Lake Auburn is at a tipping point, sitting on the edge between its current clearwater state and one where it becomes green and prone to large blooms of algae and cyanobacteria (sometimes called blue-green algae).

In its current clearwater state, water quality is good enough for trout, for our drinking water to be treated by ultraviolet light and chlorine without first being filtered and for all of us to enjoy the lake without noxious scums. In the green state, none of these things would be true.

A transition from the clearwater to the green state can happen rapidly, and the change is almost always permanent. The transition happens because nutrients such as phosphorus and nitrogen flow from the watershed into the lake, fueling algal growth. As algae decompose at the bottom of the lake, oxygen is depleted. This leads to chemical reactions that add even more nutrients, leading to a run-away cycle that lands the lake in a nutrient-rich, green state.

This is what happened to China Lake about 40 years ago; the lake now is rated as having “poor” water quality, and expensive filtration is required for the drinking water.

A view Wednesday afternoon of the giant sandpit across from Gracelawn Road, showing how close it is to Lake Auburn, background. A recent change in the watershed boundary has paved the way for development at the site. Auburn Public Works property can be seen in the foreground/ Visit sunjournal.com to watch a closer view of the area. Russ Dillingham/Sun Journal

Municipalities that can protect watersheds and water supplies have less expensive, better tasting water and they know that what they are doing is in the public good. That includes us; those of us drinking public water or using it in our businesses in the Lewiston-Auburn area are enjoying among the lowest water rates in the state — less than half that charged in many places in Maine — all because of the protection of Lake Auburn.

The filtration waiver we have — like ones for Portland, Bangor, Bar Harbor, Boston, New York City, San Francisco and Seattle — is worth protecting. Waivers prevent municipalities from having to build plants to filter the water before treatment.

In the case of New York City, they have spent $1.7 billion (yes, billion) since the 1990s to protect land and water quality in the watersheds of the reservoirs that feed the city. And they are poised to spend more because it would cost $10 billion to build filtration facilities and another $100 million each year to maintain such facilities.

This return rate of $5 for every dollar spent may even be low. The US Environmental Protection Agency estimates that we save $27 for every dollar spent on source-water protection. Most of us would be ecstatic if our savings account offered that kind of interest rate.

All this is threatened by what is happening right now in Auburn.

Part of this threat we can only distantly control — the effects of climate change with its shorter winters, rain events in winter, warmer temperatures and big storms in the summer. Each of these factors has a negative effect on water quality. Ice goes out about three weeks earlier than it did when record keeping started in 1836, and because freezing happens later, we have nearly 40 days less of ice than we did in 1953 when they started recording ice-in. This means there is more time with lots of light and warmth for organisms to grow in the lake.

Algae and cyanobacteria also grow more because there are more nutrients washing in from the watershed, and the intense summer storm events are particularly big sources of sediment and nutrients to the lake. We have seen algal blooms and water transparency become much worse after big storms in 2011, 2012, 2018 and 2021. There are good data to suggest that big storms washing nutrients from the watershed are the cause of this. Combine more nutrients with warmer temperatures and you have a recipe for worse water quality and those feedbacks that lead to a green lake. The fish kill in 2012 was the result of this kind of cycle.

But there is part of this we can stop. The sediment and nutrients washing into the lake come in largest quantities from where the soil is disturbed and where surfaces such as pavement prevent the water from filtering into the ground rather than running into the lake.

What if we put all of our efforts into working with everyone in the watershed to reduce or halt activities that add nutrients and worked collectively to figure out how to keep sediment and nutrients from flowing into the lake in such large quantities?

This is a hard problem that would require a lot of collaboration, but we then stand a chance of keeping our filtration waiver, continuing to drink good water, and saving ourselves both the tens of millions of dollars it would take to build a filtration plant and the extra couple million dollars a year it would take to run such a plant.

The city of Auburn asked consultants to study the environmental, economic and regulatory impacts of water-supply protection in the Lake Auburn watershed. One conclusion was that it would not benefit the city financially to maximize development in the watershed, but there is movement toward changes that will maximize development anyway.

If we want to preserve Lake Auburn, our drinking water and our money, we all need to wake up, listen, watch, engage in conversations with our neighbors, ask a lot of questions and think hard about how any proposed change might influence water quality in the lake. Any answers need to be tested against scientific evidence, not general assurances. The drinking water for tens of thousands of people is on the line.

Holly Ewing is a professor at Bates College who collaborates with students, water quality managers, and academic colleagues to understand Lake Auburn and other lakes in the Northeast.