In the shallows of south Lake Tahoe, diver Brandon Berry is slurping up clouds of algae with an underwater vacuum cleaner. Snorkeling above, I can hear his Darth Vader breaths better than I can see him—both researcher and lake bed are shrouded in a green muzz of metaphyton. The filamentous, cotton-candy-like algae is a persistent affliction here, where it intermittently grows, dies, washes up on the shoreline, and rots in unsightly, smelly piles.
A long string runs up out of the gloom from Berry’s contraption to the Tahoe Environmental Research Center (TERC) boat. There, chemist Michael Welsh is stationed by the on/off switch to wait for tugs on the line, as if he’s patiently fishing for research divers. It’s July, and the weather is balmy—but the researchers will be out here through the Sierra winter, huddling for warmth and wind protection in the boat’s tiny cabin and diving in heavy-duty dry suits. All to figure out how and why the famously pellucid waters of Lake Tahoe are glowing with the eerie emerald of algal overgrowth.
On the lake bed, the pale shells of one suspect shimmer through the haze: Asian clams (Corbicula fluminea), first discovered on Tahoe’s south shore in 2002 and now legion. The algae aren’t invasive, but the clams are—and the two seem to be in cahoots. As clam infestations have crept up the east side of the lake, the algae has followed.
Aquatic invasive species (AIS) have been making trouble for Lake Tahoe’s ecosystem ever since people started sticking them in there in the mid-1800s. Invasives crowd out native plants, starve out or prey upon native animals, and kick off disastrous ecological cascades. Increasingly, limnologists are finding alliances like that of clam and algae—in which aquatic invasive species create conditions that help other undesirables spread. They’re aided by a third accomplice: climate change. Warmer waters are worse for native species, and better for invasives and potentially harmful algae. At Lake Tahoe, native fish stocks have declined, toxic algae alerts have closed down beaches, and the celebrated waters are about 30 percent murkier than they were 50 years ago. The lake’s ecosystems, along with its multibillion-dollar tourism industry, rely on clear, clean, cool water. Tahoe—jewel of the Sierra, sacred space of the Washoe Tribe, and destination for nearly six million vacationers each year (including about a million from the Bay Area alone)—is at risk.