Can Oysters Solve the Nitrogen Problem?

One single oyster can filter about 50 gallons of water per day. This kind of filtering power is being looked at by researchers as a potential solution to absorb harmful amounts of nitrogen entering coastal waters via wastewater treatment facilities and stormwater runoff from urban areas.

“We need nitrogen, so let’s not demonize it, it’s just we can’t have too much of it, like chocolate cake. One piece is okay but five pieces is a bad idea,” said Dr. Robinson Fulweiler during a Coastal State Discussion presentation at the University of Rhode Island Graduate School of Oceanography this past March.

BU ecologist Robinson W. Fulweiler, has spent the better part of her career studying the impact of human activities on coastal ecosystems, such as Narragansett Bay. Photo by Melody Komyerov

Fulweiler, an associate professor  from Boston University who specializes in nitrogen dynamics, and her team have been investigating how nitrogen is cycled through coastal water environments, and specifically whether oyster reef restoration or aquaculture may combat those extra pieces of the nitrogen “cake” either by directly pulling nitrogen from the system and/or enhancing natural processes to sequester and recycle nitrogen.

The extra nitrogen, she said, can alter the environment in such a way that new types of phytoplankton (microscopic marine plants) that can produce toxins may be present in the water. It could also mean more fuel for extra algal growth that causes large mats, which block out the sunlight and kill submerged plants such as eelgrass.

When the phytoplankton that makes up these large algal mats die and fall to the bottom, animals, and microbes in the sediment of the seafloor ramp up oxygen use to decompose this extra debris, and in turn cause low (hypoxic) oxygen or no oxygen (anoxic) conditions within the water.

“Once this happens you can see things like fish kills with fin(fish) and shellfish, and eventually losses of biodiversity,” said Fulweiler, adding that reports of these kinds of low-oxygen conditions have increased substantially over the last 40 years and are directly connected to areas of high population. “Where you have a lot of people, you have a lot of nitrogen and this sort of sequences of events.”

The hardest hit coastal habitats, however, have been oyster reefs.

“Unfortunately, the oyster reef is arguably the number one hardest hit, where on the high end we’ve lost about 85 percent globally,” she said.

This loss is in part due to overharvesting, pollution from excess nitrogen, oil, PCBs, or heavy metals, as well as major hurricane events. And it’s not just this filtering service that’s been lost. Oysters provide habitat for other plants and animals, protect coastlines from heavy wave energy, storm surges, and erosion, as well as a provide a high protein food source with a high market value.

“The market value [of oysters] is estimated between $5,500 to 99,000 per hectare. That’s a pretty big range, but if we begin to look at oysters not just as important for filtering nutrients or as a food source for all of these other things, but take the whole value in together, they have a lot of monetary value,” explained Fulweiler, adding that a restored reef could pay for itself in about 10 years.

                             Almost 85% of the world’s oyster reefs have disappeared. 

Where Does the Extra Nitrogen Go?

Aside from the high return on investment and the multitude of benefits oysters provide, Fulweiler’s team is specifically interested in how different oyster habitats — natural reefs and aquaculture farms —  process nitrogen and whether oysters do in fact hold the potential to balance the nitrogen budget in coastal waters.

One way in which oysters can remove nitrogen is through “bioextraction.” That is, as the oyster is growing it takes up nitrogen in both its shell and tissues.

“When we harvest this oyster shell and it’s delivered to your plate, you’re removing that nitrogen from the system,” she said.

Another way to think about it, Fulweiler added, is that oysters are like us in that they have their own microbiomes of bacteria living inside and outside of their bodies. And just like the plaque on our teeth, which convert one form of nitrogen (nitrate) to nitrogen gas that is released back into the atmosphere, oysters can do the same thing.

But while oysters may be filtering and transforming one type of nitrogen (nitrate), they’re also producing another form of nitrogen as a waste product — ammonium. This is calculated in the overall nitrogen recycling rate to help calculate how efficient an oyster is at removing nitrogen from the system, which is about 20 percent, according to Fulweiler.

Recycling vs Efficiency

Their hypothesis was that oyster reefs would have the highest nitrogen recycling rates followed by aquaculture and then the bare sediment of the seafloor.

“We were wrong,” she said, noting that while the results showed a lot of variability, the sites didn’t differ much nor were they increasing the recycling rate. What they did find was that generally the sediments under aquaculture sites with bag and rack setups in Ninigret Pond were removing more nitrogen (almost 400  micromoles per squared meter per hour), compared to bare sediment nitrogen removal rates in Narragansett Bay (40 to 85 micromoles in mid-Narragansett Bay and the Providence River estuary, respectively).

The efficiency at which they remove nitrogen paints a different picture. This view takes on a new meaning when significant amounts of nitrogen loading are measured in the form of ammonium as a waste product.

“Like that lone oyster, which is 20 percent efficient, we can see this efficiency can be really high in some cases. The highest rates seen in a previous study in 2002 were around 50 percent. So, some of these sites are really efficient at removing nitrogen. On the other hand, some of these sites are really good at adding nitrogen but not at a rate that’s higher than other subtidal areas of Narragansett Bay,” she said.

Fulweiler also accounted for varying ages of aquaculture farms in Ninigret Pond from one, three, and seven years but didn’t observe a clear pattern or impact for how long a farm was in place.

“We don’t really see a major stimulation of [nitrogen removal] when we’re adding rack and bag aquaculture to the site, statistically … there’s tons of variability, but I think overall, that these rates are generally higher than the bare sediment,” she said, adding that aquaculture is either having a neutral or positive effect on the ecosystem. “With the data at hand, it doesn’t look like these oyster habitats are removing lots of nitrogen, but it also doesn’t look like they’re harming the system in any way.”

But there is a seasonal aspect she has observed where the highest rates of nitrogen removal appear to be in the early spring, and she believes oysters may also help prime the environment to be more efficient in cycling nitrogen long-term.

Role in Greenhouse Gas Removal

One ecosystem service from oyster habitats and marine sediments that is overlooked, said Fulweiler, is the removal of nitrous oxide — a greenhouse gas 300 times more powerful than carbon dioxide. It is often referred to as laughing gas and is the number one offender in depleting the ozone layer that protects us from the sun. This gas can be produced in several fashions, including in marine environments where there’s a lot of nitrate.

Generally, the more nitrate, the more microbes produce nitrous oxide and consume it, said Fulweiler, explaining that the same process that can make nitrous oxide can also remove it.

“Most of the nitrous oxide fluxes in Ninigret Pond and across sites in Narragansett Bay are consumption … so there’s this whole ecosystem service that coastal marine sediments, including those impacted by oysters, are doing,” she said. “They’re consuming nitrous oxide for us and doing so at considerable rates.”

Fulweiler and her team have also been looking at the greenhouse gas footprint of growing oysters and compared that to other protein sources of food.

They looked at greenhouse gas concentrations that come from manure, fodder production, and within the animal’s gut — excluding greenhouse gas production from transportation, refrigeration, etc.

“We wanted to be comparing apples to apples … what’s the greenhouse gas footprint for growing a cow in terms of how much they’re producing from their gut, their manure, and how much is produced from feeding?” she explained.

Compared to cows, sheep, goats, pigs, and chickens, oysters account for a fraction of the greenhouse gases produced.

“Oysters are a friendly alternative if you want to eat protein and not contribute to the greenhouse gas footprint,” said Fulweiler. “If we replaced 10 percent of our red meat protein with oysters, it would be the equivalent of keeping about 14.8 million cars off of the road.”

Why We Need Oysters

As such, there are a billion reasons to restore oyster reefs and establish aquaculture sites, she said.

“I think it’s very important as we move forward and begin to offer ways of either increasing shellfish aquaculture or restoring oyster reefs, how we might look at that in terms of this entire picture,” she said.

And restoring a reef can take time and be costly.

“We have to think hard about how and where to do that because you can’t just put an animal or a plant back into a system that is subpar and expect them to do well. We may have to figure out areas that may have some improved water quality to allow them to flourish and improve the water quality even more,” she explained.

As for aquaculture, more work needs to be done to better understand which conditions they’ll be the most efficient at recycling nitrogen. “Maybe we can say, this rate was so much higher over here because the flow rate is a lot higher or this type of sediment is different. In doing so, we might be able to suggest the best areas to put aquaculture if your goal is to increase nitrogen removal.”

Fulweiler is currently planning a workshop to be held in September 2019 to better understand the range of nitrogen removal rates observed over the last decade to figure out the major patterns in oyster systems.

“We want community involvement to look at the literature and the patterns to help come up with this range based on the data … and then we’ll have to be adaptive in our management plan because the number might change in three years because of better techniques or more data,” she said. “We need to take an approach that, ‘this is the state of science right now, this is what we recommend, and we’ll revisit in two, three, or five years.”

IN THE NEWS

+ Oysters: Good for us, good for the environment | Narragansett Times

 

The Coastal State Discussion Series is sponsored by Rhode Island Sea Grant with the support of the Coastal Institute, the College of the Environment and Life Sciences, and the Graduate School of Oceanography at the University of Rhode Island.