Excess nitrogen comes into Casco Bay from three different sources, almost in equal proportion—from sewage, from stormwater runoff, and from the sky (see pie chart).
When we look at the relationship between nitrogen and salinity (see graph below), we see high levels of nitrogen closer to shore where salinity is lower. This is evidence that much of the excess nitrogen found in Casco Bay is coming from land-based activities, such as polluted stormwater runoff and sewage treatment plant discharges.
Salinity is the measure of how salty water is. Salinity of the open ocean is 35 parts per thousand or 3.5% saltiness. Casco Bay is less salty and averages around 31 parts per thousand, mainly because of stormwater and fresh water from rivers and streams flowing into coastal waters. The water in Casco Bay, in general, is saltier farther from land, though fluctuations do occur.
The time of year also has a big impact on the salinity of Casco Bay. Salinity plummets every spring as snow melts, flooding rivers and streams that run into the Bay. Heavy rains also reduce salinity in the Bay.
Read the next section of the report The Impact of Coastal Acidification—It Shucks to Be a Clam