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Decoding Diatoms Sediment records of past algal communities inform today’s climate change investigations by Margaret Nagle
One of the diatoms studied in the fossil record is Cyclotella bodanica.

One of the diatoms studied in the fossil record is Cyclotella bodanica.

For the past decade, much of Saros’ research has focused on six lakes — three above and three below the tree line — in the central Rockies in Wyoming and Montana, outside Yellowstone National Park. Lakes in this area have had few disturbances, such as development on the water- sheds. Nevertheless, across the lakes, there has been a synchronized increase in some species of diatoms, suggesting “a larger-scale driver.”

Using field surveys and on-site experiments, including annual sampling for water chemistry and biodiversity, she has looked at how changes in nitrogen and phosphorus alter the structure of aquatic communities.

Sediment cores from the area lakes revealing a 300- to 400-year-old record show that two diatoms, Asterionella formosa and Fragilaria crotonensis, are indicators of nitrogen enrichment.

But unlike their incidence hundreds of years ago when they made up less than 1 percent of the phytoplankton population, lake sampling done since 1999 shows that the two species are now dominant in the water column.

It appears that increases in nitrogen deposition are not only stimulating diatom species changes, but also driving phosphorus limitation in at least three lakes. One of the concerns is that lakes that are limited by phosphorus are more prone to the acidification effects of air pollution.

Acidification in lakes changes the transparency of the water column, altering the depth that ultraviolet (UV) radiation reaches. Alpine lakes are particularly vulnerable to UV radiation, yet another environmental variable with the potential to change diatom communities.

Saros now plans to study the alpine lakes transitioning to phosphorus limitation, looking at what such chemistry means to the transparency of the water column and biodiversity of the ecosystem.

“What’s kept me going back over the decade are the pretty dramatic changes now occurring,” Saros says. “Since 2001, the lakes we’re studying have progressively gone from nitrogen limited to phosphorus limited, and it’s unclear how this major change across these lake ecosystems will alter the ecology of these systems.”

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Fall 2009

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