ROWLEY — The air was brisk Wednesday morning on Plum Island Estuary when Woods Hole Research Center biologists ventured by boat into a nearby section of the Great Marsh to examine its slow recovery from a decade-long controlled pollution experiment.

Thirteen years ago, Woods Hole researchers began polluting some of the marsh's creeks with nitrates — which can be found in fertilizer, animal manure and sewage — to study its harmful effects. They stopped adding nitrates in 2016, and this month kicked off a $1.6 million study of the recovery response of the ecosystem's microbes, plants and fish.

The study is part of the TIDE Project, launched in 2002  to help researchers understand the long-term, cumulative effects of moderate increases in nutrients and changes in species on the productivity, food web and physical structure of salt marshes.

Adding excess nitrates simulates the kind of nutrient pollution that could come from agriculture, leaking septic tanks and sewage systems, fertilized lawns and the burning of fossil fuels, which has increased dramatically over the last century as a result of development and sea level rise — damaging coastal ecosystems.

While polluting the marsh on purpose might seem strange, Hillary Sullivan, a research assistant who runs the project, explained that the results will help researchers at marshes along the Atlantic coast that are hit with much greater levels of nutrient pollution.

Sullivan emphasized the study's goal of bringing attention to climate change's impact on valuable marsh ecosystems.

"Our ultimate goal is to publicize and reach policymakers and people who have the money and power to limit nitrogen that goes to marshes or make regulations to stop pollution to marshes and also to get more funding for marsh restoration," she said. 

Previous Woods Hole research has shown excessive nutrient pollution leads to changes in marsh microbes, algae and plants that can lead to the marsh edges collapsing, a drop in fish populations, and compromising the salt marsh's ability to keep up with rising sea levels.

This new research is supported by a grant awarded this year by the National Science Foundation. It comes at a critical time, with more than $200 million proposed in the next five years to restore salt marshes along the mid-Atlantic coast in the wake of massive losses from Superstorm Sandy in 2012.

Sullivan said that while a lower concentration of nitrates is beneficial to plants and algae, large amounts are detrimental, and researchers hope to find a "tipping point" where the amount of pollution begins to have negative effects on the marsh.

"We know that the marsh can withstand some nutrients, but we want to know where it's too much — where we overload the system," Sullivan said. "We actually saw stimulation of the growth of the plants and algae until a certain point, then the marsh started to fall apart."

During Wednesday morning's site visit to the marsh near Woods Hole's research center in Rowley, Sullivan and Justin Lesser, a doctoral student assisting with the project, took water samples from the affected areas.

The high tide made it hard to see the pollution's effects, but beneath the surface, Sullivan said the walls of the polluted creeks were slumped and cracked from years of being overloaded with nitrates. 

Sullivan stressed the importance of marshes, noting they act as a buffer against storms for coastal environments and communities, filter nutrients from water, provide nursery habitats for fish and birds, and also serve economic and recreational purposes for people. 

She explained that the study's harmful effects give a glimpse of what could happen to Atlantic salt marshes as the sea level rises, and also warn of the effects of increased coastal development on marshes' health. 

"As we're relying on the coast for development, we're infringing on these marshes," Sullivan said. "They're so important to us, but if we destroy them, we're going to lose those benefits." 

For more on the project, visit http://thetideproject.org/.

For more Woods Hole Research Center news and resources, visit http://www.whrc.org/news.    

Staff writer Jack Shea can be reached via email at jshea@newburyportnews.com or by phone at 978-961-3154. Follow him on Twitter @iamjackshea.