TRURO — Kettle ponds, the glittering pools of fresh water that are one of the Outer Cape’s most prized natural features, are facing a growing threat caused not only by excess nutrients but by warming temperatures.
Two of these ponds, Snow Pond in Truro and Gull Pond in Wellfleet, were temporarily closed in late June due to what the Association to Preserve Cape Cod (APCC), an environmental nonprofit, expects will become an increasingly common hazard: cyanobacteria blooms. The ponds reopened in mid-July, after the blooms dissipated.
Marshall Watnick, a summer visitor from Connecticut, goes to the Wellfleet ponds for quiet swims and to be with his grandchildren. He has noted an alarming trend.
“When we first started coming 40 years ago, there was never any bacteria,” he said as he looked out over Gull Pond in Wellfleet.
Actually, cyanobacteria, a single-celled microorganism, is native to the kettle ponds and survives by converting sunlight to food.
“As a group, they’re not a bad thing,” said Sophia Fox, an aquatic ecologist for the National Park Service, emphasizing that cyanobacteria have always been a part of the kettle pond ecosystems. It’s the blooms that are new. When conditions allow cyanobacteria to reproduce rapidly, they form a green scum on the water’s surface.
It is these scum-like blooms that can produce cyanotoxins, which can cause respiratory and neurological problems for humans who encounter them, says Kevin Johnson, an ecologist and the coordinator of the APCC’s cyanobacteria monitoring program.
The Heat Factor
The algae’s rapid reproduction and subsequent blooming is made possible by two factors, say Johnson and Fox: warm water and high nutrient levels, including nitrogen and phosphorus. The nutrient buildup that stimulates algal blooms is part of an ecological phenomenon called eutrophication.
While blooms in ponds on other parts of the Cape have lingered for weeks at a time, National Seashore ecologists discovered this year that many of the blooms on the Outer Cape appear and disappear rapidly, sometimes in a matter of hours. Fox said scientists do not yet know why this is happening.
They do know that, on the Outer Cape, sparser development and remote pond locations significantly decrease the issue of eutrophication, Fox said.
Fox believes that on the Outer Cape, an increase in water temperatures — a symptom of global warming—is responsible for the blooms, rather than nutrient loading. For example, in Snow Pond and Ryder Pond in Truro and in Gull Pond and Herring Pond in Wellfleet, blooms have been spotted over the past three years, despite no excess nutrient buildup.
“Cyanobacteria may be more tolerant of higher temperatures than some of the other phytoplankton, which allows them to dominate more easily,” said Fox. Other plankton that normally compete for resources struggle in warmer temperatures, allowing cyanobacteria to thrive and reproduce quickly.
Know It When You See It
The APCC began closely monitoring 22 of the Cape’s 900 ponds for algal blooms in 2017, offering an interactive map with color-coded ratings. But it wasn’t until 2019 that the first blooms were observed on the Outer Cape by aquatic ecologists from the Cape Cod National Seashore. They first spotted the blooms along the shores of Gull Pond in Wellfleet. Since then, the National Seashore, the Outer Cape towns, and the APCC have collaborated to monitor bloom activity in local ponds.
Responsibility for monitoring the water quality of the ponds depends upon who manages the access point, said Fox. With most of the Outer Cape’s public beaches managed by individual towns, the towns are responsible for testing water quality of these ponds. Five National Seashore ecologists, led by Fox, monitor water quality at Gull Pond and Snow Pond, as they both can be accessed via trails within the Seashore Park.
“Our team is out there at each pond that we monitor every other week, so if we make any observations of cyanobacteria activity, then we work with our partners in the towns and at the APCC to trigger a cyanobacteria protocol,” said Fox. The protocol involves sampling the water and sending it to an APCC laboratory for testing, and then, if needed, temporarily closing the ponds.
One problem, said Fox, is that “we know we can’t be there every time to notice a bloom, because we now know they’re happening on such short time scales.” That means it’s important that people enjoying the ponds know what to look for, Fox said. The Seashore and town conservation commissions are now creating educational posters and presentations to help pond users recognize blooms.
Fox said to look out for a jelly-like green scum along shorelines or a collection of green dots floating on the surface of the pond. Some blooms are benign, but it is impossible to tell if a bloom is toxic just by looking. For this reason, visitors to the kettle ponds should report any sightings of blooms to the National Seashore, the APCC, or the town health department.
Eutrophication Watch
The Cape Cod Commission, the regional land use planning, economic development, and regulatory agency, is continuing to monitor eutrophication. Tim Pasakarnis, a water resource analyst there, said eutrophication is typically measured by an increase in algae or other types of plant growth on the water surface, or by submerged plant growth.
Even in healthy ecosystems, eutrophication may occur eventually, converting a pond to a marsh, then to a meadow, and eventually to a forest over the course of a thousand years. But human-caused pollutants from fertilizers and septic systems trigger rapid eutrophication.
Phosphorus input, in particular, is a concern in freshwater ponds, said Pasakarnis. He cited faulty septic systems near ponds, along with runoffs from roadways, stormwater, and fertilizer, as common sources of human contributions to nutrient loading.
On June 8 the new Cape Cod Pond Network had its first meeting. The network was established by the Cape Cod Commission this summer as a single meeting place for groups across the Cape to join forces and share strategies for improving pond health.
