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As the climate warms and nutrient inputs shift, algal communities in cool, clear mountain lakes like Lake Tahoe will likely experience seasonal changes, according to from the ɫ��ɫ. 

Periphyton, that fuzzy layer of attached algae covering the rocks as you step into the water, is a healthy and critical part of a lake’s food web. Periphyton blooms, however, signal changes that can degrade both water quality and a shoreline’s natural beauty.

Climate change is projected to increase global water temperatures by 1.8 to 7 degrees Fahrenheit by 2100. It’s also expected to increase nutrients to lake waters through increased runoff from higher intensity storms and more precipitation falling as rain rather than snow.

“A majority of lakes globally are warming as a result of climate change,” said lead author Nick Framsted, a master’s student in the department and when the study was conducted. “With their clear, cold waters, mountain lakes are exceptionally sensitive to changes in temperature and nutrients.”

Attachment issues

To understand how warming and nutrient inputs are expected to affect periphyton growth at such lakes in the future, ɫ��ɫ scientists conducted lab experiments using periphyton-covered rocks collected from Lake Tahoe’s west shore. They exposed each rock to two of four warming treatments and two nutrient treatments and analyzed their effects. 

Their results, published in the journal Water Resources Research, show the effects of climate warming were especially pronounced in the colder months, with little to no additional effects in summer.

“Warming in the summer, surprisingly, didn’t have an effect on periphyton growth,” Framsted said. While more research is needed, the authors suggest several possible reasons for that result:

“Periphyton growth may respond more strongly to winter warming because small temperature increases can cause large boosts in metabolic rates,” said ɫ��ɫ Associate Professor Steven Sadro, the principal investigator on the project. “During the winter, nutrients that would otherwise remain unused can be taken up and converted into new growth. In summer, periphyton may already be near their thermal and nutrient limits, so additional warming has little effect.”

Seasonal differences

The relative importance of warming and nutrients on the metabolic rate of periphyton shifts seasonally, the results show. Nutrients had a higher effect than warming during the fall months, while warming had a greater effect in the winter. Understanding this seasonality can help lake managers respond to climate change and help prevent periphyton blooms. 

Framsted said that while Lake Tahoe presented an ideal study system, the results can be applicable to other cool, clear, low-nutrient lake systems, which scientists refer to as “oligotrophic” lakes.

The research was funded by the Lahontan Regional Water Quality Control Board. Adrienne Smits, who oversees the ɫ��ɫ Tahoe Environmental Research Center’s periphyton monitoring program, co-authored the report.