Why is Cayuga Lake Green?

If you’ve ever visited Cayuga Lake during late summer, you might have noticed something strange: the water isn’t always blue. Sometimes it looks murky, even bright green. This shift in color isn’t just a trick of the light — it’s a real environmental issue that locals and scientists pay close attention to.

So, what causes Cayuga Lake to turn green?

The answer lies in the growth of algae — tiny aquatic organisms that thrive under certain conditions. During the warmer months, the lake can experience algal blooms, where algae multiply rapidly, changing the color and clarity of the water. These blooms are most often made up of cyanobacteria, also called blue-green algae, which are known for turning lakes like Cayuga various shades of green.

The main reason for these blooms is a combination of heat, sunlight, and an overload of nutrients — especially phosphorus and nitrogen. These nutrients often come from nearby farmland, lawns, failing septic systems, and stormwater runoff. When they wash into the lake, they create the perfect environment for algae to flourish.

While algae are a natural part of any freshwater ecosystem, excessive growth can become a problem. Some algal blooms produce toxins that are harmful to humans and animals. These are known as Harmful Algal Blooms (HABs). When a HAB occurs, it can make the water unsafe for swimming, fishing, or boating. Pets are particularly vulnerable, and even brief contact with toxic water can lead to illness or worse.

This is why you’ll sometimes see warnings or advisories posted along Cayuga’s shoreline during the summer. Officials and scientists from organizations like the Cayuga Lake Watershed Network and New York State DEC monitor water quality closely, especially during peak bloom season. When a bloom is detected, they alert the public and often recommend avoiding contact with discolored or scummy-looking water.

Climate change may be making the situation worse. Longer, hotter summers mean longer periods of warm, still water — exactly the conditions algae love. At the same time, development around the lake continues, increasing the risk of nutrient-rich runoff entering the watershed.

Despite the challenges, there are efforts underway to protect Cayuga Lake. Farmers are being encouraged to use better land management practices. Septic systems are being upgraded, and public education campaigns are helping lakeside property owners understand their role in keeping the water clean.

As a visitor or resident, there are small things you can do to help. Reducing fertilizer use, planting native plants along the shoreline, and supporting local conservation efforts all contribute to a healthier lake.

So yes, Cayuga Lake sometimes turns green — but it’s not a mystery. It’s a mix of natural biology and human impact. Understanding why it happens is the first step toward protecting one of the Finger Lakes’ most beloved waters.