Algae also feel effects

Sockeye salmon aren't the only species that might be feeling the effects of rising temperatures in Lake Washington.

New studies by University of Washington researchers have found that algae, the foundation of the lake's food chain, appear to be responding to rising temperatures in ways that could affect the many animals that rely on it.

The population of diatoms, vital single-celled algae, normally explodes in the spring, but the blooms are happening nearly three weeks earlier than they did in the late 1970s as average spring air temperatures grow warmer. After the bloom, the algae population drops off.

Water fleas are tiny crustaceans that feed on the algae, but their life cycle hasn't adjusted to these early blooms. So the water fleas emerge after the algae population has peaked. As a result, water fleas are in decline in Lake Washington, said Monika Winder, a researcher at the UW's School of Aquatic Sciences and Fisheries, who co-authored several studies on the phenomenon.

Water fleas, or daphnia, are an important food source for young sockeye and other fish such as trout and smelt. So the trend could be a problem for them, said Winder.

More broadly, it illustrates how climate change can produce minute, local shifts that ripple through an ecosystem.

The algae bloom is closely linked to when the lake, heated by spring weather, separates into two layers, one upper layer of warmer water and a lower layer of cool water. That's also happening earlier.

And there are some worrisome signs that the lake could be encountering other problems. Blue-green algae, which can produce a toxin and is often a sign of poor water quality, is returning in increasing numbers, said Winder.

The lake's warming trends match predictions about how global warming could affect the Northwest, said Nathan Mantua, a research scientist with the University of Washington's Climate Impacts Group.

But he cautioned that computer models used to track global climate change aren't specific enough to conclusively blame global warming for what's happening in Lake Washington.

"We're still not at the point where we can say this regional warming in the Northwest is a sure sign of global warming," he said.

Warren Cornwall: 206-464-2311 or wcornwall@seattletimes.com