Current Nature: Green Water: HAB You Seen Any?

Kristin Bullett •

A pond edge is in the foreground, with a thick green color in the water’s edge and along the shoreline. Trees are in the background.
A telltale sign of a harmful algal bloom is green scum along the water’s edge, as shown here. Photo courtesy of The Linda Loring Nature Foundation

Nantucket may be known for its beaches, but our inland water resources are spectacular too. If you have visited any, you already know our ponds are gems. They are a great place to see birds, interesting plants, and cool insects. Many of Nantucket’s ponds were once treasured swimming holes. Although that legacy remains, today these ponds are unfortunately becoming known for their algal blooms, which typically ramp up this time of year and can persist through fall.

Algae is a normal and important part of aquatic ecosystems. “Algae” describes a diverse assortment of organisms that photosynthesize (like plants) but lack the parts of "true” plants like roots and stems. Algae range from microscopic, single-celled organisms to large seaweeds.

We often hear that trees are the planet’s lungs, but technically they are just one lung. The other lung is cyanobacteria, a two-billion-year-old group of bacteria that function like algae and produce oxygen. These tiny organisms produce nearly half the oxygen on Earth! Cyanobacteria were the first organisms to produce oxygen and established Earth’s atmosphere, so we have them to thank for getting this whole party started. While cyanobacteria are essential to ecosystems and humans alike, they can be too much of a good thing.

A photo of a pond’s edge, with a blue sky reflecting on the water. A clump of vegetation is seen on the right side of the shoreline.
The North Head of Long Pond on a gorgeous fall day. Photo courtesy of The Linda Loring Nature Foundation

Under the right conditions (like high nutrient levels and warm temperatures) cyanobacteria can grow out of control. Some cyanobacteria species produce toxins that can kill fish, mammals, and birds. Even species that do not produce toxins are harmful to the ecosystem because when they eventually die, its decomposition uses up a lot of oxygen in the water. This creates a hypoxic, or low oxygen, an environment that can be deadly for aquatic life.

Harmful Algal Blooms (HABs) can occur anywhere, as cyanobacteria have colonized every biome on earth. One type of HAB you may be familiar with is a “red tide,” which occurs in salt water with a reddish species of algae that produces lethal toxins. This commonly causes shellfishing closures, as the toxin makes the shellfish unsafe to eat.

It is impossible to tell if a HAB is producing toxins just by looking at the water, so if you see signs of a bloom, you should assume it is toxic. Stay out of the water and away from the shoreline. That means pets too! The toxins that blue-green algae can produce may cause liver, kidney, and neurological issues in humans and pets.

Two photos are side by side for comparison. On the left, the photo shows two yellow and white signs that read ATTENTION with small black text about algal blooms. On the right, the photo shows two red signs that read CAUTION with small black text advising people to stay away from the pond.
All of the monitored ponds on island have these yellow signs posted to alert people to the potential of a harmful algal bloom. When a bloom is actually detected, these signs are swapped out for red signs that advise people on the status of the pond and to stay away. Photo courtesy of The Linda Loring Nature Foundation

What are we doing about HABs on Nantucket? For starters, conservation organizations and town agencies collaborate on weekly monitoring of the major ponds on island. The Nantucket Land Council, Mass Audubon, Nantucket Land Bank, UMass Boston Field Station, Nantucket Conservation Foundation, the Town’s Department of Natural Resources along with us at the Linda Loring Nature Foundation check 16 ponds across the island for HABs.

Signs of a Harmful Algal Bloom:

- Green or blue-ish scum anywhere- in the water, on the surface, or in sand on shore

- Water appears overall green

- A foul smell

When a pond has an active HAB, the monitoring team is notified and the Town website is updated. Signs are posted at the pond, and every organization in the monitoring team posts on social media to inform the community. We also take extra outreach opportunities (like the very article you are reading) to share the message. Education and information sharing is the best tool we have in keeping people and pets safe, so share what you know with anyone spending time outdoors.

Although many of Nantucket’s ponds are monitored, a pond could bloom at any time. Monitoring is performed weekly and at specific locations and times, and blooms may occur and dissipate throughout the week. Always keep an eye out and report any signs of a HAB through the Town website here[SE1] .

In addition to visual monitoring, Nantucket scientists are conducting more in-depth research on Harmful Algal Blooms and water quality. The Nantucket Land Council collects water samples from some HABs to determine what algae species are present and if the algae have produced toxins. The Land Council has studied Nantucket’s ponds for years, and has compiled data on measurements like depth, pH, salinity, and clarity. The Town of Nantucket also has a robust water quality program, sampling for bacteria and nutrients in addition to standard water quality parameters like dissolved oxygen and temperature.

Photo shows a pond’s edge, with a wide band of green-blue discoloration in the sand.
Keep an eye out around the shoreline- algae can accumulate here and alert you to a bloom in the pond. This particular algae species is a striking teal color. Photo courtesy of The Linda Loring Nature Foundation

At LLNF, we have recently launched a research program that conducts regular monitoring of the North Head of Long Pond. We are collecting a baseline dataset of standard measurements to track the health of the pond. Our property comprises much of the pond’s watershed, and the pond itself is a major habitat that we are proud to be stewards of.

It has been exciting to get to know “our” pond a little better! The water quality project is a favorite among interns and volunteers at LLNF, and is a great hands-on learning opportunity for students. The next time you drive on Madaket Road, look to your right at First Bridge - you just might see our colorful kayaks out and about on the pond!

Image shows four species of algae under a microscope. Their shapes are varied, one being c-shaped, star shaped, circular and even a long squiggly shape.
An up-close view of some of the algae species that have been found in Nantucket’s ponds. From left to right, Dolichospermum solitarium, Asterionella, Microcystis aeruginosa, and Dolichospermum mucosum. Aren’t they beautiful? Photo courtesy of The Linda Loring Nature Foundation

Nantucket’s ponds are beautiful and full of life. They are a fantastic place to spend a nice day. The more we know and understand about our ponds, the better we can protect them and keep people visiting them safe. Although HABs may occur at different ponds throughout the summer, that does not mean that you need to avoid all pond interactions. Keep a note of the weekly updates and check ponds for signs of HABs before recreating in them and you are sure to have a healthy and happy summer.

Two people in kayaks on a pond under a blue sky smile.
When conditions are right, Nantucket’s ponds are a joy to explore. Photo courtesy of The Linda Loring Nature Foundation

Stay tuned for more editions of Current Nature, a bi-weekly column featuring seasonal topics, natural history information, and advice on the outdoors from the staff at the Linda Loring Nature Foundation.

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