Pond in Davis, Calif., at various times of the year. During the summer, a type of phytoplankton poor in omega-3 fatty acids called cyanobacteria dominated the pond and the Daphnia suffered with an energy-conversion rate from plants to animal of 5 to 26 percent. During the winter and spring, however, a type of phytoplankton called diatoms dominated. Diatoms are rich in omega-3 fatty acids and, although the diatom concentration was lower than that of the summer phytoplankton, the Daphnia flourished with an energy-conversion rate of 50 to 65 percent.
The reason for the difference may lie in the role omega-3 fatty acids play in maintaining healthy cell membranes and helping form important hormones in animals, Brett said. “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.”
Also participating in the study was Anne Liston, UC Davis research technician.
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For more information, contact Brett at (206) 616-3447, (206) 543-2549 or mtbrett@u.washington.edu. M?Navarra is currently in Germany and can be reached at dcnavarra@ucdavis.edu or (011-4940) 4808802.
