Christmas Post Redux: Letter Home Explains Carbon Cycle via Christmas Ornaments

Happy Holidays from the CFL! Last year, graduate student, Julia Hart, was so ready to head home and see family that she wrote this great primer in advance to tell her Mom what, exactly, she studies here at the UW-Madison. It’s a letter worth reading again.

Photo by Paul VanDerWerf via Flickr
Photo by Paul VanDerWerf via Flickr

by Julia Hart
Dear Mom,
How’s it going? Are you busy decorating the house? Are you ready for the whole family to visit? Have you prepared your standard answer for “What are the kids doing these days?”
Well, I’m here to help with that last one—by telling you what exactly it is I do. I know you’re proud of me—that “Julia’s busy studying lakes in Wisconsin.” But what I really do is study the carbon cycle in lakes. I know, I know. Scary words, “the carbon cycle”—nobody ever explains what it really is! It’s a pretty important process—especially when it comes to water quality and climate change. But after you read this, you’ll be able to tell Uncle Steve, Aunt Jan, and all the cousins about your daughter who studies the carbon cycle in Wisconsin lakes. You’re welcome.
Let’s back up a little and start with the big picture. Everything is made up of carbon—this tiny little atom is the building block of life. You are made up of carbon. Trees are made of carbon. Even fish are made of carbon. It’s in our bodies, atmosphere, and lakes. And it’s constantly moving. So how does it even get there? What happens while carbon is in a lake? And where does it end up?
The answer might be in the bins and bins of holiday ornaments you’ve probably dredged up from the basement in the last few days. Let’s think about all of those ornaments as carbon. And our house as the lake.
How does carbon come into lakes?
Forget the confusing diagram and think "Christmas ornaments." Image: Northern Arizona University
Forget the confusing diagram and think “Christmas ornaments.” Image: Northern Arizona University

Do you remember that one Christmas when your co-worker gave you that ugly squirrel ornament? How about that creepy baby ornament we inherited from Nana? Whether gifted, inherited, or purchased, ornaments keep finding their way into our house. And we can’t seem to stop the flow.
Carbon enters lakes via similar sources: streams, stormwater runoff, and groundwater. All of these sources dump water into a lake, bringing loads of carbon with it. And it comes in several forms: ugly squirrels, creepy babies…kidding! It comes into the lake as organic carbon like leaves or particulates from the landscape or as inorganic carbon like carbon dioxide—the same gas that we exhale—dissolved in the water. We call these “external” sources of carbon to the lake, just like your co-worker and Nana are external sources of holiday ornaments to our house.
But some carbon is also created within the lake. These are like the ornaments that Corinne, Annie, and I made as kids—produced internally, right there at our dining room table. When we were little, you provided the construction paper, glitter, and glue, and just let us go to town creating our own ornaments. The more glitter, the better! “Yes, honey, that looks beautiful…”
With the right amount of nutrients and carbon dioxide, lakes can produce their own “internal” carbon, too—in the form of algae. But we have to be careful with this carbon pathway. Here, nutrients are like the glitter. The more nutrients in the lake, the more algae (and, thus, carbon) the lake is going to create.
As you know, too much glitter means a Christmas tree covered in construction paper ornaments and hardwood floors that sparkle until St. Patrick’s Day. Sometimes nutrients are a good thing, as algae are an important food source for many fish and other organisms. But with too many nutrients, lakes produce too much algae, turning the water green and blocking light and oxygen for the plants and animals that need it. Just as you had to limit our use of glitter for the sake of your hardwood floors (and sanity), we need to limit the amount of nutrients that enter our lakes for the health of the ecosystem.
What happens once carbon is in the lake?
Julia Hart.
Julia Hart.

Every year, there is a growing pile of ornaments that we retire and no longer hang on the tree. I know this because you frequently take ornaments I’ve just put on the tree back off, deeming them old, ugly, or outdated. And those unwanted relics of holidays-past go back to the depths of the basement, to be buried among the Easter Bunny table runners and Halloween candy buckets forever and ever.
A lake does this too. When algae die, they fall to the bottom of the lake and are buried in the mud. We call this process “sedimentation” since all of that carbon sinks down to become part of the sediment. It’s a pretty slow process, but once that carbon is buried, it stays there! Sometimes carbon preserved at the bottom of the lake can tell us about the climate 10, 20, or even 100 years ago. Just like our growing pile of retired ornaments can tell you what our tree looked like in 2005 or 1995.
How does carbon leave the lake? Where does it end up?
Funny story, do you remember when I dropped that crystal star your boss had given you last year? A thousand little pieces! Oh man, that was so funny. Are you laughing? No?
Well, broken ornaments happen, okay? Your unusually clumsy daughter drops them, or the kitten knocks them off the tree, and suddenly, their form is forever changed. And those ornaments must find their way out of the house, usually via the trash man.
Carbon is also often transformed before it leaves the lake. Aquatic organisms (like fish) breathe out just like you and I do, releasing carbon dioxide into the water. That carbon dioxide is then released into our atmosphere. But there are also some places in the lake, like the deepest parts, with no oxygen. There, carbon is transformed into methane, which can also be released to the atmosphere. We refer to this as respiration, just like the lake is exhaling. Both carbon dioxide and methane are considered greenhouse gases. You know, the ones that are really good at trapping heat in our atmosphere, leading to warmer global temperatures. Turns out lakes are pretty important for climate change!
Lastly, carbon can leave the lake just like it came in – through streams. You can think of this as carbon “re-gifting.” So your co-worker gave you that really ugly squirrel ornament—what if you just decided to re-gift it? You could re-gift any ornament you like really. But you can’t re-gift my homemade cotton ball, glitter bomb snowman, Mom. That’s a keeper.
This is what lakes do with carbon as well. Whether it was originally external or internal carbon, streams carry carbon out of one lake and deliver it to another. Boom! Re-gifted! And the cycle is ready to begin again.
So what do I do again?
So there you go, Mom. That’s what I do. I study the carbon cycle in lakes. While it’s not as charismatic as studying baby pandas or dolphins, it’s important because carbon is crucial to water quality and climate change. And it happens everywhere. Every lake in Wisconsin, every lake in the US, every lake in the world.
So when Uncle Steve wants to know what I’m doing in graduate school, you’re answer is going to be: “Oh, you know, she likes lakes…and holiday ornaments are like carbon…oh, and do you remember when she broke that crystal star last year?”
Nailed it.