Even as overall lake clarity continues to improve, scientists are seeing a worrisome trend with another key measure of the lake's health: lake mixing.

For the first time scientists came up with an index for how long the lake stays in it's "summer state,"  where the warm top layer and the cooler deep layer remain stratified. Tahoe Environmental Research Center's Geoff Schladow says they used data dating back to 1968.

SCHLADOW: "So what we're able to see is that over that 43 year period, the length of time that the lake exhibits this summer-like state has increased by almost 20 days."

Right now the lake mixes every 4 to 5 years, but Schladow says scientists expect as climate change warms the upper layer, it will mean less lake frequent lake mixing.

Mixing is important to because it brings oxygen to the lower depths of the lake.  Less mixing could mean a cascade of changes to the ecosystem, and it could also reduce Lake Tahoe's famous water clarity. 

The same report also brings good news about the Tahoe region's battle with one invasive species.  Scientists have been using a new technique to combat the Asian clam, a species that has plagued the lake for the last three or four years. 

Schladow says scientists cooperated with public  agencies to stretch a rubber tarp over about half an acre of lake's bottom. The tarp smothered the clams: 

SCHLADOW: "Once we've removed the barrier, the rate at which Asian clams returned to that has been very slow-- so this is incredibly heartening."

Schladow says the clams are trickier to remove from Lake Tahoe's Emerald Bay.  That's because it has a rockier bottom and it's harder to completely block the flow of water and oxygen.