The first surprise was the colors: brilliant yellows and greens, electric in the sunlight, a world apart from the drab Pumice Plain. They belonged to aquatic plants, thick, vinelike macrophytes stretching ten feet from lake bed to surface. Mossy clumps were suspended above the silt. Everywhere I turned were fish, hook-jawed and fat, all of them 20 or more inches. I swam after them. They didn't spook. The sunken jungle, I noticed, was only in the shallows. In deeper water it was gone—and so were the fish.
Before the eruption Spirit Lake was, like many subalpine lakes, unproductive and nutrient-poor, with clear water and few shallow spots. When the volcano top slid into it at 150 miles an hour, it became choked with what Crisafulli terms "pyrolyzed forest constituents"—organic material burned in the blast. The water was warmed to body temperature, filled with dissolved carbon, manganese, iron, and lead. Visibility went from 30 feet to six inches. Bacteria flourished. The first scientists to take water samples came down with unexplained ailments. There was a rapid succession of microbes: aerobes, which quickly used up all the oxygen; anaerobes, which require none; then nitrogen-consuming bacteria; and then forms that fed on methane and heavy metals. For 18 months Spirit Lake was ruled by chemistry, home to "hundreds of millions of bacteria per milliliter," Crisafulli says. Finally, the microbes had consumed so much that they began to die off, and streams and snowmelt came in, and the water cleared.
Once light penetrated Spirit Lake, algae and other phytoplankton colonized, followed by zooplankton, which fed on the phytoplankton, followed by aquatic insects and amphibians. By the early 1990s, macrophytes grew in shallow shoals—ideal trout habitat that didn't exist before the eruption. Gorging on tiny midges and freshwater snails, the rainbows were reaching a record four or five pounds in two or three years. The post-eruption lake followed a pattern Crisafulli would see many times in the blast zone. New organisms colonize the virgin environment with dramatic success, only to burn themselves out or be checked by predators, parasites, or competitors. This was the second revelation of St. Helens: When there's a blank slate, ecological succession is a cycle of boom and bust.
Spirit Lake's richness is spilling over. When a tadpole dies as a frog on the Pumice Plain, when an insect hatched in the lake is deposited in the ash, their nutrients are transferred to land. Slowly this process undoes what the eruption wrought. "Before the eruption, the terrestrial environment was superproductive," Crisafulli says, with "lots of nutrients and carbon tied up in the old-growth forest. In comparison, the lake was impoverished. After the eruption they flip-flopped." Now the landscape is going back from gray to green, and the lake is becoming more like its former self.