Nutritional imbalance in Flathead Lake — ScienceDaily

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As any gardener or farmer can tell you, nitrogen and phosphorus are chemicals found in soils and fertilizers that plants need to grow. They also know that different nitrogen and phosphorus ratios are ideal or detrimental for different types of plants and crops.

Nitrogen and phosphorus also play an important role in lakes and can alter the crystal clear waters of nutrient-poor lakes. But while considerable effort has been made to monitor the amounts of each nutrient separately, limited research has assessed how the ratio of nitrogen to phosphorus supplied to lakes might also alter algal growth and water quality. water consistently.

Today, a team of researchers led by scientists at the University of Montana’s Flathead Lake Biological Station examined nearly 40 years of nutrient dynamics in Flathead Lake. This unique dataset, assembled by the FLBS Flathead Monitoring Program, documents a sustained nitrogen-phosphorus imbalance that likely has significant ecological consequences in Flathead Lake, as well as other low-nutrient ecosystems. .

Their work was published on July 11 by the Proceedings of the National Academy of Sciences.

“Since the early 1990s, I have worked to better understand when and where nitrogen and phosphorus limit the growth of lake organisms, such as plankton,” said Jim Elser, director of FLBS, member of the National Academy of Sciences and lead author of the study. “It turns out that strong imbalances in the ratio of nitrogen to phosphorus in ecosystems and organisms can have big impacts. I wanted to see if this was happening at Flathead Lake.”

For more than a century, FLBS research and monitoring programs have served as the first line of defense against ever-looming threats to the renowned water quality of the Flathead watershed. The main threats of nutrient pollution and invasive species have remained the biological station’s oldest enemies in the fight to maintain the state of the lake and the excellent water quality.

Flathead Lake is known for its clean, clear water, largely because the geology encompassing its watershed is ancient and poor in nutrients, especially the nutrient phosphorus. This means that there are very low levels of nutrients that can be weathered from bedrock to reach the lake through rainstorms and snowmelt. As a result, there are naturally low levels of nutrients available for lake algae growth, and Flathead Lake remains clear and blue instead of green and cloudy.

This low level of naturally supplied nutrients makes Flathead Lake very sensitive to human nutrient inputs. These human-caused nutrient inputs to Flathead Lake and the associated algal blooms caused concern in the 1970s and 1980s. Subsequent research by FLBS scientists led to measures to reduce nutrients in the Flathead watershed, including one of the nation’s largest bans on phosphorus-containing laundry detergents and a multimillion-dollar overhaul of local wastewater treatment facilities to remove phosphorus to levels very low.

But in recent years, Elser and his colleagues have begun to question whether monitoring nitrogen and phosphorus in isolation is enough. Given his long experience in developing and testing the theory of ecological stoichiometry – the study of the balance of multiple chemical elements in ecological interactions – Elser was eager to find out.

“We found that overall nitrogen and phosphorus levels in Flathead Lake and its surrounding rivers and streams, while variable from year to year, are low but not increasing,” Elser said. “In fact, the nitrogen and phosphorus levels entering Flathead Lake from its larger rivers actually appear to be slowly declining. This is great news for the water quality and clarity of our well- loved Flathead Lake, while the water quality of many lakes around the world is declining due to increased nutrient inputs.”

Then came a surprising development. While overall nitrogen and phosphorus levels in Flathead Lake did not increase, researchers found that the lake maintained a high nitrogen to phosphorus ratio over a four-decade period, often reaching values which greatly exceed the normal ratio between nitrogen and phosphorus. phosphorus recipe that meets the needs of most phytoplankton, the lake’s microscopic floating algae.

In other words, just as humans benefit from a well-balanced breakfast or farmers apply fertilizer with the appropriate ratio of nitrogen to phosphorus for specific crops, the microorganisms that form the basis of a lake’s food web depend on a very specific ratio. of nutrients. When the ratio of nitrogen to phosphorus is high, as is the case in Flathead Lake, plankton growth is likely limited by the lack of available phosphorus for much of the year.

Through a series of experiments, the research team showed that Flathead Lake phytoplankton are phosphorus-limited. This means that the algae are forced to build low-phosphorus cells, which makes them low in nutrients. For the tiny animals of the lake, the zooplankton, which eat this phytoplankton and thus maintain the high transparency of the lake, this amounts to a “junk food” diet. As a result, zooplankton also become phosphorus limited and their abundance is low.

Finally, the team showed that the strong imbalance between nitrogen and phosphorus in Flathead Lake sets the stage for potential production of methane, a greenhouse gas. This happens when phosphorus-hungry microbes start scavenging phosphorus from organic molecules and producing methane as a byproduct.

These findings have implications not only for Flathead Lake, but also for lakes around the world. Wastewater treatment systems, agricultural runoff, and urban influences are increasingly recognized as contributing to nitrogen-phosphorus imbalance in various situations.

“At Flathead Lake, implementing wastewater treatment processes that remove nitrogen more efficiently would help balance the lake’s nitrogen-to-phosphorus ratio,” Elser said. “On a regional scale, a reduction in atmospheric transport of nitrogen, which occurs through the burning of fossil fuels or the volatilization of agricultural fertilizers or animal waste, would also help reduce nitrogen inputs to the lake. “

In other words, when it comes to the building blocks of our lake ecosystems, nutrient balance matters.

Besides Elser, study authors include FLBS lake ecologist Shawn Devlin, Nanjing Institute of Geography and Limnology scientist Jinlei Yu, FLBS laboratory director Adam Baumann, FLBS microbial ecologist Matthew Church , Montana State University research professor John Dore, FLBS flow ecologist Robert Hall, FLBS student and UM researcher Melody Hollar, Oklahoma State University scientist Tyler Johnson, Trista Vick-Majors, assistant professor at the Great Lakes Research Center, and Cassidy White, FLBS student and researcher at UM.

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