Pyramid Lake in northwest Nevada, just over the California border, took on the appearance of a spooky witch’s brew several weeks before Halloween, with striking swirls of green that were visible from space. The colorful bands in the popular 125,000-acre lake, about 35 miles northeast of Reno within the Pyramid Lake Paiute Tribe Reservation, came courtesy of cyanobacteria, also known as blue-green algae. … Also on the rise in California in recent years, cyanobacteria can produce toxins that pose serious health risks to people and animals — including the thousands of recreation-goers and their pets who visit Pyramid Lake each year.