NEW YORK—New York City marshes are not only impacted by storm surge and rising sea levels, they are also threatened by the outflows of sewage and stormwater that the city releases into the waterways during rainstorms, as well as the high nitrogen levels present in treated water.

The amount of inorganic sediment—sand, silt and clay—in the marshes, particularly those in Queens, is decreasing. Due to the changes humans have made to the natural flow of sediment in the New York City area, marshes are not receiving enough sediment from land upstream to fight erosion.

The Natural Areas Conservancy, a conservation group that helped create the city’s framework for managing and restoring its wetlands, as well as the scientists who study the wetlands, have described these changes as sediment starvation. 

A deficiency like this can weaken the structure of a marsh, making it more prone to erosion through consistent waterlogging on the coast.

“With sea level rise, you’re basically getting marshes that, with the tides, are exposed or flooded,” said Helen Forgione, the senior manager of conservation science at the Natural Areas Conservancy. “You’re getting them flooded for a much greater period of time with the rising sea elevation.”

In her 2018 study, Dr. Dorothy Peteet, a senior research scientist with the NASA Goddard Institute for Space Studies who has studied the marshes for over 30 years, found that the organic material, or plant growth, on top of many of the marshes in Jamaica Bay was increasing, all while the marshes were starving for sediments. 

Sewage is very high in nitrogen. When sewage consistently flows onto marshes, it fertilizes the plants over and over again. Like many older cities, New York uses a combined sewer system that sends sewage and stormwater runoff into the same pipes. To keep the system from backing up and flooding streets in periods of heavy rain, the system is designed to overflow at discharge points, sending untreated sewage directly into streams, rivers and the marshes. 

Such inundation “tells the plants that they don’t need to make many roots,” said Peteet. “So then it’s just wimpy little roots in the bottom that don’t hold on very well.”

The long roots of healthy marsh plants, like Spartina grass, help strengthen the marsh against erosion from storm surges and rising sea levels. When they are repeatedly fertilized, their ability to help mitigate erosion is limited, particularly in a marsh already weakened and at low elevation due to a lack of inorganic sediment. 

Higher levels of nitrogen can also cause an algae to bloom over the marsh, often choking marine animals and aquatic plant species of oxygen. 

“It’s an algae bloom that’s just so big because there’s so much fertilizer in the water,” said Peteet.

“If you get too much algae in the water then you get things that start to die because they don’t have enough oxygen underneath.”

According to the city’s Department of Environmental Protection, it has invested approximately $1.3 billion to upgrade nitrogen removal infrastructure at eight wastewater resource recovery facilities along the East River and Jamaica Bay, ensuring that they considerably reduce the nitrogen levels in treated water. 

“The upgrades, even in the last couple decades, have made a huge improvement in the amount of nitrogen and phosphorus and so on that is put into the system,” said Forgione. “Just looking at pollutant levels or pollution levels in the water column, the water quality is definitely much better than it was 20, 30 years ago.”