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Too Much of a Good Thing: Why Phosphorus Matters

  • Gracie Grimes ~ ECO Americorps Member
  • Aug 18
  • 4 min read
Lake Champlain via the South Hero causeway
Lake Champlain via the South Hero causeway

You’ve probably heard people talk about phosphorus in Lake Champlain in the news, at a public meeting, or even during a walk along the shore. Words like “legacy phosphorus” or “cyanobacteria” get tossed around a lot, but what do they actually mean? And why should we care? Together, we’ll walk through what phosphorus is, how it ends up in the lake, and why too much of it can cause problems. It’s a big topic, but I promise to keep things easy to follow!


So, what is phosphorus?

Phosphorus is a naturally occurring nutrient that plants and animals need to grow. It’s an important part of healthy ecosystems and is commonly found in fertilizers, where it helps crops thrive. But when there’s too much phosphorus in lakes and rivers, it can throw things out of balance. It can attach to tiny particles of soil and sediment. When that sediment settles to the bottom of a lake, it takes the phosphorus with it. Over time, this builds up and becomes what’s called "legacy phosphorus", old phosphorus that stays in the lakebed and can continue to feed algae blooms long after new sources are reduced.


How does it enter our waterways?

Tributary of the Missisquoi River
Tributary of the Missisquoi River

Most of the phosphorus in Lake Champlain comes from the land around it. Phosphorus is found in soil, fertilizers, manure, and even wastewater. When it rains or snow melts, water runs over fields, roads, and other surfaces, picking up phosphorus along the way. This runoff flows into storm drains, streams, and rivers, and eventually into the lake. The amount of phosphorus that enters the lake each year changes depending on the weather. Rainy years with heavy storms can wash in more nutrients, while dry years tend to bring in less. Things like snowpack, droughts, and soil conditions all play a role.


What's the impact?

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When too much phosphorus enters the lake, it can lead to serious water quality problems. Just like garden plants, algae and cyanobacteria (also known as blue-green algae) love phosphorus because it helps them grow fast. In Lake Champlain, there’s already plenty of other ingredients that algae need to grow, like nitrogen, carbon, and sunlight. So, phosphorus is the key that really drives blooms.

 

Some types of cyanobacteria are harmless, but when they grow into visible colonies, or blooms, they can release toxins that are dangerous for people, pets, and wildlife. These blooms can also use up the oxygen in the water, which can lead to fish kills and other ecosystem damage. Even if you don’t live on the lake, these blooms can impact recreation, tourism, drinking water, and the overall health of the watershed.


What's already being done?

Many groups are working hard to reduce phosphorus and improve water quality in Lake Champlain. The Lake Champlain Basin Program partners with organizations like the Lake Champlain Committee and state agencies in Vermont and New York to monitor water quality at over 130 locations around the lake.

 

In 2015, Vermont passed the Clean Water Act, which set new rules for phosphorus control and increased funding for clean water efforts. In 2021, Vermont and Quebec renewed a cross-border agreement originally made in 2002 that sets phosphorus limits in Missisquoi Bay—one of the areas most affected by blooms.

 

The state’s Clean Water Initiative Program works with local groups to support, fund, and track clean water projects. If you're curious to learn more about what clean water projects look like, check out this blog post by FNLC’s Project Manager, Josh Serpe.


What can you do at home?

Even small actions make a big difference, and there are lots of ways you can help reduce phosphorus pollution.

 

First, you can help track harmful blooms. The Vermont Department of Health’s Cyanobacteria Tracker lets you report blooms and see where others have been reported around the state.

 

You can also become a volunteer cyanobacteria monitor. These volunteers check lake conditions every week—whether there’s a bloom or not—and their observations help scientists understand how often blooms occur and how they change over time.

 

Want to take it a step further? You can become a lay monitor and collect samples and measurements of lake conditions like phosphorus levels, temperature, and water clarity. These long-term data are key to tracking trends and planning for the future.

 

If you live near a lake, the Lake Wise Program offers guidance on how to care for your property in a way that protects water quality. Even if you don’t live near the water, managing stormwater runoff on your property—like redirecting rainwater, planting more vegetation, or reducing paved surfaces—can help. Most of the water that falls on Vermont land eventually ends up in our lakes or rivers.

 

Lastly, get involved in your community! Your town or municipality may already be working on stormwater planning, road improvements, or conservation efforts. Showing up, asking questions, and supporting local clean water efforts is a powerful way to protect Lake Champlain. If you live in Franklin County, take a look at the Franklin County Stormwater website to stay up to date with community events and cleanup efforts.


A Healthier Lake Starts Here

Phosphorus might not seem like a big deal at first, but it plays a major role in the health of Lake Champlain. By understanding where it comes from, how it affects the lake, and what’s being done to manage it, we can all play a part in keeping the lake clean and healthy. Whether you volunteer, report a bloom, or make small changes around your home, your efforts matter. When communities come together to take care of the land and water around them, real progress happens. Lake Champlain is a vital part of our region, and it’s up to all of us to help protect it now and into the future.

 
 
 

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