As Casco Bay warms, climate change alters its chemistry in unexpected ways

A lobster boat heads out in Casco Bay at sunrise on Oct. 18. Gregory Rec/Staff Photographer, file

Heavy rains most likely caused low salinity and dissolved oxygen rates in Casco Bay this year, raising long-term pollution and productivity concerns for the local section of the fast-warming Gulf of Maine, according to the Friends of Casco Bay.

Casco Bay remains healthy overall, but the near-shore waters face a plethora of threats, especially stormwater pollution, that are exacerbated by the extreme rain events and rising ocean temperatures caused by climate change, Baykeeper Ivy Frignoca said.

“A marine environment is dynamic, changing with every tide and season, but climate change is speeding up those natural changes to alarming rates,” Frignoca said. “The changes are coming too fast now. Nature can’t keep up. We aren’t keeping up. A way of life is vanishing.”

Casco Bay is warming by about a degree every decade, according to three decades’ worth of data from two dozen seasonal and real-time monitoring sites. Once-productive mudflats are devoid of clams. Eelgrass beds are disappearing. Near-shore lobstermen now fish 24 miles out to sea.

The region is responding to the changes as best it can, Frignoca said – building oyster shell reefs to prevent salt marsh erosion and studying how to develop heat-resistant strains of eelgrass – but this year’s heavy rains show the unexpected impact that climate change can have on the sprawling inlet.

The salinity level was too low to have been caused by direct rainfall alone, Chief Scientist Mike Doan said. Such swings could only be caused by massive amounts of stormwater runoff cascading off the impervious surfaces of Maine’s most densely developed communities.

And with that much more stormwater runoff – about 250 million gallons of it from Portland alone compared with 70 million gallons in a dry year – comes all the pollutants that stormwater runoff is likely to carry, from forever chemicals to microplastics to excess nitrogen.

Nitrogen is necessary for plant growth, but when stormwater runoff dumps too much sewage and agricultural or lawn runoff into the bay, it can trigger a population explosion of phytoplankton that smothers mud flats, closes clam beds, and chokes out eelgrass beds.

Doan is anxiously awaiting the results of this year’s nitrogen tests to come back from the lab.

While reduced salinity is a warning sign about polluted runoff and could post longer-term threats, it hasn’t declined enough yet to have direct effects on marine life.

Dissolved oxygen is often used as an overall indicator of the health of the bay, Doan said. In its pristine state, Casco Bay is a cold, oxygen-rich home to 850 species of marine life. As the bay warms, oxygen rates are expected to decline, he said – warm water can’t hold as much oxygen.

Continuous sensors deployed off Portland, Falmouth and Harpswell showed the dissolved oxygen rate was below the six-year daily average in January and February, from April to June, and from July through September. For weeks at a time, this year’s oxygen rate was the lowest recorded in that time.

Although lower than normal, Doan said the oxygen rates were not so low as to pose an immediate threat to marine life, Doan said. He said it is too early to tell if this one-year decline – possibly caused by a decline in photosynthesis brought on by overcast skies – will turn into a long-term trend.

The problem is expected to intensify with climate change as the Northeast region of the U.S. sees more rainfall and more frequent extreme precipitation events that dump 2 or more inches of rain in a day, according to the latest National Climate Assessment.

Frignoca plans to present the Friends of Casco Bay’s findings to the Maine Climate Council, which is working on an update to the state’s climate action plan, and to the Maine Department of Environmental Protection, which is about to start work on an overhaul of the state’s stormwater runoff regulations.

The new regulations should require developers to prepare for more frequent and intense rainfall in a hotter, wetter Maine, Frignoca said. To address Portland’s needs, the regulatory scope should be broadened to apply to redeveloped properties and lots with less than an acre of paved surface.

She thinks the regulations should promote low-impact development and require projects to treat stormwater runoff before it can be discharged into a water body, like is currently being done on many newly built schools and in the parking area around The Maine Mall.