An undergraduate research project has verified concerns that a cyanobacterial algal bloom in Oyster Pond this summer emitted potentially dangerous toxins.
Kristy Sullivan, a junior at Wheaton College, undertook the study as a participant in the Semester in Environmental Science program at the Marine Biological Laboratory in the fall.
Under the guidance of molecular biologist Kristin E. Gribble and with help from members of the Oyster Pond Environmental Trust, Kristy analyzed samples from Oyster Pond to identify the different types of algae present in the water and sediment.
Kristy gathered samples between November 14 and December 5, during which time the algal bloom had already subsided. However, by taking the samples to the laboratory, adding excess nutrients and incubating the water and sediment, she was able to quickly grow the already-present algae enough to identify the various algal genera.
She discovered five types of freshwater algae: two of those were Microcystis and Aphanocapsa, which are cyanobacterial species that create microcystin toxins. Those toxins can cause nausea, vomiting, paralysis and acute liver failure in large doses or in the case of long-term exposure.
Based on her cell counts, Kristy said that a child would have to drink a few liters of water from Oyster Pond before reaching the dangerous toxicity levels set by the World Health Organization, but she did note that individuals with liver conditions may be at a higher risk of harm by the toxins.
Kristy also tested the salinity of Oyster Pond and reported that in December the salinity measured 0.16 parts per thousand more than it had in August, at the height of the algal bloom.
However, salinity levels remained below the suggested threshold of the Oyster Pond Environmental Trust—between two and four parts per thousand—which would limit freshwater algal growth.
Although the pond has a working weir built to limit the amount of inflowing saltwater, sediment build-up in Trunk River and the connected lagoon has blocked the flow of water from Vineyard Sound into the brackish pond.
An “emergency” dredge to remove 16 cubic yards of sediment from the lagoon at the end of the summer did increase the inflow of saltwater and alleviate the algal bloom, but Oyster Pond Environmental Trust director Wendi B. Buesseler said salinity levels still remain too low.
In her report, Kristy said that the strong presence of Microcystis algae in the sediment samples suggested that the species is overwintering in pond sediment, and she concluded that another algal bloom could arise in the summer of 2017 if salinity levels do not increase.
Beyond concerns about the harmful toxins, trust members also worry that another algal bloom could cause a fish kill, depleting the already-fragile herring population in the pond.
“We’re so worried that this will happen again, even though it was sort of a freakish confluence of the blockage, the drought and the weather,” Ms. Buesseler said.
In order to prevent a future bloom and potential fish kill, Ms. Buesseler has been collaborating with the town marine and environmental services department to dredge the lagoon.
Work began in February and Ms. Buesseler said deputy director R. Charles Martinsen III planned to remove an estimated 750 cubic yards of bottom layer “gunk.”
“It desperately needed to be done,” she said, adding that the nutrient-rich sediment would be relocated to the town compost facility, so it could be reused as fertilizer in local gardens.
If the dredge does not prevent a cyanobacterial algal bloom, Ms. Buesseler said the trust would once again have to warn residents against touching the pond water.