As a fisheries and aquaculture specialist at the Barnstable County Cooperative Extension, Abigail Archer spends a lot of time trying to help the public connect the dots between shellfish, nitrogen, and healthy estuaries. This relationship starts when nitrogen travels through freshwater streams and runoff into our marine environment.
"Oysters are kind of like sheep grazing out in a field. And so, you know, the sheep are not standing in a field absorbing the nutrients from the grass. They're actually munching on the grass and then eating that and then getting the nutrients in the grass," she explained.
"And so it's the same thing for the oysters. So the nitrogen that's coming through our groundwater is then fertilizing basically the algae, the phytoplankton that's in the water. And then the oysters are filter feeders. And so they're they're kind of grazing on all of that yummy algae."
And this is how things are supposed to work. But as Abigail’s colleague and fellow fisheries and aquaculture specialist Josh Reitsma explained to me, in many places, the system has gotten out of balance.
"So nitrogen in and of itself is a good thing. We all need it. It's an essential element for all of us, including oysters, or quahogs for that matter. But the challenge is, in the marine environment when too much nitrogen is entering the system, it overloads the system," Josh said.
Mostly in the Cape’s marine systems, this is a problem in estuaries — partially enclosed bodies of water where freshwater from the land meets the saltwater of the sea. And this is because increasingly, this freshwater carries more nitrogen than it has historically — and as a result more nitrogen-fertilized algae grow than the marine system’s filter feeders can eat.
"It's roughly, I think, 80% of the nitrogen in our waters on Cape Cod is coming from humans. So mostly septic systems. But there's also, you know, some wastewater treatment systems, and they all input nitrogen that eventually reaches the estuaries," Josh explained.
This might sound gross, like human waste is flowing through the ground and settling on top of our oyster beds. But that’s not how septic systems work. Their job is to break down and filter human waste and especially to make sure that harmful pathogens don’t make it into drinking water, streams, or estuaries. And I cannot emphasize this enough — our septic systems do this well. But when they break down and filter our waste, they break it down into its core elements — one of which is nitrogen — and as the population of humans on the Cape grows, our septic systems aren’t adapting — they don’t do anything to stop excess nitrogen from making it into our waterways. Here’s Abigail again.
"It's a balance thing. So as long as there’s enough food for the oysters so they can grow and support our industry then that’s great but if there's too much algae, then you get the nuisance algae blooms that that's some of the other areas on the Cape are seeing."
These nitrogen-caused algae blooms are similar to the problems we’re having with carbon dioxide in our atmosphere. It’s not that carbon dioxide or nitrogen are inherently bad — just the opposite, they’re critical for life — but we’re adding so much of both that we’re throwing the carbon and nitrogen cycles out of whack and changing ecosystems completely. What’s interesting about the nitrogen cycle — and excess nitrogen in our waterways is that when it comes to correcting this imbalance, shellfish have a critical role to play.
"We've known for a long time they're an integral part of the system by filter feeding. And, you know, in that process sort of taking in nitrogen from the environment," Josh said.
And because of this, starting in about 2010, many towns on the Cape began trying to increase shellfish populations and credit them for their nitrogen-removal work — sort of like how organizations use tree planting to offset carbon emissions. But it’s been complicated — because planting a bunch of oysters and clams basically means getting involved in the aquaculture industry.
"It got complicated when we started wanting to, credit shellfish for the job they're doing, for extracting that nitrogen from the environment and pulling it out with the harvest of the shellfish."
There are so many layers to this — not only do towns have to figure out where to get seed and how many oysters they can handle growing and selling but there’s also the math, which with any living system is never as simple as we like to think. For now, fine-tuning how oysters and other shellfish fit into nitrogen mitigation plans is an ongoing challenge for most towns. Still, they’re an important piece of the puzzle. Because even though it might seem like we’re growing lots of shellfish today, populations up and down the East Coast are significantly down from historic levels. In Wellfleet where I live, current populations are estimated to be roughly 10 percent of what they were in the 1800s — which you could think of as meaning that we all need to eat a few more oysters and clams for the health of local waters.
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Learn more:
Cape Cod Commission on Estuaries: https://www.capecodcommission.org/our-work/estuaries/
The Massachusetts Estuaries Project: https://www.mass.gov/guides/the-massachusetts-estuaries-project-and-reports
A study on nitrogen removal potential of oysters and quahogs: https://www.capecod.gov/wp-content/uploads/2022/07/Nitrogen-extraction-potential-of-wild-and-cultured-bivalves-harvested-from-nearshore-waters-of-Cape-Cod-MA-Marine-Pollution-Bulletin1.pdf
