Basin Watch: Studying ultraviolet light in Lake Tahoe |

Basin Watch: Studying ultraviolet light in Lake Tahoe

Five years ago, I was lucky enough to be cruising Lake Tahoe aboard the John LeConte, the Tahoe Research Group’s research vessel.

My fellow passengers were two scientists from Finland. Pirjo Huovinen and Markus Soimasuo, with the Department of Biological and Environmental Science at the University of Jyvaskyla, were studying the effects of ultraviolet light (UV) on aquatic life in Lake Tahoe.

Since my research, once upon a time, was about UV, Freon and stratospheric ozone, I was intrigued. And, as a plus, I got to steer the boat while Bob Richards, the skipper, helped the researchers prepare their samples. Huovinen and Soimasuo went on to study UV in other lakes and oceans around the world and I often wondered what they learned. Well, this year, I found out. Pirjo Huovinen published “Ultraviolet Radiation in Aquatic Environments,” and thoughtfully sent me a copy. It includes the Lake Tahoe study.

A couple of years later, U.S. Geological Survey scientists discovered a class of chemical compounds called polycyclic aromatic hydrocarbons (PAHs) floating around in the lake. Then, J.T. Oris, of Miami University, learned that some of those PAHs in Lake Tahoe became toxic when they were irradiated with UV-A. The longer wavelength UV-A radiation is generally thought to be nonhazardous. Yet, those photo-toxic chemicals could be fatal to small aquatic critters. So, UV studies by the scientists from Finland are even more pertinent to Tahoe.

While they were here, Huovinen and Soimasuo investigated the effect of UV on our algae. They were particularly interested in a narrow band of the spectrum known as UV-B, which is biologically harmful. Shorter wavelengths of UV (UV-C) are more hazardous, but fortunately, they never reach the ground. Some UV-B, however, does reach the ground. In addition, it’s predicted that UV-B radiation at ground level might increase as stratospheric ozone is depleted.

Water in lakes and streams generally contain more organic material than is found in ocean water. Consequently, UV-B penetrates much deeper into ocean water than into fresh water. Generally, fresh water penetration (to 90 percent attenuation) is less than a meter, while in the Sargasso Sea and near Antarctica, penetration has been measured at 20 to 25 meters. Because of Tahoe’s clear water, it was believed that UV-B penetration would rival that of the ocean. Huovinen and Soimasuo calculated that 90 percent of UV-B radiation is attenuated at depths of 10 meters in Lake Tahoe’s water. UV-A was calculated to penetrate as deep as 30 meters.

Samples of Lake Tahoe water (including algae) were taken at depths from the surface to 50 meters. They were put in glass flasks, along with carbon-14, and lowered back down to incubate for a while. One Pyrex flask at each depth was covered with opaque plastic as a dark control. Another Pyrex flask was covered with Mylar film, which together with Pyrex, filters UV-B out of the sunlight reaching the flask. The third flask was quartz glass, which passes UV-B; so algae were exposed to all the sunlight (including UV-B) that penetrated to that depth.

The object was to see if UV-B inhibited the production of algae. The intake of carbon-14 by algae (which can be measured) is related to algal productivity in the flasks. In their June experiment, Huovinen and Soimasuo found that algal productivity was inhibited to some extent down to about 20 meters. In their two July experiments, they found productivity inhibition down to 50 meters. However, the most significant reductions in algal productivity occurred in the top two meters in the lake. The researchers noted that productivity is inhibited down to some 25 meters in the ocean, while in Lake Tahoe, the magnitude of UV-B effects on algae dropped off quickly below two meters. Comparing their results with measurements on lakes elsewhere led Huovinen to conclude that the algae in Lake Tahoe had acclimated to UV-B exposure.

In later work at Bodega Bay, Huovinen and Soimasuo found that UV-B could also inhibit the development of kelp, which are giant algae. In another study, they found that retene, one of the PAHs, became toxic to daphnia, tiny aquatic critters, when irradiated with UV-B. The experiment wasn’t done at Tahoe, but it suggests that both UV-A and UV-B can photosensitize some of the PAHs in Lake Tahoe, perhaps making our potential problem with PAHs worse.

The objective of research done by Huovinen and Soimasuo is to determine what potential increases in UV-B radiation, caused by stratospheric ozone depletion, might do to aquatic life. What they found is that aquatic critters might be at risk, at Lake Tahoe and elsewhere, unless they learn to live at greater depths.

(Leo Poppoff is a retired atmospheric physicist with NASA and has been a member of the Tahoe Regional Planning Agency’s advisory planning commission since 1983. He is also a former member of the Lahontan Water Quality Control Board.)

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