Crater Lake’s Hidden Moss Garden: How Plants Thrive in One of the Clearest Lakes

Crater Lake, in southwestern Oregon, is famously clear. That clarity comes from a lack of nutrients that would otherwise feed algae and plants; scientists call such lakes oligotrophic. Yet deep below the surface is an abundance of plant life, and a puzzle that a microbiologist, recently retired from Southern Oregon University, spent her research trying to solve. The National Park Service's science engagement coordinator interviewed her; this is what she found.

Crater Lake from the West Rim, near the Lightning Springs picnic area. National Park Service.

The garden

The hidden garden is mostly one moss, Drepanocladus aduncus, with some other mosses and liverworts. It grows from about 30 m (98 ft) to about 100 m (328 ft) deep, in about 1 percent of the light at the surface, dark enough that divers need lamps.

The puzzle: the moss is the largest mass of living matter in the lake, but the water is extremely poor in nutrients, with dissolved nitrate below one microgram per liter. How does so much plant life grow?

A clue from the Arctic

Underwater mosses are found in almost all deep, oligotrophic mountain lakes, from Lake Tahoe to Crater Lake and probably Lake Baikal, but, being hidden, they are little studied. Land mosses in the nutrient-poor Arctic are better known: they carry cyanobacteria that take nitrogen from the air and turn it into a form the moss can use, which is how they carpet the forest floor. Were the deep-water mosses doing the same?

The nitrogen cycle, briefly: most nitrogen exists as nitrogen gas (N₂), which also dissolves in water, but plants and animals cannot use it. Bacteria convert it to ammonia (NH₃), which is converted further to nitrate (NO₃). Plants can use both, most preferring nitrate, and the nitrogen ends up in proteins and DNA.

Searching the moss

She looked for cyanobacteria growing inside the leaves (endophytes), as in Arctic mosses, and on them (epiphytes). The best collecting tool turned out to be hooks on the boat's anchor, dragged a few yards along the bottom at different depths and hauled up with moss attached. In the lab, the team identified organisms under the microscope and by DNA, comparing sequences of a ribosomal gene (for algae and fungi) and the nif gene, used in nitrogen fixation, to focus on nitrogen-fixing bacteria. To wash the epiphytes off without collecting too much leaf DNA, she used a water flosser as a high-pressure jet: an example, she said, of the creativity science needs.

What lives there

Even on the boat, the moss teemed with mites, nematode worms, copepods, algae filaments and gelatinous balls about half a centimeter across, likely the cyanobacterium Nostoc. Under the microscope there were many diatoms, algae and swimming protozoa. DNA identified nitrogen-fixing cyanobacteria, including Nostoc and Calothrix, growing on the outside of the leaves, but no sign of bacteria inside them.

The open water of Crater Lake is remarkably free of microbes, she noted; right beside the moss, the community is entirely different, an ecosystem the moss supports.

Does the moss get the nitrogen?

Probably, but it is not proven. In land mosses, scientists have traced labeled nitrogen gas into nitrate made by cyanobacteria and then into the moss's leaves. Underwater, nitrate might simply diffuse away. A lab experiment with aquatic moss and cyanobacteria could test it; she was able to keep Crater Lake moss growing, at least for a time, in test tubes, with cyanobacteria on it. The bacteria benefit too: the moss holds them at a depth with the right amount of light, since cyanobacteria are harmed by too much sun and UV.

What is left to find

Where in the lake is moss growing and where is it dying? Does new moss grow on old, dying moss and feed on it? Does it do better deeper or shallower? A video camera lowered on a winch could film the moss fields. Her advice to young scientists: go out and look, and one discovery will lead to the next.

She also saw something many visitors think is a myth: the "Old Man of the Lake," a log with a face-like end that has floated upright in Crater Lake for a hundred years, turning up unpredictably beside the boat.

Sources

• National Park Service, "Researching the Riddle of Crater Lake’s Secret Garden", an interview by the NPS science access and engagement coordinator; rewritten in hubnx's own words. Photograph: National Park Service. Microscope images and a portrait from the researcher are not reproduced here.