Nearly everyone has heard of the threat of global warming—as more carbon dioxide is released into the atmosphere through the burning of fossil fuels, the planet’s climate is changing. Not all carbon dioxide released into the atmosphere stays there; scientists estimate that nearly a third is absorbed by the oceans. As marine water absorbs this carbon dioxide, it forms carbonic acid, making the oceans more acidic. Scientists have discovered that over the past two hundred years, the average pH of the ocean has dropped 0.1 pH units. This means that the world’s oceans have become 30% more acidic than they were before the start of the Industrial Revolution. As more and more carbon dioxide is released into the atmosphere, the pH of the oceans continues to decline. This is known as Ocean Acidification.
Casco Bay suffers from a double whammy of carbon dioxide resulting from human activities. First, there are the effects on our local waters from Ocean Acidification. But we are finding excess nitrogen coming from onshore sources, namely polluted rainwater running off fertilized lawns and city streets, emissions from smokestacks and tailpipes, and sewage discharges, all of which send an overdose of nitrogen into our coastal waters. This nitrogen bonanza can stimulate the growth of large blooms of algae, beyond what animals in the ecosystem can consume. Much of these blooms end up dying and settling on the mud. This organic material decomposes; the bacteria responsible for decomposition respire during this process, removing oxygen and adding carbon dioxide. The carbon dioxide and seawater combine to form carbonic acid, lowering the pH of our water and our clam flats. This is called Coastal Acidification.
As more nitrogen pollution enters the Bay, more algae are produced, resulting in more decomposition, which adds more carbon dioxide to the water and sediments, lowering pH. This increased acidity can mean dissolution and death for young clams and other shellfish.
We See a Disturbing Trend in the pH of Bottom Water
Measurements at our sentinel sites show a decline in the pH of the bottom water. The points on the graph to the right show annual mean pH for each of thirteen years and illustrate high variability; the dots bounce all over the graph. While this is not surprising, given that coastal systems everywhere exhibit high variability, we did not expect to see this statistically significant downward trend in pH, with the overall slope of the line dropping 0.014 pH units per year over the thirteen-year period. This is a serious and disturbing trend.
Read the next section of the report Nitrogen—Can’t Live Without It, Can’t Live With Too Much of It