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Oysters and Nitrogen Removal from the Water Column

View of a section of the BOP Community Reef at Bush Terminal Pier Park in Sunset Park, Brooklyn.

View of a section of our man-made BOP Community Reef at Bush Terminal Pier Park in Sunset Park, Brooklyn.

The BOP Curriculum Team is hard at work creating restoration education curriculum for our BOP teachers- our full curriculum archive lives on the BOP Digital Platform here, and it’s growing every day!  Last month, they rolled out the Nitrogen Cycle Investigation, which uses a classroom oyster aquarium as a microcosm for examining the global nitrogen cycle and the impact of nitrogen on estuaries.  The nitrogen cycle has real and potentially deadly consequences for organisms living in the classroom tank, and this motivates students to learn about different nitrogen molecules and how to test for their presence, followed by daily testing and data collection, research, and a final analysis to answer the question, “Is our tank ready for animals?”  Throughout the unit, students consider the cycling of matter through ecosystems at the tank, neighborhood, city, national, and global scale.  

“Oysters and Nitrogen Removal from the Water Column” is a student-friendly text the Curriculum Team created for this unit.  It appears as a part of a topic library, “Nitrogen and Estuaries,” from the lesson “Nitrogen Pollution.”

oyster-reef

A natural oyster reef from a post by Northeastern University, “Excess nitrogen removal by oysters depends on where a reef is located.

In the past few decades, New York City has made efforts to reduce the amount of nitrogen in New York Harbor.  But to fix our nutrient pollution problem we’d need to spend $200 million a year to continue to upgrade our wastewater treatment systems, and sometimes it’s a challenge to convince people that it’s important.  (This is one reason why it’s important for all of us to learn about our local environmental issues- so we can help the city make informed decisions about how to spend our tax dollars.)

Since oyster reefs in other parts of the country have been shown to remove nitrogen from the water, researchers are now studying whether oysters could help reduce the amount of nitrogen in New York Harbor.

Different forms of nitrogen
To understand why researchers hypothesize that oysters might help remove nitrogen from the Harbor, it’s useful to know a little bit about the different forms nitrogen takes.  In the environment, nitrogen goes through a series of transformations that we call the “Nitrogen Cycle.”  In different stages of the cycle, nitrogen combines with other elements, like oxygen and hydrogen.

Some of these forms of nitrogen are harmful to aquatic life, while others are relatively harmless.  Nitrogen gas (which makes up 79% of our air!) is harmless.  But some nitrogen compounds, like ammonia (a compound of nitrogen and hydrogen) can kill fish and oysters if there is too much of it in the water.

Ammonia is not all bad- we use it to fertilize plants to make them grow.  Plants then incorporate that nitrogen into plant proteins, which we and other animals eat.  Some of that protein then becomes a part of our bodies.  Your body is around 3% nitrogen! And you got that nitrogen from…

When animals digest protein, some of the nitrogen from that protein ends up in our poop and pee.  Bacteria break the waste down, turning that nitrogen back into ammonia.

Could oysters help?
Sometimes there’s so much nitrogen in a waterbody that it sets off a chain of events that uses up the oxygen in an area, creating what’s called a “dead zone.”  (To learn more, look at the reading “Nitrogen, Dead Zones, and New York Harbor.”)  Since oysters need oxygen to survive, if they’re caught in the dead zone, there’s not too much they can do to help!

However, in places where there is a lot of nitrogen, but not so much that all the oxygen gets used up, there are several ways that scientists are studying, to find out if oysters might be able to help remove nitrogen from the water.

1) Oysters incorporate nitrogen into their bodies and shells.
Remember how we said that ammonia is a fertilizer for plants on land, and that your body is 3% nitrogen?  In the water, ammonia acts like a fertilizer for tiny microscopic organisms called phytoplankton.  Like plants, phytoplankton turn ammonia into protein.  When oysters eat the phytoplankton, that protein becomes a part of the oysters’ bodies and shells.  Oysters have nitrogen in their bodies, just like you! But oysters also excrete nitrogen waste, just like you.  And when they die, the nitrogen in their bodies returns to the water as they decompose.Oyster reefs are also a habitat for lots of other organisms whose bodies contain nitrogen.  So scientists have found that some nitrogen is removed from the water just by being incorporated into the oysters’ and other organisms’ bodies.  But those animals also excrete nitrogen waste.  And when they die, the nitrogen in their bodies also returns to the water as they decompose.

Scientists are still trying to determine if oysters and their reef associates in New York Harbor could remove more nitrogen from the water (by incorporating it into their bodies) than they put back (through excretion and decomposition).

2) Oyster poop and other “biodeposits” can settle on the bottom of a waterbody and get buried.
When oysters filter in little bits of rock or other materials that aren’t food, they cover it in mucus (basically snot!) and spit it out.  These are called “pseudofeces.”  (“Pseudo” is from a Greek word that means “fake,” and the word “feces” means “poop”- see how scientists came up with that term?)  Oyster feces and pseudofeces are called “biodeposits.”Both real oyster poop and pseudofeces contain nitrogen, like human poop.  But because oysters coat their feces and pseudo feces in mucus, oyster biodeposits settle on the bottom.  Sometimes their biodeposits get buried under sediment instead of floating back into the water.  Scientists aren’t sure how much nitrogen can be removed from the water column this way, or how long the nitrogen can stay buried.  But it might play a small part.

3) Oyster reefs create a special habitat for specific bacteria that can turn ammonia back into nitrogen gas.
Bacteria can turn nitrogen from forms that are harmful to fish and other wildlife (like ammonia) back into a form that is harmless, nitrogen gas.  Scientists are learning a lot right now about how oyster reefs can create good conditions for bacteria to carry out this process, and it has a lot of them excited about the possibilities.

Oysters alone won’t solve the problem of nitrogen overloads in New York Harbor  But they might help!  That’s one reason why your Oyster Restoration Station data is so important.  Your work will help our scientists find the best spots to put oyster reefs in New York Harbor!

Adapted from: https://www.billionoysterproject.org/can-habitat-restoration-improve-water-quality-in-urban-estuaries-september-2016-bop-ccers-colloquium/

For more information, check out: “Setting Objectives for Oyster Habitat Restoration Using Ecosystem Services, Manager’s Guide,” pages 36-39 from The Nature Conservancy.

Posted 3/6/17 at 4:10 PM