The Ponds of Martha's Vineyard: Great Ponds, the Cut, the Blooms and Where to Meet Them (2026)

Look at a map of Martha's Vineyard and the south shore looks like a comb: a long barrier beach with a row of ponds behind it, each one a drowned valley the glacier left, sealed off from the Atlantic by sand. The north shore has its own set, with permanent openings to the Sound. Islanders call all of them ponds. The oysters and scallops grow in them, the herring run through them, the kayaks and the summer houses face them, and the island's hardest environmental argument is about them. The sections below cover the three kinds of pond, the odd ritual of cutting a great pond open to the sea, why some of them bloom green in August and what the color codes mean, what nitrogen has to do with your rental's septic tank, and where a visitor can swim, paddle and watch it all work.

Filmed June 2023The island moves on: shops, signs and prices may differ from what you see here. The text of this guide was checked October 2026.

Three kinds of pond

The cut: opening a pond to the sea

Several times a year, on a schedule set by the town shellfish constable and a committee of the pond's shoreline owners, an excavator drives onto the barrier beach and digs a trench from the pond to the surf. The pond, which has risen with winter rain, pours out through the trench in a torrent that can be heard from the parking lot; the ocean pours back in on the next tides; and for days or weeks the pond breathes salt water until the surf rebuilds the beach and seals it again. The cut does three jobs. It lowers the pond before it floods the low roads and cellars. It brings in the salinity that oysters, eelgrass and the herring need, and lets the herring in to spawn. And it flushes out the nitrogen that has built up since the last opening, which is the job that matters most in August. How long a cut stays open decides the pond's year: Edgartown Great Pond was open 92 days in 2018, 15 in 2022, 16 in 2025, and 19 days in March and April of 2026, which the Great Pond Foundation called a good start; Tisbury Great Pond is cut too, but as the 2026 summer showed, a cut alone does not keep a pond clear. The cuts are dramatic to watch and dangerous to approach: the current through a fresh opening runs like a river, the banks collapse, and the towns close the beach around it, so the view is best from a distance (the beach safety guide has more).

Why the ponds bloom, and what the colors mean

Cyanobacteria, the blue-green algae, live in every pond and become a problem when three things line up: warm water, low salinity (below about 15 parts per thousand) and an excess of nitrogen and phosphorus. Then they bloom into green scum, clumps and paint-like streaks along the shore, and some strains produce toxins that damage the liver or the nervous system and can kill a dog that drinks the water. Since 2021 the island's boards of health and the Great Pond Foundation have run MV Cyano, a monitoring program that samples eight ponds routinely, fourteen in 2025, at eleven stations, and posts a color for each: green for no bloom, yellow for an alert, orange for a bloom watch, red for a bloom advisory. The 2025 season brought the most blooms since the program began, with record concentrations at Edgartown Great Pond, routine early-summer blooms and high toxin readings at Chilmark Pond, one of the highest liver-toxin readings the lab had ever seen at Squibnocket Pond in June, and repeated blooms at Mink Meadows Pond in Tisbury. The 2026 summer told a different story: Tisbury Great Pond and Chilmark Pond bloomed again by late July, while Edgartown Great Pond stayed clear at every station, which the foundation credited to the spring cut and to eelgrass that had come back and was eating the nutrients before the algae could.

For a visitor it comes down to a few habits: checking the current colors at greatpondfoundation.org/mvcyano before a pond swim or paddle, staying out of water with green clumps, scum or streaks (and out entirely near an open cut), rinsing in fresh water afterwards, and keeping dogs away from a pond under advisory, because they drink it and lick their coats. A red pond is not closed to boats, but it is no place to swim or let a dog drink.

Nitrogen: the argument under the houses

The nitrogen that feeds the blooms comes, on this island, mostly from under the houses: the state's standard Title 5 septic systems, which were designed to remove bacteria and do nothing about nitrogen, plus lawn fertilizer and road runoff. Every summer house on a pond watershed adds its share, and the Massachusetts Estuaries Project set each pond a target: Lagoon Pond needs its nitrogen load cut by about 35 percent, Lake Tashmoo by 32, Menemsha Pond by 47, Tisbury Great Pond by 19. The eelgrass that scallops and young fish need is the first casualty, and there is little of it left in Katama Bay or Sengekontacket. The fixes are expensive and slow: Edgartown's wastewater treatment plant is the single biggest thing that town has done for its great pond; Tisbury's State Road sewer project and its 2023 Lake Tashmoo watershed plan are the down-island equivalents; nitrogen-removing septic systems are required on some new houses; and the Shellfish Group's eelgrass restoration and the oysters themselves, which filter water and take nitrogen out with them when they are harvested, are the biological half. Our shellfish guide explains that side. None of it is fast, which is why the cut on the beach still matters every spring.

Where to meet the ponds

What a visitor can do

• Before a pond swim, the color is worth a look: MV Cyano’s map at the Great Pond Foundation, and the state beach dashboard for bacteria; the beach safety guide links both.
• The salt ponds are the island’s best flat water for paddling; the kayaking guide has launches, rentals and the wind warning. Boats with heads use the harbor pump-outs, never the pond.
• Renting on a pond comes with a few easy habits: no fertilizer on the lawn, nothing down the drain that should not go there, and the septic pumped on schedule; the rental guide explains why the lease mentions the septic system.
• Shellfishing takes a town permit: quahogs year-round, scallops from October, oysters in winter, each under the town’s rules; the shellfish guide has the permits and the seasons.
• A pond cut is best watched from the top of the beach, behind the town’s tape, because the trench banks move and the current in it is fast.
• Dogs are safest on a leash and out of green water, since bloom water can kill a dog that drinks it.
• October is a lovely time to see them, with scallopers on the Lagoon, ospreys leaving, the great ponds full and clear after the autumn cut, and the Milky Way over the water.

At a glance

How we checked this

The cut history of Edgartown Great Pond, the 2026 opening, the eelgrass return and the wastewater plant are from the Vineyard Gazette of April 19, 2026; the 2025 MV Cyano results and the 2026 blooms from the Gazette of May 12 and July 28, 2026; the program’s structure, color codes and health guidance from the Great Pond Foundation’s cyanobacteria resource; the nitrogen reduction targets from the Massachusetts Estuaries Project as reported by the Gazette and the Martha’s Vineyard Commission; the Tashmoo watershed plan from the Martha’s Vineyard Times of July 2023. Pond conditions change with every rainstorm, and the foundation’s map is the only current source. The photographs are ours.

Frequently Asked Questions

What is a great pond?

Tisbury Great Pond, Edgartown Great Pond and Chilmark Pond: drowned glacial valleys behind the south-shore barrier beach with no natural inlet. Their water is brackish and their health depends on being cut open to the sea by an excavator several times a year.

What is the cut?

On a schedule set by the town shellfish constable and the pond’s shoreline owners, an excavator digs a trench from the pond to the surf. The pond pours out, the ocean pours back in on the next tides, and for days or weeks the pond breathes salt water until the surf seals the beach again.

Which ponds are open to the sea?

The salt ponds: Sengekontacket, Lagoon Pond, Lake Tashmoo, Menemsha and Nashaquitsa Ponds and Katama Bay, with permanent openings, bridges or jetties and real tides. They hold bay scallops, quahogs, oyster farms, ospreys and most of the island’s kayaking.

Can I swim in a pond?

Seth’s Pond in West Tisbury has a town swimming beach and Blackwater Pond has a Land Bank loop and boardwalk; the other kettle ponds are for looking at. The salt ponds are sheltered enough for children, though the openings have tidal currents.

Why do some ponds turn green in August?

Cyanobacteria bloom when warm water, low salinity and too much nitrogen and phosphorus line up, and much of that nitrogen comes from septic systems. MV Cyano posts a color for each monitored pond, worth checking before any pond swim and especially after heavy rain.