Salt marshes may not be the "nutrient sponges" that they were thought to be, according to a new study led by Dr. David Samuel Johnson of the College of William & Mary’s Virginia Institute of Marine Science.

Salt marshes are intertidal grasslands that grow at the interface between land and sea. These ecosystems can receive excess concentrations of nutrients such as nitrogen from wastewater and runoff of agricultural fertilizer. This “eutrophication” affects coastal waters and estuaries worldwide and can lead to fish kills, harmful algal blooms and areas of low oxygen.

Salt marshes can receive excess concentrations of nutrients such as nitrogen from wastewater and runoff of agricultural fertilizer. Image credit: Pixabay.Johnson and his team wanted to know how eutrophication impacted salt marshes. To do so, they conducted an unusual experiment, flooding several acres of salt marsh in northeastern Massachusetts with fertilizer-rich water for almost a decade.

“When we were able to mimic a eutrophied estuary at an ecosystem scale—quite a challenge I must add—we found that salt marshes did not respond as you might have predicted from fertilization experiments done over the past half a century or so," says Scott Warren, professor emeritus of botany at Connecticut College and study co-author.

Despite the abundant supply of nitrogen, a key plant nutrient, vegetation in the fertilized marshes did not grow much larger than that in unfertilized marshes. Also, unlike previous studies, fertilizer did not change species composition and cause some species to outcompete others.

One reason the team’s results differed from previous studies may be their use of nitrate fertilizer, which is the most common form of nitrogen in eutrophied estuaries. Much of the previous work used ammonium fertilizer. Another reason the plants may not have responded strongly was the way the fertilizer was delivered—with flooding tidal water—which meant that less fertilizer reached the plants compared to previous studies, in which fertilizer was added directly to the marsh surface.

The mild response of plants doesn’t mean that salt marshes are safe from eutrophication, however. The research team found that fertilizer treatments caused the marsh edges to collapse and erode away—again the opposite of what they had predicted.

“We hypothesized that the grass would grow taller, which would trap more sediment and help the marsh grow,” says Johnson. Instead they found that plants in fertilized marshes had fewer roots and rhizomes than those in non-fertilized ones, which may have contributed to the collapse.

“Our work underscores that we can’t simply rely on salt marshes to clean up nutrient pollution," says Johnson. "We need to do a better job at keeping nutrients out of the water in the first place."