Thursday, July 12, 2007

A Previously Unknown Source of Nitrogen in Estuaries, But What Does It All Mean?

Two researchers at the University of Rhode Island have discovered a previously unknown source of nitrogen in Narragansett Bay – the bay’s sediments. They published their work in Nature, which is the big time for peer-reviewed papers, and so I have to believe that their findings are important. I also think their work might have implications for the cleanup of Narragansett Bay and Long Island Sound. But having read a summary, and a newspaper account, and the comments of the researchers, I can’t say for sure, and I wish they had addressed that issue more directly.

I don’t have access to Nature but I’m going to quote from a press release that URI put out, since the scientists presumably approved it and therefore it presumably explains things in a way they consider adequate:

Estuaries have long been considered nitrogen “sinks” or filters, whereby bacteria in the sediments remove substantial quantities of nitrogen through a process called denitrification.

But a new study in Narragansett Bay by researchers at the University of Rhode Island and published this week in the journal Nature has revealed a surprising reversal in the nitrogen cycle. Instead of removing nitrogen, the sediments have become a source of nitrogen through a bacterial process called nitrogen fixation.

According to URI researchers Robinson Fulweiler and Scott Nixon, chlorophyll concentrations in mid-Narragansett Bay appear to have been declining since the 1970s. This has resulted in a decrease in plankton sinking to the bottom. This is an important change because the plankton are an important food source for the benthic community and are essential for the denitrification process.

We know that organic matter affects the rate of denitrification, but no one else has observed a switch from denitrification to nitrogen fixation (production), and no one predicted that the absence of organic matter would lead to nitrogen fixation,” Fulweiler said.

Added Nixon, “Instead of removing some of the nitrogen we put into it, in the summer of 2006 the Bay sediments brought nitrogen into the system. In fact, last summer, the Bay’s sediments added 1.5 times more nitrogen than the direct discharge of sewage.”

You can read the rest yourself, here.

Now here’s my problem. We know that nitrogen from sewage plants is the trigger that starts the hypoxia process in Long Island Sound and presumably in Narragansett Bay too. Or at least that’s what the entire cleanup of Long Island Sound and Narragansett Bay is based on. Here on the Sound, we have a 58.5 percent nitrogen reduction target; Rhode Island’s is 50 percent (more here).

So my question is, if nitrogen is being added through the sediments, and if New York, Connecticut and Rhode Island are spending more than a billion dollars combined to improve sewage treatment plants to remove nitrogen, is it possible we’re wasting our money?

That’s a fairly important public policy question and while it’s nice that Professor Nixon talks about global climate change in the press release, I would have liked to hear his opinion on whether his findings might be applicable to estuaries other than the Bay, and if so what are the implications for our clean-up plans.

I should add, by the way, that while Nixon is a top scientist, I’m not inclined to cut him any slack on non-science issues, solely because he wrote a tepid (to say the least) review of my book back in 2002 or 2003. My recollection is that he thought Bobby Kennedy’s name was too big on the cover and that my writing was melodramatic and that I didn’t provide some of the environmental history that he thought was important. You can read it here (and for balance you can read a review by another scientist, John Waldman, here.)

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Blogger Sam said...

When waters such as the Narragansett undergo eutrophication, sediments can release all kinds of stuff. That issue has been around for at least 50 years - I still have my college textbook from the 1970's entitled "Limnology." Nitrogen pathways are but one of many that include carbon (as methane, inorganic, and organic C), phosphorus, calcium, silica, and all kinds of things that make aquatic life possible. The point is that just like trees that undergo photosynthesis during the day and respiration at night, sediments can uptake or release nutrients just as well.

I wouldn't read much into the offending article other than if People hadn't caused the eutropication of the bays, such as by releasing untreated wastewater and toxic compounds, we wouldn't be having this discussion.

Some contributors to release of nitrogen from the sediments may indeed be mechanical, such as by the huge dredging and bridge projects in the area. If the sediment micro-zone layer is destroyed, all kinds of compounds will attempt to seek their ionic balance in the water depending on physical inputs such as pH, temperature, pressure, and so forth.

So as usual, it is probably good work on its own merits, but once the authors start to BS about its implications outside their narrow scope of research they usually screw up.

As you found out! Sam

4:28 PM  
Blogger Tom Andersen said...

I don't think they screwed up. I just wish that if they were going to discuss things with public policy implications, they talked about possible implications for other estuaries and and possible implications for our very expensive nitrogen removal programs.

5:12 PM  
Blogger Sam said...

I caught that undercurrent (ha-ha) cut also found his statement rather shocking, that for removing all that nitrogen from wastewater plants 1.5 times as much was released by the sediments. That means that all that money cleaning up 'Gannsett Bay might have been a total waste.

That is an untrue, specious, unscientific argument that cannot be deduced from the author's body of work. I certainly hope you don't agree that it was a waste of money. I would draw the opposite deduction, that MORE work needs to be done to control nutrient inputs to the estuary.

As to implications for the LIS, I would day it is a mixed bag, since the Sound is blessed with very deep canyons at its western limits. However, shallow-water areas which have undergone hypoxia and advanced eutrophication could act in a similar manner to what was seen in 'Gannsett Bay. Many of these "hot spots" are in the eastern reaches of the LIS.


9:58 AM  

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