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How is climate change impacting Casco Bay?

Some people call it climate change. Some call it global warming. Others call it climate chaos. No matter what you call it, changes are happening more rapidly than anyone thought they would.

Scientists, policy makers, and long-time residents of Maine are worried about rising sea levels, warming waters, an increase in nuisance and harmful algal blooms, invasive species, and the changing chemistry of the ocean.

Climate Change is a unifying thread and threat underlying much of our work to protect the health of the Bay, as we confront concerns about acidification, invasive species, coves covered in green algae, and the potential impacts on wastewater infrastructure from sea level rise.

The chemistry of the Bay is changing

Seawater is becoming less alkaline and more acidic.

One of the largest impacts of climate change in the Bay are Ocean Acidification and Coastal Acidification.

Friends of Casco Bay has measured an average drop in pH (meaning a rise in acidity) of 0.01 units per year over the past 15 years.

When carbon dioxide from smokestacks and tailpipes mixes with water, it can make the water more acidic. This is called ocean acidification.

More carbon dioxide ends up in coastal waters as a result of nitrogen pollution from fertilizers, pet wastes, and sewage carried into the sea by rivers, rainwater, and snow melt. This nitrogen overdose stimulates massive algal blooms. When these blooms die and decay, decomposing bacteria consume oxygen and release carbon dioxide in bays and coves. This creates even more acidic conditions for coastal critters. This is called coastal acidification.

“Globally, the oceans have become about 30% more acidic over the last century… a rate that is faster than anything the marine realm has experienced in millions of years…Coastal areas also are more affected by acidity, due to runoff of pollutants from land, depending on local factors such as upwelling, river discharge, and water quality.” ◊

The shells of sea creatures can become weakened by ocean and coastal acidification. Ocean scientists are finding that as seawater becomes more acidic, the shells of clams, corals, and tiny creatures at the base of the marine food web can weaken and actually dissolve. Ocean acidification impairs the ability of sea creatures, from fish and squid to sea urchins and copepods, to grow, reproduce, and fight off disease.

Coastal acidification is one more stressor for shellfish species already challenged by other impacts, such as predation by milky ribbon worms and gluttonous green crabs that flourish in warming waters.

Longer and more severe red tides and other harmful algal blooms can be triggered by excess nitrogen and warmer ocean temperatures. Red tides and other harmful algal blooms, exacerbated by nitrogen runoff, may close clam flats to shellfish harvesting for weeks or months and aquaculture operations, affecting jobs and our economy.

Nuisance algal blooms may be the result of the combination of warmer water and excess nitrogen washing off the land. This combination creates ideal growing conditions for thick mats of green algae that can choke our coastal sediments and the creatures that live there.

Algal bloom in Back Cove in July 2017 Photograph by Deb Dawson

The water is getting warmer

Water temperature in the Gulf of Maine may rise 2 to 8 degrees Fahrenheit over the next century.  

“Sea surface temperatures in the Gulf of Maine have already increased, as demonstrated by the 100-year record from Boothbay Harbor. Regional sea surface temperatures have increased almost 1.1°C (2°F) since 1970, and could rise another 3-4°C (6-8°F).” ♦

More severe storms are fueled by warm water. We are experiencing more intense rainfall in extreme events. Heavy downpours lead to sewage overflows, and increased nitrogen runoff.

Population shifts are occurring. Harvesters will see more changes in the seafood they catch, as some cold-water-loving species such as cod and lobsters move further north and species such as red hake, turbot, black sea bass, blue crab, butterfish, and summer flounder, move up from mid-Atlantic. We are also seeing more warm-water creatures, such as seahorses and ocean sunfish. ◊

Disruptions in the foundation of the ocean food web already are being observed. Data collected by NASA satellites and researchers at Bigelow Laboratory for Ocean Sciences have found that as water temperature warms, phytoplankton in the Gulf of Maine have declined significantly during the past 20 years.

Phytoplankton constitute the base of the marine food chain. These microscopic plants produce half of the oxygen we breathe! Not only do they nurture all ocean animals (directly or indirectly), but they also absorb carbon dioxide, a by-product of burning fossil fuels, which is the main cause of global warming.

Changes in water temperature can influence the strength and path of ocean currents, further disrupting the ocean food web. In the Gulf of Maine, a circular current carries blooms of phytoplankton to where copepods—zooplankton—have just hatched. The copepods feast on the phytoplankton and then the current carries them to where herring larvae are hungrily emerging.  Any disruption in this ocean conveyor belt can threaten the food chain.

Infectious diseases spread faster in warmer water, which is worrisome to lobster harvesters who fear that shell-wasting disease, now common in southern waters, will spread here. Lobster-shell disease has contributed to the demise of lobstering in places such as Long Island Sound. Maine lobster biologist Diane Cowan warns that female lobsters need cold water for their eggs to develop.

Invasive species that prefer warmer water, such as green crabs, devour juvenile clams and mow down eelgrass. In recent years, we seen an increase in invasive species like tunicates or sea squirts. Some invasive species may take over the niche of our native species.

Sea level is rising 

Sea level in Maine will likely rise between three to five feet by 2100 based on a intermediate scenario. Beyond 2100, sea levels in Maine are expected to continue rising.

In its 2020 report, the The Scientific and Technical Subcommittee of the Maine Climate Council recommended “that the Climate Council consider committing to manage for 1.5 feet of relative sea level rise by 2050, and 3.9 feet of sea level rise by the year 2100. Additionally, the [subcommittee] recommends that the Climate Council consider preparing to manage for 3.0 feet of relative sea level rise by 2050, and 8.8 feet of sea level rise by the year 2100.” Later the committee adds, ” Abrupt sea level change on the order of months, rather than years, can also occur on top of the long-term rise.” ◊

Beach erosion will strip away dunes and sandy beaches, leaving ever smaller sand barriers for sunbathers and shorefront property owners. As warmer water temperatures fuel stronger storms, storm surges will pound the coast, driven by high winds and high tides.

Landslides can occur when clay cliffs are undercut by wave action. That’s bad news for those who live on top of soft, glacial marine along the coastline and islands of Casco Bay. Scientists predict that rising sea levels could cause many coastal homes to slide into the sea.

A report on sea level rise prepared by the Maine State Planning Office and US EPA projected that, based on a three-foot sea level rise, 200 existing homes in Falmouth, Freeport, and on Casco Bay islands could be lost by 2100. Even envisaging a more conservative two-foot sea level rise, University of Maine geologist Joe Kelley predicts that none of the shorefront houses on Long Island will be there in 100 years.

Drowned wetlands are going to be a problem. The Maine coast will lose valuable nesting and nursery habitats as rising sea levels inundate salt marshes. Natural protective buffers against flooding upland areas will be threatened. Freshwater bogs near the ocean will be converted to salt water as the ocean creeps inland.

Drowned infrastructure as our aging water transport facilities and wastewater treatment systems are inundated, we are worried about more sewage overflows, broken pipes, and costly repairs.

What is Friends of Casco Bay doing to tackle the effects of climate change?

Working at the state level

MOCA Partners
The Maine Ocean and Coastal Acidification Partnership (MOCA) is asking – and answering – the hard questions: “Why is ocean acidification happening?” and “What can we do about it?”

We helped form and lead the Maine Ocean and Coastal Acidification Partnership to coordinate the work of government agencies, academic institutions, resource harvesters, and nonprofit organizations that are studying ways to reduce the impact of—and help adapt to—the changing chemistry of our ocean.

In 2014, our members helped convince the Maine Legislature to pass a bill to establish a state Ocean Acidification Study Commission, the first on the East Coast. Casco Baykeeper Joe Payne was one of 16 commission members who issued a report to the state Legislature that called for more data collection and education to reduce nitrogen pollution in 2015. The State did not act on its recommendations. 

To respond to the need for action, Friends of Casco Bay catalyzed meetings to discuss how to fulfill recommendations of the Maine Ocean Acidification Study Commission report by creating an on-going volunteer partnership of those concerned about ocean and coastal acidification.Friends of Casco Bay, the Island Institute, and University of Maine/Maine Sea Grant formed Maine Ocean and Coastal Acidification partnership (MOCA)  to coordinate the efforts of governmental agencies, private organizations, and citizens.

In March of 2016 Friends of Casco Bay organized the first official meeting of the Maine Ocean and Coastal Acidification partnership to share on the causes and effects of ocean and coastal acidification in Maine waters. Since its first meeting in 2016, MOCA has held several scientific and policy making workshops, organized working groups, met with coastal legislators, hosted outreach events, and coordinated water monitoring efforts all along the coast.

Casco Baykeeper Ivy Frignoca served as Coordinator of Maine Ocean and Coastal Acidification partnership from 2016 to 2019.

In 2019, our bill to create a state-funded Science and Policy Advisory Council on the Impact of Climate Change on Maine’s Marine Species was integrated into the Governor’s omnibus bill to address climate change and establish the Maine Climate Council. An action plan developed that summer with MOCA helped inform the work of the Coastal and Marine Working Group of the Maine Climate Council, of which our Baykeeper is a member.

Continuous Monitoring of Casco Bay

Photograph by Kevin Morris

Our Continuous Monitoring Station is using the latest technology to collect data every hour, 24 hours a day, 365 days a year. This station will help us better understand the changing chemistry of our coastal waters over time.

Water Reporter

We rely on volunteers to be Water Reporters to report conditions around the Bay, especially areas with nuisance algal blooms, which result in increasing ocean acidification. They use a smartphone app, Water Reporter, to document changing conditions in the Bay through photos taken over time that record the location and date of the observations. We are always looking to grow this network of observers. You can find out more and join here: cascobay.org/water-reporter.

In 2019, volunteer Water Reporters tracked 18 algal blooms that appeared all around Casco Bay from. The photos they took on their smartphones documented changes throughout the spring and fall as the algal blooms expanded to worrisome levels in many coves from Cape Elizabeth to Harpswell and West Bath.

Water Reporters also take photos of sea level rise. Sarah sends volunteers an email alert of upcoming dates when especially high tides are predicted. She requests that they take photos of areas that show flooded streets, coastal erosion, and other indicators of what the coast may look like as sea level continues to rise.

Supporting One Climate Future

We are also supporting the work of the cities of Portland and South Portland to create a joint climate and action plan: One Climate Future. Troy Moon, Portland Sustainability Coordinator, and Julie Rosenbach, South Portland Sustainability Director, jointly lead the One Climate Future initiative, which they believe is the only dual community effort in the country to address climate change. 

They are pooling financial resources and working together on multiple fronts to promote more sustainability now and into the future. They collaborate on energy, LED street light upgrades, waste reduction, pesticides use, and  energy benchmarking ordinance toward attaining climate action goals of 100% clean energy by 2040 and 80% reduction in greenhouse gases by 2050. 

Teaching the teachers: Casco Bay Curriculum

We developed the Casco Bay Curriculum: A Changing Estuary to help teachers connect the classroom with our coastal waters to increasing students’ awareness and knowledge about climate change.

Casco Bay Curriculum: A Changing Estuary addresses two major topics:

  • What is an estuary and how has it changed over time?
  • How is climate change impacting us locally in Casco Bay and the Gulf of Maine?

The curriculum activities use data on Casco Bay to cite local evidence of climate change, including sea level rise, warming waters, the fractured food web, and the changing chemistry of our oceans. Supporting the 20 stand-alone, hands-on activities are PowerPoint presentations, activity sheets, references to children’s literature, readings from scientific and news media articles about the impacts of climate change on Casco Bay and the Gulf of Maine, and links to relevant videos and web pages. 

Twenty curriculum activities, resource materials, and  professional development workshops explain about our estuary and  illustrate the regional impact of rising sea level (King Tides), invasive species (Attack of the Green Crab), a changing ecosystem (Winners and Losers Bingo), and how carbon dioxide  is a game changer for shellfish and other sea creatures (Acid Ocean).

We encourage school systems to make use of service learning and community-focused stewardship activities in the Casco Bay watershed, foster a sense of stewardship for the Bay and the environment, and demonstrate how environmental issues facing our communities may be resolved.


Advocating for the health of Casco Bay has resulted in lasting, permanent, and positive changes for the Bay. We work with diverse partners to find reasonable solutions to issues that can negatively impact the health of the Bay, such as dredging, oil spills, and wastewater discharges. We have been working with communities, especially the City of Portland, for over 25 years to push for upgrades to wastewater transport and treatment systems, which are increasingly vulnerable to the impacts of sea level rise.

What can you do to address climate change?

♦ Jacobson, G.L., I.J. Fernandez, P.A. Mayewski, and C.V. Schmitt (editors), 2009, Maine’s Climate Future: An Initial Assessment. Orono, ME: University of Maine. http://www.climatechange.umaine.edu/mainesclimatefuture/ 

◊ Fernandez, I., R. Marvinney, (Editors and Co-Chairs), 2020, Scientific Assessment of Climate Change and its Effects in Maine, Augusta, ME: The Scientific and Technical Subcommittee of the Maine Climate Council. https://www.maine.gov/future/sites/maine.gov.future/files/inline-files/GOPIF_STS_ClimateImpactsMaineReport_Final.pdf

Cover photo: Photograph by Dave Dostie

Read more about climate change and Casco Bay:

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