Nutrients coming into Flathead Lake are low, but imbalanced
When it comes to lake ecosystems, nutrient balance matters.
And a nutrient imbalance in Flathead Lake is alarming scientists.
While the overall levels of nitrogen and phosphorus in Flathead Lake have not been increasing, researchers at the Flathead Lake Biological Station discovered that the lake has sustained a high ratio of nitrogen to phosphorus in the last 40 years. The imbalance has exceeded the normal nitrogen-to-phosphorus ratio that matches the needs of most phytoplankton, the lake’s microscopic floating algae.
When the ratio between nitrogen and phosphorus is high, as it is in Flathead Lake, plankton growth is likely limited by lack of available phosphorus for much of the year.
The University of Montana’s Flathead Lake Biological Station has examined nearly 40 years of nutrient levels in Flathead Lake. This dataset documents a sustained imbalance between nitrogen and phosphorus that likely has significant ecological consequences in Flathead Lake, as well as other low-nutrient ecosystems.
Flathead Lake Biological Station director Jim Elser, a member of the National Academy of Sciences and the lead author on the study, said strong imbalances in nitrogen and phosphorus in ecosystems can have big impacts.
Flathead Lake is known for its clean and clear water, largely because the geology encompassing its watershed is low in nutrients, especially phosphorus. This means there are very low levels of nutrients that can be weathered from the bedrock to reach the lake through rainstorms and snowmelt. Therefore, there are low levels of nutrients available for lake algae to grow, and Flathead Lake remains clear and blue instead of green and murky.
This low background of naturally supplied nutrients makes Flathead Lake very sensitive to human-driven inputs of nutrients, Elser said.
Human inputs of nutrients into Flathead Lake raised concerns in the 1970s and 1980s. Research conducted by Flathead Lake Biological Station scientists led to measures in the Flathead Valley watershed, including one of the nation’s largest bans on phosphorus-containing laundry detergents and a multimillion-dollar overhaul of upstream wastewater treatment facilities in Kalispell to remove phosphorus to very low levels.
But in recent years, Elser, said, “We found that the overall levels of nitrogen and phosphorus in Flathead Lake and its surrounding rivers and streams ... are low but not increasing,” Elser said. “In fact, nitrogen and phosphorus levels coming into Flathead Lake from its larger rivers actually appear to be slowly declining. This is great news for the water quality and clarity in our beloved Flathead Lake, while water quality in many of the world’s lakes is declining due to increasing nutrient inputs.”
The research found the nitrogen-to-phosphorus imbalance in Flathead Lake sets the stage for potential production of the greenhouse gas methane. This occurs when phosphorus-hungry microbes start to scavenge phosphorus from organic molecules and produce methane as a byproduct.
These findings have implications not only for Flathead Lake but also for lakes globally. Wastewater treatment systems, agricultural runoff and urban influences are increasingly recognized as contributing to nitrogen-to-phosphorus imbalance in a variety of situations.
“At Flathead Lake, implementation of wastewater treatment processes that more effectively remove nitrogen would help balance the lake’s nitrogen-to-phosphorus ratio,” Elser said. “Regionally, a reduction of the atmospheric transport of nitrogen, which occurs through fossil fuel combustion or volatilization of agricultural fertilizers or animal wastes, would also help reduce nitrogen inputs to the lake.”
To learn more:
For the complete study, visit the Proceedings of the National Academy website at https://www.pnas.org/eprint/BTJ6FEQZNWYCSCB93R2U/full.
Montana Living photos by David Reese