Water quality in Mashpee’s freshwater ponds is declining even as the town has been unable to stave off water quality decline in coastal embayments, which have been the focus of years of study and planning.
Ashley K. Fisher, the town’s director of natural resources, told the Mashpee Select Board during a joint meeting of regulatory boards in the Waquoit Meeting Room at Town Hall on Monday, November 8, that she had no good news.
“I wish I had some good news for you, but I do not,” she said. “We have no good water quality left in Mashpee.”
Both Popponesset Bay and Waquoit Bay in Mashpee have impaired water quality throughout, Ms. Fisher said. She referenced a report from University of Massachusetts Dartmouth scientist Brian L. Howes, who has studied both bays for decades. This summer, Dr. Howes told the town that “for the first time in 20 years, I have nothing good to say.”
Eelgrass, once abundant throughout both bays, has been extirpated by pollution, Ms. Fisher said.
“I for one have seen the eelgrass completely disappear,” she said. “The last known patches [are] now covered with macrophytes and filamentous algae, and they’re suffocating.”
Parts of the Mashpee River, an estuary of Popponesset Bay, have such poor water quality that oysters grown by the town had to be moved earlier this year due to dissolved oxygen levels “so low that the oysters were seeing stress and they started to die,” Ms. Fisher said. “In these areas the species diversity is completely gone. The dissolved oxygen levels are so low we did see a mass blue crab die-off this year.”
Septic systems, which discharge nitrogen-laden effluent, are the primary source of pollution in the town’s saltwater embayments. Decades of study have shown that the nitrogen-laden effluent travels easily through Cape Cod’s sandy soils and winds up in the estuaries and bays. There, nitrogen acts like a fertilizer, fueling the out-of-control growth of algae. The blooms of algae block sunlight needed by plant life, and as the algae dies off, the process of decay consumes dissolved oxygen from the water column, creating dead zones where most life cannot survive.
“When we have high nitrogen or high nutrients, we get excess algae blooms,” Ms. Fisher said. “With those thick algae blooms, we see no dissolved oxygen in the night, so we’ll see fish kills or mass mortalities of other species.”
Freshwater ponds in Mashpee, which have received considerably less study than saltwater embayments, are also in decline due to excess nutrients, Ms. Fisher said. Phosphorus, as well as nitrogen, is a concern for freshwater ponds.
Similar to how nitrogen fuels out-of-control growth of algae in the bays, nutrients like phosphorus spur the growth of cyanobacteria, a microorganism sometimes referred to as blue-green algae. Cyanobacteria can produce toxins harmful to humans and animals.
Santuit Pond, the shallowest of the major ponds in Mashpee, “is our worst pond in town,” Ms. Fisher said.
Advisories warning people not to swim or make contact with the water in Santuit Pond have been posted for each of the past five consecutive summers. Cyanobacteria, which is usually fluorescent green in color, fills the water column and forms a scum layer at the surface of Santuit Pond.
From May 30 to November 3, cell counts of cyanobacteria in Santuit Pond averaged above 70,000 cells per mililiter, the state threshold for a public health advisory, Ms. Fisher said.
“We also know that there are abnormal fish deformities, lesions, tumors,” Ms. Fisher said of Santuit Pond. “That’s most likely due to the cyanobacteria toxins that are being produced.”
The natural resources director categorized Santuit Pond as “eutrophic and trending toward hypereutrophic.”
Eutrophic is a scientific term for a water body so rich in nutrients that plant and algal growth overwhelm the aquatic system and consume dissolved oxygen from the water.
Ms. Fisher used the trophic index to categorize the water quality of the major freshwater ponds in Mashpee. Eutrophic and hypereutrophic describe ponds with “poor” water quality, while mesotrophic describes ponds with “fair” water quality and oligotrophic describes ponds with “good” water quality, she said. None of the ponds was categorized as oligotrophic.
Ashumet Pond is the second-most-impaired pond in Mashpee, Ms. Fisher said. She described the pond as “between mesotrophic and eutrophic” but said that the only reason the pond is not eutrophic is because it has been treated with aluminum sulfate to reduce nutrient levels.
Cyanobacteria cell counts in Ashumet Pond averaged just 3,500 cells per milliliter through the summer, with several events where the counts were above 10,000 cells per milliliter, Ms. Fisher said. While these levels remain below the 70,000 cells per milliliter threshold for a health advisory, the presence of a scum layer of cyanobacteria led an advisory to be posted from mid-July to early August of this year.
At Ashumet Pond, “we’re seeing elevated levels of these cyanoblooms occurring,” Ms. Fisher said.
“What the Air Force base did, and what the town did in conjunction, is treated the pond with this aluminum sulfate, and that has really kicked it down, the blooms, in the summer months,” she said. “But because the treatment is now wearing off and they might need to re-treat.”
High levels of phosphorus visible in water samples taken at the surface of Ashumet Pond suggest that the nutrient loading in the pond is a result of land-use practices and runoff from roads and fertilized lawns, she said.
Ashumet is also polluted by chlorinated solvents from Joint Base Cape Cod that are causing lesions and deformities in fish, she said. Largemouth bass in the pond are not safe to eat due to mercury contamination.
A permeable reactive barrier in place to eliminate chlorinated solvents coming from the joint base is also helping to reduce phosphorus from reaching Ashumet Pond, the natural resources director said.
Johns Pond is the next-most-impaired freshwater pond in Mashpee and can be considered eutrophic, Ms. Fisher said. While Johns Pond did not have any cyanobacteria advisories posted this past summer, poor oxygen conditions have been noted, she said.
The pond is host to invasive species, including koi fish and milfoil, and is contaminated by PFAS chemicals and chlorinated solvents from Joint Base Cape Cod, Ms. Fisher said. PFAS and mercury contamination have led to fish consumption advisories from the Massachusetts Department of Public Health at Johns Pond.
Mashpee/Wakeby Pond, which Ms. Fisher classified as two separate ponds, was the least-impaired pond in town, she said. Still, the natural resources director described the Mashpee side as mesotrophic but “trending toward the eutrophic state” and the Wakeby side as eutrophic.
“Both total nitrogen and total phosphorus are increasing on the Mashpee side of the pond,” Ms. Fisher said. “When we see the bottom water being so high for phosphorus and total nitrogen, we can almost determine that that is coming from groundwater seepage and our septics.”
Cyanobacteria are present in the Mashpee side of the pond only at low levels, she said. Cyanobacteria occur naturally at low levels in ponds and lakes.
A scum layer of cyanobacteria that “was significant enough to be concerned” led to a health advisory being posted on the Mashpee side of Mashpee/Wakeby from June 24 to July 7, Ms. Fisher said.
The water clarity in the Mashpee side of the pond has reduced to the point where “we can no longer see below eight feet, which means that the turbidity of the water is increasing” and becoming more eutrophic, she said.
On the Wakeby side of Mashpee/Wakeby Pond, total nitrogen and total phosphorus levels are even higher than on the Mashpee side, Ms. Fisher said. Water clarity in the Mashpee side is lower than on the Wakeby side, and a cyanobacteria advisory was also posted from June 24 to July 7, she said.
Mashpee/Wakeby Pond also has a fish consumption advisory due to the presence of PFAS chemicals and mercury.
Ms. Fisher said the data on nutrient loading in the ponds is taken only once annually through the Ponds and Lakes Stewardship Program on Cape Cod.
“We should do some more sampling throughout the whole summer instead of just one profile sampling so that you can get a better data set,” she said. “That is what we’re trying to do, [but] obviously those samples are expensive to run.”
The natural resources director said the town is doing the best it can to collect data on the ponds. Town Meeting last month approved funds for diagnostic study for Mashpee/Wakeby Pond to gather further information about nutrient loading, she said.