Probing – 12 Remarkable Limnological Features

Probing the Depths of Crater Lake: A Century of Scientific Research by Douglas Larson

 As reports about the remarkable limnological features of Crater Lake began to appear in the scientific literature, scientists were drawn to the lake to study it further. On July 18, 1940, a team of oceanographers from the University of Washington-led by Clinton Utterback, Lyman Phifer, and Rex Robinson-explored the lake with state-of-the-art oceanographic equipment. The team lowered a light meter (a Weston submarine photometer) into the lake and for the first time measured the vertical penetration of blue, green, and red bands of light in the visible light spectrum. They collected water samples to a depth of 1,400 feet and analyzed the samples on-site for oxygen, carbon dioxide, pH, and silica (they preserved samples for phosphorus analysis at the University of Washington). The test results were “remarkably similar” to those obtained in 1913 by Kemmerer and others. Phifer, an algae expert, examined portions of the samples under a microscope to determine how many and what kind of phytoplankton were present. The team also caught phytoplankton by towing a plankton net from depths of 330 and 660 feet to the surface.

Utterback, an expert on the optical properties of natural waters, observed that the lake’s highly transparent water transmitted blue light much deeper than either green or red. The blue light penetrates deeper because of the lake’s extreme water purity. As sunlight travels down through the water, it is gradually absorbed by the water itself and by the dissolved and particulate materials in the water. The light is also scattered back to the lake surface (called back-scatter) by particles that are suspended in the water, such as phytoplankton and soils. Consequently, the intensity of the sunlight is gradually reduced the deeper it penetrates into the lake, eventually fading to complete darkness. In most lakes, particularly those that produce large amounts of algae and other vegetation, the depth of sunlight penetration is greatly restricted. In Crater Lake, however, where the water is pure and less fertile, sunlight especially the blue segment-can penetrate to depths greater than three hundred feet. Further, because of the scarcity of phytoplankton and other suspended particles, penetrating sunlight is scattered largely by water molecules. This back-scattered light is what gives Crater Lake its exceptionally blue color. Phifer discovered that about 90 percent of the lake’s phytoplankton was concentrated between 230 and 490 feet below the surface, with the highest concentration found at 250 feet, a few found at 985 feet, and none found at 1,400 feet.

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