What’s that in the water? A quick guide for lake users

I was out on the lake this week for routine sampling when I noticed a familiar sign of summer: tiny yellow-green dots floating just below the surface.

I’ve seen them on Winni year after year, and researchers have documented their presence in the lake since at least the 1970s. It was my first sighting of Gloeotrichia (pronounced GLEE-oh-TRICK-ee-ah) this year, a type of cyanobacteria that tends to appear mid- to late-summer when conditions are right. Seeing it reminded me how easy it is to overlook, and it got me thinking about what to write for this week’s column.

Chances are, most people wouldn’t know what they were looking at, or how many other strange-looking but often harmless things show up in the lake this time of year — including green filamentous algae, iron bacteria, periphyton or my personal favorite, bryozoans. All of these, including Gloeotrichia, are part of a lake’s natural ecology.

While cyanobacteria are a natural part of the lake, it’s their rapid growth and bloom formation, some of which can produce toxins, that signal the need for caution. Like all plants and algae, cyanobacteria need sunlight and nutrients to grow, and when both are abundant, blooms can form quickly. Knowing what to look for, and when it might be cause for concern, helps protect not just our lake, but also ourselves, our kids, and our pets.

Gloeotrichia is often described as tiny, fuzzy spheres that float just below the surface and can be seen with the naked eye. Each colony is about the size of a pinhead, typically 1-to-3 millimeters in diameter, and contains around 5,000 cells. Unlike other types of cyanobacteria, Gloeotrichia tends to stay suspended in the water column rather than forming the thick, paint-like surface scums people usually associate with blooms. What makes it especially interesting is its life cycle. Gloeotrichia starts its growth in the sediments at the bottom of the lake.

In early summer, it uses gas-filled vesicles to rise up into the water column. This kind of vertical migration is unusual among cyanobacteria and may help explain why Gloeotrichia tends to appear in clearer, deeper lakes like Sunapee, Moosehead, and Winni.

While it doesn’t usually form thick surface scums, this type of cyanobacteria can still build up along shorelines when the conditions are just right.

As NHDES explains, “Under certain conditions, they can accumulate along shorelines in more dense concentrations of material, occasionally forming green or yellow clouds or surface scums. Oftentimes these accumulations do not last long and become mixed back into the water to lower densities. These blooms can rapidly change and accumulate in various locations around a waterbody.”

NHDES encourages residents to keep an eye on their own shoreline conditions and to avoid contact with areas where the water appears unusually dense or cloudy. They offer a helpful visual: “If it looks like a starry night, the risk is low. But if it looks like a galaxy, consider staying out of the water and keeping pets out as well.”

Not all unusual-looking things in the water are cyanobacteria, though. Many natural substances are often mistaken for harmful blooms. Knowing what you are looking at can help reduce unnecessary concern and make sure that when there is a problem, it gets reported quickly.

One of the most common harmless lookalikes is green filamentous algae. This type of algae grows in long, stringy strands that often cling to rocks, docks, and submerged surfaces. It tends to be slimy or fuzzy to the touch and often floats in loose mats that resemble green cotton candy. It does not produce toxins and is not harmful. If you are unsure what you’re seeing, try the stick test: if you can lift it out of the water with a stick and it clings together like stringy hair, it’s probably filamentous algae, if it breaks apart, clouds the water, or looks like paint, it could be cyanobacteria.

Iron bacteria often show up near culverts, wetlands, or other areas where groundwater seeps into the lake. These naturally occurring bacteria create an oily or rainbow-colored sheen on the water and are frequently rusty orange or reddish-brown in color. They may produce a metallic or swampy smell. Although they may look like pollution, they are harmless and non-toxic.

If you’ve ever felt a slimy coating on submerged rocks or noticed a fuzzy green film near the shoreline, you’re likely seeing periphyton, a natural biofilm made up of algae, bacteria, fungi, protozoa, and tiny invertebrates that attach to underwater surfaces. This slimy layer plays an important role in the lake’s ecosystem by producing oxygen, recycling nutrients, and providing food for insects and other aquatic creatures. While it might feel unpleasant underfoot, periphyton is a normal part of lake life and isn’t a sign of pollution or harmful blooms. However, excessive growth can sometimes indicate increased nutrient levels or other environmental changes in the lake.

And then there are the truly odd surprises, like bryozoans. These gelatinous, blob-like colonies sometimes attach to docks, logs, or rocks and catch people off guard. They’re not usually confused with cyanobacteria, but they still spark plenty of questions. Up close, they have a jelly-like texture and sometimes a honeycomb pattern. Bryozoans are actually harmless filter feeders that help clean the water by straining out tiny particles, even if they do look like something out of a sci-fi movie.

So how do you know when to be concerned? Cyanobacteria blooms can look different depending on the type and conditions. While some look like blue-green paint, others appear as cloudy or discolored water, surface streaks, foam, scum, or floating clumps near shore. They may also have a strong earthy or musty smell. If you see anything that fits this description or makes you second-guess the water quality, it is always best to err on the side of caution. “When In Doubt, Stay Out!”

NHDES tracks cyanobacteria blooms and issues public health warnings when cell counts are found to exceed the state threshold of 70,000 cells/mL. However, they rely on reports from the public to detect potential blooms.

If you see something unusual, take a few photos and submit them using the NHDES Bloom Report Form, available at des.nh.gov/water/healthy-swimming. If you’re on Winni, you can also text or call me at 603-619-8703. LWA and NHDES coordinate bloom sampling efforts on the lake. For the most up-to-date information on cyanobacteria blooms and E. coli advisories, visit the NHDES Healthy Swimming Mapper, at des.nh.gov/water/healthy-swimming/healthy-swimming-mapper.

With the help of shoreline residents and lake users, we can continue protecting water quality and keep Lake Winnipesaukee safe, not just for the people who enjoy it today, but for everyone who depends on it during the height of the summer season and beyond.

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Bree Rossiter is the conservation program manager for the Lake Winnipesaukee Alliance. She leads the water quality and cyanobacteria monitoring efforts, coordinates the LakeSmart program to promote lake-friendly practices among property owners, and supports community education and outreach initiatives. Bree can be reached at brossiter@winnipesaukee.org. The Lake Winnipesaukee Alliance is the only nonprofit dedicated to protecting the water quality and natural resources of Lake Winnipesaukee and its watershed. To learn more visit winnipesaukee.org.