Activity under our frozen lakes

Chel Anderson is a botanist and plant ecologist. She lives here in Cook County and joins us periodically to talk about phenology or what’s going on in the woods right now. Welcome, Chel.

Anderson: Hi!

Well, a couple of weeks ago, some volunteers jumped into Lake Superior to help raise money for a charity. Yes, they did. They got out right away, leaving unanswered the question of what’s going on in the dark below the ice in our frozen lakes. So, tell me, what’s going on under the ice?

Anderson: It is cold in the water these days. Not as cold as the air temperature, though, right? At least most of the days recently. It’s not only cold, cool to cold, down there, but it’s also very dark, because, right, all the light from the sun, for the most part, is bouncing back into the sky and so, it’s pretty darn dark down there. But, it’s not all quiet down there. Things are still moving around. When fetching water from the water hole at our place, it’s not at all uncommon for us to see diving beetles moving around, giant water bugs have been at the water hole, dragonfly nymphs are being their whiz-bang predators that they are; all that is going on. Water scorpions have shown up at the water hole. So, lots of, you know, life is happening. It’s happening at a slower pace, but things are going on. Not just aquatic insect life, but also small mammals, like water shrews and star-nosed moles. They spend a lot of time, this is where they do most of their feeding, is in the water. One of the key things that’s going on, though, right now under water is that because the lakes are sealed tight with ice, the oxygen that the lakes went under the ice with is slowly being consumed by all the things that live there, except the plants, which normally take in carbon dioxide and give off oxygen, adding to the oxygen, but they’re not doing that, because there’s no light. So, the lakes have this kind of total budget of oxygen that’s there and until the ice leaves, that’s what everything has to live on. So, it’s being consumed, it’s not being created, so that, over the span of the winter, begins to have an effect on the life there. One of the really interesting ways to actually see the impact of oxygen changing over the course of the winter is to look at a little zooplankton that are called Daphnia—that’s the genus that they’re a part of—and they are planktonic crustaceans. They’re also called water fleas, but I’m not going to call them that, because they’re not fleas.

There have been some reports in the news lately about Daphnia as having a genome.

Anderson: Yeah, bigger than anything. Yeah, right.

Bigger than anything, and they’re hardly big enough to see.

Anderson: Yeah, right. Exactly. Yeah, good, I’m glad you brought that up. It’s very fascinating that this tiny little thing—so think, when I’m talking tiny, I’m talking the size of an apostrophe on your keyboard. So, these are tiny, but they are visible. You can actually see them.

And we have them here.

Anderson: And we have them. They’re ubiquitous in all freshwater ecosystems. They have tree-like tentacle arms that they flail over the tops of their heads which sends them spinning in kind of backwards circle, which seems kind of silly, except for it also drives water through their outer shells. Then, the filter algae, which is what they eat, and oxygen out of the water that passes through them. So, that’s why they do all this flailing. But, they have this amazing ability that is probably part of this huge genome that they have that’s given them the capacity to adapt to the really dramatically-shifting conditions that occur in their aquatic habitats, which can range from lots of water at one time of the year to no water sometime else, but also to changes in oxygen levels. The adaptation that they have that relates to oxygen is that they can make hemoglobin. So, during the summer when there’s lots of oxygen, they are translucent. But, during the winter or any other time when oxygen begins to be depleted, they begin to make hemoglobin for themselves so that they can survive in the depleted oxygen condition.

What does the hemoglobin do?

Anderson: Hemoglobin is the best way to move oxygen from a source of oxygen to where it’s needed that any life form can make. So, it’s the thing that makes our blood red, it’s the thing that moves oxygen from our lungs to the rest of our body, to our organs, to our muscles in our bloodstream. So, they are making this for the same purpose, because, in the summer when there’s lots of oxygen, it’s just moving right through them, there’s plenty of it. They don’t need any special mechanism to bring oxygen from water into their systems to use. Once the oxygen starts to get depleted in the water, then they need hemoglobin to do that transfer more effectively and efficiently. So, because they’re making hemoglobin, as the winter goes on and the dissolved oxygen in the water becomes diminished, they begin turn color, because the hemoglobin they make is red, too. So, as the oxygen becomes lower, they get pink, and when it gets really low, then they are red. And, how long the ice lasts has a big impact on how quickly that oxygen begins to be replenished. If the ice lasts way, way late into the spring, well, of course the oxygen then gets even lower in the lakes and ponds than it does if the ice goes off early.

Before we end the discussion, I know we agreed we’re not going to talk about fish, because we talk in all other sets of circumstances, but do fish slow down in the winter?

Anderson: Yes, fish slow down. I read some great descriptions of the people diving under the ice and actually approaching groups of sunfish that are just totally still, barely moving in the water. They were described as like autumn leaves suspended in the water, and as the person went by them or dove through them, they could just move their hand to part them like you’d part leaves on the ground, and they’d just kind of shift over, not moving on their own, but just moving because the diver was moving the water to push them aside. But, yeah, they’re just conserving, conserving, conserving so that they’re just sipping, sipping oxygen so that they can make it through.

That explains my luck ice fishing. Chel Anderson, botanist and plant ecologist, thanks for helping us understand what’s going on under the ice.

Anderson: You’re welcome.

Photo courtesy of Brynn via wikimedia.