Climate change is triggering population booms and declines for certain types of algae in Lake Tahoe, according to a recent study from the UC Davis Tahoe Environmental Research Center.
The shifting makeup of these algae communities could affect biological and physical conditions in the lake, including the food web and water clarity.
“We saw that smaller sized species are more abundant now, but diversity did not change,” said Monika Winder, a postdoctoral researcher and lead author of the study. “That shows us that the change in size structure is really driven by something else.“
Since 1970, the average water temperature of Lake Tahoe has increased nearly one degree while the surface water temperature increased nearly two degrees, according to TERC’s State of the Lake Report 2008.
The stronger warming trend in the surface compared to deeper water has changed the mixing pattern of the lake by decreasing circulation of water between the upper and lower depths.
Single-celled algae called diatoms depend on turbulent water mixing to remain suspended in the upper depths of the lake where there is enough light for them to grow, Winder said.
“The warming leads to conditions that are making it more difficult for larger diatom species to stay afloat,” said TERC director and contributing author Geoffrey Schladow in an e-mail interview. “So instead, the smaller species that were always present but in low numbers are filling the gap and taking over.“
The researchers looked at data collected from Lake Tahoe since the early 1970s, from which they could document changes in diatom species composition and size structure, water temperature and strength of water mixing. They found that while the overall amount of diatoms did not change significantly, there was a shift toward abundance of smaller sized species and decline in larger species.
The study confirmed previous hints from fossil evidence that warmer water temperature shifts the relative distribution of diatoms in favor of smaller sized species, Winder said.
The change correlated with a trend toward weaker water mixing over the years, an effect that can be explained by simple physics. Larger diatom species sink faster in the calmer waters compared to their smaller, lighter counterparts. Meanwhile, the smaller species flourish because they can remain in the upper light-reaching water layer for longer periods of time.
The structure of smaller species also makes them more efficient in taking up nutrients, which tend to accumulate at lower depths when there is less mixing, Winder said.
The amount of nutrients in the lake remained relatively stable over the years, and changes in water clarity did not appear to drive the change in size structure, she said.
“The size change in diatoms will impact those species that consume them,” Schladow said. “These feeders include tiny animals called zooplankton, which in turn are eaten by small fish, which are eaten by larger fish and birds, and so on up the food chain.“
The researchers agree that more studies are needed to determine how the change will affect the Tahoe food web.
It could change the abundances of zooplankton that select certain size diatoms for consumption, Winder said. She has observed a decrease in one zooplankton species during the study, but whether the decline is related to the change in diatom species or some other factor is a subject of ongoing research.
The proliferation of small diatoms in the upper water layer may also have indirect consequences for the lake by affecting carbon sequestration and cycling and reducing water clarity, Winder said.
The study was published several months after an earlier TERC report announced that ongoing loss in Lake Tahoe’s water clarity has slowed since 2001.
ELAINE HSIA can be reached at firstname.lastname@example.org.