A large algal bloom has spread along a portion of the Edgartown Great Pond, and it’s causing concern among local scientists, according to a report from the Edgartown Great Pond Foundation.
An algal bloom is a rapid increase of algae in freshwater and saltwater that comes to dominate an ecosystem. An algal bloom is dangerous because when the large biomass of algae dies, it sinks and blankets the bottom of the pond, where it is then broken down by bacteria. Those bacteria take oxygen out of the pond estuary that other organisms need to survive, according to water-quality information on the foundation’s website.
The most dense bloom is lining a pocket of the southeast corner of the pond. The bloom appears to spread from Crackatuxet Cove along the eastern shoreline, and into Slough Cove.
Algae in the pond is nothing new, and in small doses is a normal part of the cycling in the ecosystem, the report details.
Eutrophication — the buildup of excess nutrients — in the pond is the main culprit behind the bloom. Most estuaries like the pond are dealing with nitrogen, which enters the pond through rainwater, decomposition of biological matter, animal waste, septic sources, or fertilizer, according to the foundation’s website. Phosphate, nitrate, and ammonia are other nutrients known to cause blooms. In a nutshell, excess nutrients cause blooms, blooms cause oxygen starvation, and oxygen starvation harms other organisms in the ecosystem.
“It’s when the macro algae dies we start to be concerned,” foundation director of science and education Emily Reddington told the Times in a phone conversation. “Things can get out of balance in an ecosystem when there is a change to the environment.”
Woods Hole Oceanographic Institution research specialist Mindy Richlen and her colleagues, who are doing work at the pond, believe the algae to be ulva. The exact species of Ulva is still being determined.
To tackle the issue, Reddington, foundation interns Sam Hartman and Spencer Goldsmith, Edgartown shellfish constable Paul Bagnall, drone photographer David Welch, and the Martha’s Vineyard Shellfish Group have been taking water samples around the bloom, monitoring its growth and expansion, and manually removing the current bloom.
Manual removal is the foundation’s best treatment at this stage, as they continue to study the bloom. Removing clumps of the algal bloom, however, is not practical.
“It’s just manual removal, but it’s not very practical on this large scale. It’s hard to remove that much biomass, but that’s the best management treatment right now. We need to continue to monitor to understand what drove the bloom to happen in the first place,” Reddington said.
Algal blooms have happened in the pond before. In 2008, a particularly terrible bloom damaged oyster populations in the pond. A restoration project for the oysters, which naturally improve the quality of the water, is still going on today. The decade-old project has seen the oyster population go from unhealthy to supporting commercial harvests in recent years.
Overall, the pond is in good shape, Reddington said; the water clarity and water oxygen throughout most of the pond are good. Nitrogen levels are also decreasing to target levels, and eelgrass beds are expanding.