David Robinson and Jeff Droker set off in a boat on Lake Sammamish last week, the sun warming their backs, its blaze making them squint.

Like doctors performing a routine exam, the two environmental scientists from King County checked the vitals of the lake — temperature and oxygen levels — and collected samples to test later. Normally at this time of year, they do the job in the rain, drenched and cold. But there's been nothing normal about the arid weather taking over the Pacific Northwest this winter. And that's got scientists concerned over what may be in store for Lake Sammamish, which they say is more vulnerable than other bodies of water in the region.

"The streams are down; there's been no rain," said Robinson, who works in the county's environmental lab. He eyed the depth of the water. "The lake is usually about 3 feet higher at this time."

ALAN BERNER / THE SEATTLE TIMES King County environmental scientists Jeff Droker, left, and David Robinson bring their gear to Lake Sammamish, where they're monitoring water quality.

It's hard to complain about a winter filled with sunshine and dry skies. But scientists are keeping a close watch on Lake Sammamish, King County's second-largest lake, to see how the effects of the dry weather will play out this year.

If it persists, scientists worry the lake could stagnate, halting the flow of nutrients and constricting oxygen at deeper levels.

This would spell bad news for native inhabitants such as the kokanee, landlocked salmon whose numbers are already a fraction of what they used to be. The summer run is already extinct, and the fall run is on the decline.

At just 100 feet in depth, Lake Sammamish is considered shallow. With no mountain snowpack to feed it, the lake relies on creeks to circulate new water through.

Heavy residential development has had an impact, too. Runoff from lawn fertilizers and car washings have made the lake more prone to higher levels of phosphorous, which spurs algae growth. Too much algae rob water of dissolved oxygen, endangering fish.

"It's a fragile lake," Droker said. "Whatever comes in here will have an effect on it."

King County environmental scientists David Robinson, left, and Jeff Droker bring up the mini-probe that is used to monitor Lake Sammamish water.

Higher temperatures

Lately, the lake's temperature, about 52 degrees at the surface, is up by a few degrees, Robinson said. During the transition from winter to spring and summer, the lake's waters separate into layers. Higher temperatures warm the top of the water, and oxygen levels drop at the bottom.

With this "oxygen squeeze" going on in the lake, kokanee would be forced to swim in a restricted band in the middle layer, making them easier prey for other fish like cutthroat trout or yellow perch, said Hans Berge, a fisheries biologist and environmental scientist at the county's Department of Natural Resources.

Although kokanee are not on the endangered list, their numbers have reduced dramatically in Lake Sammamish over the years from tens of thousands to about 3,000, Berge said. He's concerned that the warm temperatures and drought will only hurt the population more.

"It's been great spring weather for us, but it won't be for the lake," Berge said. "This could have long-lasting effects."

The separation into layers is a normal process, said Sally Abella, program manager of the county's lake-stewardship program. But dry weather accelerates the process. If the oxygen is constricted too much, that could lead to fish dying off, she said.

Charting patterns

Back at Lake Sammamish, Robinson casts a net to capture zooplankton, microorganisms that live in the water, while Droker carefully pours the samples into marked jars.

There is a certain feeling of protection that comes with charting the minute details of the lake's life patterns every month, Droker said. Both men know that underneath the lake's sparkling waters beats the pulse of a fragile ecosystem, susceptible not only to weather but, most of all, to human behavior.

Robinson sees the condition of Lake Sammamish as part of a larger environmental question.

"One of the discussions you have to have is, 'What is healthy?' " he said.

It's a fluid definition, Robinson said, subject to the dynamic shifts between people and nature.

Sonia Krishnan: 206-515-5546 or skrishnan@seattletimes.com