Nitrogen: Not (as) guilty in Westport marsh collapse mystery?

WRWA study suggests other causes to blame for marsh loss

By Bruce Burdett
Posted 5/11/19

WRWA study suggests other causes to blame for marsh collapse

By Bruce Burdett

It remains bad for the waterway, but nitrogen overload may not be the culprit that is causing marshes to collapse …

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Nitrogen: Not (as) guilty in Westport marsh collapse mystery?

WRWA study suggests other causes to blame for marsh loss

Posted

WRWA study suggests other causes to blame for marsh collapse

By Bruce Burdett

It remains bad for the waterway, but nitrogen overload may not be the culprit that is causing marshes to collapse into the lower branches of the Westport River.

So suggests the first year’s worth of a Westport River Watershed Alliance-sponsored study meant to answer the question — “What’s going on? Why are Westport’s marshes falling apart?

The study’s first phase, presented at WRWA’s recent annual meeting, concludes that what’s really damaging marshes and the spartina grasses that help bind them together is the fact that marsh sediment in the lower end of the river has become densely compacted and nearly rock hard. The underlying peat and sediment are so dense, the study presenters said, that grass roots, as well as fertilizing nitrogen, have trouble penetrating — it’s so hard, that barnacles attempt to attach and mussels cannot penetrate.

That suggestion comes as a likely surprise — and also a concern — to those who have long believed that nitrogen is at the root of what ails the “calving” marshes.

Studies in other nearby places have indicated that nitrogen spurs growth of the plants’ “shoots but not the roots” since the roots don’t need to go deep to find nutrients. And without strong root growth, there is nothing to hold the prevent the peat from collapse.”

“However, the Westport River is unique,” said Dr. Patrick Ewanchuck of Providence College, one of the study’s three principal investigators — along with Dr. Catherine Matassa of UConn and Dr. Mark Bertness of Brown University.

There is marsh collapse in the lower branches, Dr. Matassa told the audience, “but the edges of the marsh are folding over, being undermined from the bottom and putting tension on the marsh surface … like pulling on a tablecloth.”

Blame compacted sediments

When they took that hard lower-branch sediment and the plants it held back to the lab, “the plants were not doing well, whether you fertilized them or not,” Dr. Matassa said.

“Hard sediments are limiting the ability of the spartina to be resilient to other stressors. Marsh loss downriver is not due to nutrient loading,” she said. The likely culprit is sediment dynamics.”

The study concluded, “Our year one results indicate that Spartina growth is nutrient-limited in both branches of the river, despite elevated water column nitrogen levels. This suggests that the plants are somehow unable to use nitrogen in the water column to grow taller shoots. Taller shoots, coupled with shallower roots, are a hallmark feature of nitrogen-induced marsh bank collapse, but we do not observe this pattern in our experiments. Rather, the results point to sediment compaction as causing poor Spartina growth in the lower parts of both the East and West branches while also making the banks susceptible to disturbance or calving. Sediment compaction may arise due to increased water flows and/or changes in sediment dynamics possibly due to shoreline development.”

As to the actual cause of the compaction, they said, “A second summer (of study) is vital to have plants in our field experiments experience a full annual cycle.”

Beware letting nitrogen off the hook

Former New Bedford Mayor, noted environmentalist and Westport resident John Bullard responded with a cautionary comment when the meeting was opened to audience questions.

“There are enormous financial implications to all town residents” from any potential fix of the river’s problems. “The information that you present can be used or misused by people who come down on various sides of these financial decisions.

“I had always assumed that one of the problems of the Westport River was the eutrophication” (excessive richness of nutrients, especially nitrogen overload), he said.

“So one of the challenges that the Alliance has to think about is the entirety, not just the salt marshes but the health of the river itself, because while nitrogen may be good for spartina, I don’t think that it is good for the water environment that is adjacent to the salt marshes,” he said.

“It is going to be a big issue,” he added. “I can just see people saying, ‘Let’s put more nitrogen in.’”

“I agree it is troubling as a scientist to watch your data be misused,” Dr. Matassa replied. “We are not saying that adding nitrogen is good for spartina. We are simply saying that the spartina in the lower West Branch that are not doing well are nitrogen limited.”

Nitrogen, she added, is good for the invasive phragmites that choke marshes “but it is absolutely terrible for all the other plants that disappear from the system.”

“One variable that I don’t hear a lot of focus on is sea level rise,” said Planning Board chairman and audience member Jim Whitin.

If sea levels rise faster than marshes can keep up, “in the end the marshes go away. “So we can all put solar panels on the roof but it is coming and sea level rise will continue for the next century whatever we do … So what possibilities are there?”

“Quite frankly, it doesn’t look good, “Dr. Ewanchuk said. “Most of New England’s marshes have no place to migrate to. We can save the marshes but for how long?”

Asked later for reaction, Jack Reynolds, head of the former Westport Fishermen’s Association, and one of the first to sound the alarm about marsh collapse, was skeptical of the study findings.

“There are many areas that have marshes that are not in such bad shape, as in the West Branch, and the only common influence “is that there are fewer houses and less septic runoff.”

Something didn’t add up

The results are fascinating if not necessarily startling, said Robert Carvalho, science director for the WRWA, when asked later is she was surprised by the findings.

“This is part of what we have been trying to find out from the study,” she said. “Something didn’t add up.”

While nitrogen was an obvious suspect in marsh collapse, it was curious that in the lower East Branch, where marsh collapse is worst, nitrogen levels are lower than in the upper branches, where marshes remain relatively healthy, don’t exhibit the same “calving,” and the “muck is mushier.” What’s more, spartina shoots in the upper branches were found to be much taller than downriver sites.

And, like Mr. Bullard, she said it would be wrong to take from this a message that nitrogen is just fine after all.

Whatever roll it plays in marsh collapse in this particular case, the harm done by excessive nitrogen in a waterway is well documented” and far reaching, she said.

Ms. Carvalho added that this is just the investigation’s first phase. Among questions still to be answered is what is causing the marsh compaction.

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