SOUTH PORTLAND — Casco Bay is the heart of southern Maine. It acts as a refuge to wildlife, birds and aquatic biodiversity. It provides recreation and economic opportunity to Maine citizens through boating, tourism and fisheries. Our responsibility as Mainers is to protect the bay that we gain so much from.

Casco Bay is changing. Friends of Casco Bay, a nonprofit that regularly monitors the bay’s water quality, has found that over the last decade, the bay has become more acidic, and, along certain shoreline areas, oxygen levels are dangerously low and water clarity is poor. These changes are the cascading effects of climate change and other anthropogenic events. A byproduct of human activity – one that increases the acidity and decreases water clarity and oxygen levels in Casco Bay – is the dumping of excess nitrogen.

Excess nitrogen comes from three sources: runoff, sewage and air pollution. Warmer temperatures, caused by climate change, result in more frequent and heavy rainstorms. The runoff from these storms carries harmful nitrogen-containing compounds, from sources such as pesticides, fertilizers and pet waste, that end up in Casco Bay.

Heavy rainstorms cause sewage overflow via antiquated community water treatment systems: When sewage and stormwater holding tanks fill to capacity, wastewater flows directly into the bay. Lastly, gaseous nitrous oxide from car and smokestack emissions eventually settles on the surface of the water and is ultimately absorbed.

Nitrogen is necessary for the growth of oceanic organisms and drives the ocean’s food cycle. However, in excess, nitrogen can cause harmful and unsightly algal blooms. Algal blooms appear when there is a dramatic increase in algae population in the water, showing up typically as green or brown slime. When the algal bloom dies off and decomposes, the algae uses up oxygen in the water and releases carbon dioxide. This process decreases oxygen levels and increases carbon dioxide levels. Carbon dioxide causes water acidification.

Acidic water makes it difficult for juvenile and adult shellfish to access the carbonate that is needed to build their exteriors. Algal blooms not only threaten the abundance of shellfish but also can cause massive fish die-offs as oxygen levels drop.

It is unlikely that we will be able to prevent the global warming of the oceans’ water. However, we can impede significant amounts of nitrogen from entering the bay.

As citizens, there are a few things we can do to protect Casco Bay. First, consider commuting by bike, using public transit or carpooling with a co-worker or family member instead of driving alone. Decreasing the number of vehicles on the road will directly decrease nitrous oxide from being deposited in the water.

At the local level, you can contact your town or city officials to express interest in adopting a municipal ordinance on pesticide and fertilizer use. Twenty-nine communities in Maine have passed pesticide ordinances, including Portland and South Portland.

On Nov. 6, Question 2 will ask Maine voters to approve a $30 million bond package to improve coastal water treatment plants in areas where shellfish industries are at the greatest risk. Improved water treatment infrastructure will aid in preventing untreated, nitrogen-filled sewage from reaching our waterways.

Maine also will vote in a new governor. Please consider each candidate’s stance on the environment and sustainability.

At the federal level, contact your U.S. representative and senator and encourage them to ensure that the farm bill protects local insecticide policies. If passed as stands, the farm bill will prevent towns and cities from passing their own rules on pesticide use. If this provision is not taken out, the farm bill will keep citizens from deciding what they believe is best for their municipality.

Casco Bay is an invaluable resource that needs to be kept clean to ensure its prosperity. Use your voice this election season and vote to protect the bay.