A Bloom Across Chilmark Pond: What We’re Seeing and Learning

The recent cyanobacteria bloom throughout Chilmark Pond is revealing different patterns across the Middle and Lower Pond showing how this complex ecosystem changes from place to place.

Red Advisory Issued for Chilmark Pond

Over the past week, MV CYANO detected a cyanobacteria bloom across much of the Chilmark Pond ecosystem, including both the Middle and Lower basins. In response, the Chilmark Board of Health (BOH) issued a red “Bloom Advisory” on July 21, 2026, for all regular monitoring stations in Middle and Lower Chilmark Pond. As of July 21^st the Upper Pond near Lucy Vincent beach remained with lower concentrations of cyanobacteria compared to the other stations. However, cyanobacteria are still present here, and the BOH issued an orange “Bloom Watch” in this area.

Island Boards of Health use highly conservative risk levels to help keep the community safe. Under the current red “Bloom Advisory”, the Chilmark Board of Health advises the public to avoid recreational activity in the pond, with the exception of motorboat use. Please follow the MV CYANO color-grade guidance below and pay close attention to any posted signs at public launches. If you see a sign, read and follow the recommendations. It is especially important to keep pets away from bloom waters, as dogs can be particularly attracted to cyanobacteria. Keeping dogs leashed and out of the water is strongly recommended.

Bloom Dynamics

The bloom does not look the same everywhere in the pond. Depending on the location, it can appear as bright green specks, paint-like surface streaks, or a diffuse green tint throughout the water. Microscope observations also show differences in the form of Dolichospermum, the cyanobacteria genus observed in these samples.

Middle Pond: Bright Green Specks at the Surface

In the Middle Pond, near the LG-Sonic buoy, the bloom is most visible as bright green specks or clumps on the water surface. Under the microscope, these specks are made up of Dolichospermum strands that appear in a coiled form.

Specks/clumps in the Upper Pond

Upper pond water sample under the microscope showing coiled Dolichospermum (genus of cyanobacteria)

Abel’s Hill Crossing: Where Bloom Patterns Begin to Shift

Moving toward Abel’s Hill, the bloom begins to show a mix of features. Bright green specks and clumps are still present, but they are joined by vivid green surface scum that resembles paint streaks where the material has accumulated. Microscope observations from this area show Dolichospermum in two forms: the same coiled strands seen in the Middle Pond, along with straight strands that become more common farther into the Lower Pond.

Specks/clump accumulation near Abels Hill crossing Lower Pond. Green paint-like streaks on the surface.

Dolichospermum in two forms: Coiled and straight. Near Abels Hill crossing.

Lower Pond: Green Water and Reduced Clarity

Farther into the Lower Pond, the bloom looks different again. Instead of distinct green specks, much of the water has a diffuse green hue and very low clarity, with visibility limited to about 0.5 meters in many areas. In the Lower Pond water samples, with the exception of Abel’s Hill Crossing, Dolichospermum appeared only in straight strands; no coiled forms were observed.

Thumb Cove, Lower Chilmark Pond. Green-hued water with surface algal matting. The surface algal mats at this location have been confirmed to be primarily green algae, which are generally harmless; however, water samples from the area contained high concentrations of cyanobacteria.

Lower Chilmark Pond. Microscope view of straight-form Dolichospermum strands observed in water samples collected throughout the Lower Pond.

Lower Chilmark Pond. Zoomed-out microscope view showing only straight Dolichospermum strands, a pattern observed across Lower Pond samples.

What the Bloom May Be Telling Us

We will need to continue monitoring and collecting data to fully understand these patterns, but the current bloom dynamics across the Chilmark Pond system are already revealing something important. The straight Dolichospermum strands appear to be associated with the green-tinted water seen throughout much of the Lower Pond, while the coiled Dolichospermum strands seem to be associated with the speckled, paint-like bloom observed in the Middle Pond. Abel’s Hill Crossing, which sits between the Middle and Lower Pond, contains both forms of Dolichospermum, making it a possible transition zone between these two bloom forms. This is still a working theory, and continued data collection and analysis will be needed to better understand bloom dynamics throughout the Chilmark Pond system.

The Buoy and the Bloom: Early Signs of Localized Change

Many readers may be wondering: is the LG-Sonic buoy helping? Early signs are encouraging. Cyanobacteria concentrations in the southern portion of the Middle Pond, adjacent to the buoy, are noticeably lower than levels recorded at other monitoring stations, with the exception of the Upper Pond near Lucy Vincent Beach. Although cyanobacteria levels remain elevated in the Middle Pond, this lower abundance near the buoy suggests that it may be helping reduce the bloom in this localized area. We will continue monitoring throughout the season to better understand the buoy’s impact.

Continuing the Work Toward a Healthier Pond

While this bloom is discouraging, it also highlights how complex and dynamic the Chilmark Pond ecosystem is. Each monitoring season reveals new patterns, questions, and lessons. By continuing to observe, sample, and analyze these changes, we can better understand the system and work toward solutions that support a healthier pond.