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Ecology & Environmental Sciences - Decoding Diatoms

Sediment records of past algal communities inform today’s climate change investigations

by Margaret Nagle

jasmine saros

Jasmine Saros

Every summer for the past decade, paleoecologist Jasmine Saros has trekked across snowfields and horsebacked up bouldered mountain passes to reach remote, high-altitude lakes in the shadow of the Beartooth Mountains of the central Rockies. In the pristine wilderness of Montana and Wyoming, at altitudes above 9,400 feet, she arrives at the alpine lakes just after ice off to study changes in the algal community, looking for evidence of climate change and airborne pollution.

Saros, an associate professor in the University of Maine Climate Change Institute and School of Biology and Ecology, is focusing on diatoms living in the water column and preserved in the lake sediment, the remains of their silica cell walls testifying to their centuries of existence. By combining ecological field observations and bioassays with paleolimnology, Saros studies the diatom fossil records to reconstruct environmental change and to better understand the mechanisms driving the change, past and present.

Diatoms, one of the most common groups of phytoplankton, are part of the base of the food web, where changes in such fundamental species can cause ripple effects. In Saros’ research, diatoms are tools for understanding what’s happening chemically and physically in lake ecosystems.

Sediment cores reveal that in the past 10,000 years, the abundance of various Cyclotella, one of the most common diatom groups, has fluctuated in lakes in the central and northern Rockies. However, in the 20th century, the diatoms’ numbers have increased not only in the Rockies, but also worldwide in alpine and temperate lakes, as well as in the Arctic.

“Many people believe that’s a sign of global warming,” says Saros. “What we don’t know is the mechanism driving the increases, and whether it is the same catalyst across all lakes.”

alpine lakeAlpine lakes are ideal for studying climate change because of their short ice-free season, typically July to October.

“Any change in the length of that ice-free season has big impacts,” Saros says. “As we see climate changing and the ice-free season becomes longer or shorter, it has a big effect on the species in those ecosystems.”

In 2007, the fourth assessment report of the Intergovernmental Panel on Climate Change noted that there is substantial new evidence that changes in marine and freshwater biological systems are associated with rising water temperatures, as well as related changes in ice cover, salinity, oxygen levels and circulation. These shifts include increases in algae and zooplankton in high-latitude and high-altitude lakes.

Saros is investigating whether changes seen in alpine lakes are purely climate-driven. If they are, shifts in the diatom record should be evident in the sediment cores taken from lakes in the central and northern Rockies that date back 2,000 years.

To interpret the data preserved in such fossil records, researchers need to better understand the relationships between environmental variables, and the growth and distribution of diatoms. In Saros’ research, two of the critical environmental variables are nitrogen and phosphorus — essential nutrients for algae in the right quantities; in excess, the cause of algae blooms and other harmful water quality.

Alpine lakes are removed from the typical sources of human-induced nitrogen and phosphorus pollution — fertilizers, storm water and agricultural runoff, and faulty waste treatment systems. Yet in the highest elevations, humans still have the capacity to affect water quality via atmospheric nitrogen deposition, a form of air pollution resulting largely from the burning of fossil fuels.

According to Saros, human activity has led to a doubling of the amount of atmospheric nitrogen deposited in alpine lakes in the past century. Since 1980, species changes in the diatom communities have indicated nitrogen enrichment.

“We’re seeing rapid changes in the diatoms,” she says. “What’s hard to say with nitrogen is whether, in a low-deposition area like this, we’re seeing a level of saturation due to accumulation over time, or whether there’s a new atmospheric source, such as air pollution from Asia.”

diatom

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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