166 volunteers collected water samples from Portland Harbor to help measure and address nitrogen pollution. Here is what we found.
It was a sweltering summer morning on August 7, 2022 when 166 volunteers descended on Portland Harbor to collect water samples for nitrogen analysis.
After sending the 178 samples collected that day to the University of Maine Darling Marine Center Laboratory for analysis, the results are in.
The Nab data show nitrogen concentrations in Portland Harbor are generally highest near the shore. Lower concentrations of nitrogen are often observed in samples collected by boat from the mouth of the harbor and the middle of the harbor channel. These data suggest that land-based sources of excess nitrogen – a stormwater outfall, for example – enter the harbor at the shoreline and diffuse as water circulates with the tides.
“Seeing the data all together is remarkable,” says Staff Scientist Mike Doan. “Having so many samples collected at once gives us a robust image of nitrogen conditions in the harbor. Our incredible volunteers made this happen.” Mike adds that it is important to keep in mind that these data represent a snapshot of nitrogen conditions in the Bay’s dynamic ecosystem.
Mike compared data collected this summer to the data collected at our first Nab in 2016. The 2016 data also show nitrogen concentrations are highest near the shore. However, a key difference between the two Nabs was the weather.
The 2016 Nab coincided with a large storm. In fact, it was still raining when water samples were collected. Conversely, the 2022 Nab happened on a hot day during a summer drought.
“If land-based sources of nitrogen are affecting the harbor, we would expect nitrogen levels near the shore to be higher after a rainstorm than during a drought, and that is generally what we saw,” says Casco Baykeeper Ivy Frignoca. “The next step is to use these data to see if there are specific sources of nitrogen around the harbor that can be addressed and to help set limits in Clean Water Act permits that decrease nitrogen discharges to healthy amounts.”
Excess nitrogen can come from many sources. Sometimes the source is easy to identify because it comes from a specific spot, like a wastewater treatment plant, stormwater outfall, or combined sewer overflow discharge. Other sources can be more difficult to identify because they are picked up by stormwater that sheets off the land when it rains. These sources can include fertilizers, car exhaust, pet waste, and more.
Data from the Nab are being used to help the Maine Department of Environmental Protection (DEP) develop nitrogen criteria for Casco Bay. These criteria will describe the levels of nitrogen that water bodies can safely absorb. They will also influence nitrogen discharge limits in Clean Water Act permits. Angela Brewer, who leads the marine unit of the Bureau of Water Quality at DEP, is excited to have these data.
“The Nab data provide a unique perspective that is essential to understanding the nitrogen dynamics in Portland area surface waters,” says Angela. “This perspective is only possible with Friends of Casco Bay’s extensive and dedicated volunteer network.”
Thank you to the volunteers who collected water samples with us at Nabbing Nitrogen, and to Allagash Brewing Company and our members for funding for this community science event. You made this work possible.
Why Does Nitrogen Matter?
Nitrogen is naturally found in marine waters. A healthy amount of nitrogen fertilizes algal blooms that form the base of the food chain in Casco Bay. But excess nitrogen from human sources such as wastewater, fertilizers, stormwater, and air pollution can cause excessive algal growth that harms the health of the marine environment. Some of the impacts of nitrogen pollution include degrading eelgrass beds (which are critical fish nursery habitat), exacerbating coastal acidification, lowering oxygen levels, and shutting down shellfisheries.
