Finding the balance for Flathead Lake

Everything in moderation is a tough phrase for me to swallow. Especially this time of year, when the changing seasons stir a Pavlovian, pumpkin-spiced response and my brain flushes all will power to make room for the Halloween candy and Christmas cookie overindulgences that are all but guaranteed to consume my personal nutrition plan for the next three to six months.

It often takes every ounce of self-control that I can muster to reach the produce section of the grocery store, where nutrient-rich fruits and vegetables are waiting to counterbalance my sugar cravings, and do my body good.

A well-balanced diet is important for humans to maintain. When it comes to health and sustainability of our watershed, nutritional balance is essential. The food web of Flathead Lake is dependent upon a delicate and specific ratio of the nutrients nitrogen and phosphorus.

As any gardener or farmer can tell you, nitrogen and phosphorus are the building blocks of life. They’re the chemical elements found in soils and fertilizers that plants need to grow. From bountiful backyard flowers to expansive agricultural harvests, the ratios of nitrogen and phosphorus can either be ideal or detrimental to the development and health of different crops and plants. But the impact of nitrogen and phosphorus aren’t limited to the land. These elements also play a powerful role in freshwater ecosystems, with the potential to greatly alter the clear and pristine waters of low-nutrient lakes.

First ingested by microorganisms, nitrogen and phosphorus then serve as the nutritional foundation for the next tier of consumers, and then the next, and so on, until those nutrients are ultimately contained in a campfire-roasted trout on your dinner plate. Once you dine on that trout, the nutrients don’t disappear. They are expelled through our wastewater treatment systems (septic and sewer), where they find their way back into the ecosystem—for better or worse.

For much of history, human waste and wastewater went untreated. Because human waste concentrates nutrients, untreated wastewater created a tremendous influx of nitrogen and phosphorus into freshwater ecosystems. This overabundance polluted water world-wide, making it unsanitary, and causing unsightly and at times toxic algal blooms.

As a result, wastewater treatment became a priority all over the world, and wastewater treatment technologies have become extremely effective at removing phosphorus—a dominant element in human and animal waste— before returning water to an ecosystem. And it worked!

After the Bio Station received an EPA grant to build a wastewater treatment plant in the 1970s to demonstrate that low-level phosphorus removal was possible in the Flathead, other wastewater treatment plants throughout the Flathead Watershed followed suit. These municipal plants were then upgraded in the 1980s and early 1990s. Bigfork’s current wastewater treatment plant is extremely effective at removing nutrients and keeping them out of Flathead Lake. Such investments in wastewater treatment have greatly benefited Flathead Lake, keeping it clean and blue. Toxic algal blooms went away. Water became more sanitary. The problem, it seemed, was solved.

But as more phosphorus is stripped by wastewater treatment, the amount of nitrogen left behind results in a greater nitrogen-to-phosphorus ratio, and this higher ratio can be reflected in freshwater ecosystems into which wastewater is discharged.

FLBS Director Jim Elser, who specializes in nutrient ratios, began to wonder about these effects in Flathead Lake. With a long history in developing and testing the theory of ecological stoichiometry (the study of the balance of multiple chemical elements in ecological interactions) he was eager to find out.

In a recent study published by the Proceedings of the National Academy of Sciences, Elser and a team of researchers examined nearly 40 years of nutrient dynamics in Flathead Lake. This unique dataset, assembled by the FLBS Flathead Monitoring Program, documents consistently low levels of nutrients—which is great!

But the dataset also showed a sustained imbalance between nitrogen and phosphorus. This imbalance likely has significant ecological consequences in Flathead Lake, as well as in other low-nutrient ecosystems.

Organisms require a specific combination of nitrogen and phosphorus to survive, grow, and reproduce. When there is little or no phosphorus in the water, the growth and reproduction rates of organisms slow down. In Flathead Lake, there are two potential undesirable consequences of nutrient imbalance. In the short term, phosphorus limitation can result in less tiny algae-eating animals (known as zooplankton), and the clear water of Flathead Lake could begin to turn green and murky. In the longer term, if the phosphorus limitation results in fewer zooplankton (which serve as fish food), there could be fewer and smaller fish.

Through a series of experiments, the team of researchers showed that this could happen in Flathead Lake. In low phosphorus conditions, Flathead Lake algae still continue to grow, but they’re forced to build cells that have low amounts of phosphorus. This makes them less nutritious for the zooplankton, which eat those algae and thereby sustain the lake’s high transparency.

As the zooplankton eat the less-nutritious algae, which amounts to the equivalent of a “junk food” diet, they lose the ability to grow and reproduce rapidly. As populations of zooplankton stay low, the potential for the world-renowned clear, blue waters of Flathead Lake to turn green goes up.

Additionally, the team showed that the strong 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.

Maintaining a healthy balance of nitrogen and phosphorus in Flathead Lake is a worthy effort, beneficial to the ecological, economic, and recreational health of Flathead Lake. That’s why the Bio Station’s long-term Flathead Monitoring Program—which is made possible almost entirely through the generous support of our local businesses, organizations, and residents—is such an important resource.

Our understanding of this wonderful lake and any effort to protect it depend upon reliable, high quality, long-term data. Because of this data, FLBS scientists are better equipped to monitor and understand the changing conditions in Flathead Lake and beyond, giving us the best opportunity to keep our waters clean and healthy for generations to come.