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Observations of a Balanced Aquarium

A couple of months ago, I described my idea to have students create balanced aquaria and since then I've set one up and have been collecting observations so I have some idea about what to expect if I have students do it. It's been fascinating.

I initially went up to Pelham to start building my aquarium. There's a wonderful little vernal pond I know up there -- almost more of a vernal puddle. It just a depression full of wet leaves with lots of copepods and other interesting critters. I figured it might be a good place to get the decomposer part of the ecosystem. There's also a reservoir that I thought I might be able to find some nice filamentous algae in. But I had forgotten that up there, the water is practically just rain water and the reservoir is basically oligotrophic. I could see a few newts, but no algae at all.

I drove next to Podick Cole Santuary, which is surrounded by agricultural fields and has a beaver pond (with attendant swamps and marshes). I found some nice scummy floating filamentous algae and scooped it up.

I had constructed the apparatus, filled it Poland Spring water, then I added some leaves and water from the vernal pool and some water and scum from the beaver marsh, sealed up the top, and I was in business:

I observed copepods and ostracods immediately. There were some collembola on the water surface. I think there was a big planarian, although I couldn't get a good look at it: It might have been a small leach. After a few days, I saw a small dragonfly nymph.

The data are complicated and interesting. The program I'm using to collect the data is pretty minimal: eventually I want to have something set up that will send all the data to a server that aggregate and summarize everything. But for the moment I'm just collecting it over the serial monitor of the Galileo.

At first I thought the big spikes in CO2 were happening at night due to biological activity and when they stopped it was because the copepods had died. But I saw that there were still some copepods, although it was clear that there were fewer. As I caught more glimpses of the nymph (who seems bigger) it now seems more likely he's been eating them. But at first, I didn't have reliable time-stamps on the data and, once I did, I found that the big spikes were happening during the day and the variation was correlated with something else: warm sunny days. When the sun shines in, it warms up the aquarium, which reduces the solubility of gas in water and drives the CO2 in the atmosphere, where it gets measured.

Around this time, I also noticed something new: snails. At first they were tiny -- smaller than sesame seeds. They would move around on the inside clearing off the algae and aufwuchs growing on the inside of the bottle. And they grew as well: now they're peppercorn or caper-sized. (Note to self: encourage your students to actually measure things they observe, rather than to simply use cooking analogies). As they grew, they became more efficient at hoovering up all of the algae. And then the CO2 really began to spike.

The spikes of CO2 got higher and higher and higher. I began needing a log axis to plot the data. I showed them to Phil who asked "What does it mean to have more than a million parts-per-million?" Good question, I thought. I did some investigating it turns out that the sensor is really only valid between 400 and 10,000 ppm. So it's not clear what those spikes mean -- if anything. Probably, I should simply truncate the data at 10,000.

The newest excitement has been seeing a slime mold (or molds) active. I recognized it immediately as some kind of slime mold, confirmed by @Genevieve.

It moved around, but also seemed to split up and then rejoin again. I shot a few time-lapse sequences (not really good enough quality to share), but which confirmed that it was moving around in real-time.

I had seen only a very small number of copepods in recent days: almost none. But then on Thursday or Friday, I noticed a vast number of really tiny copepods. There must have been a big hatch of eggs. I also noticed the slime mold looking like it was growing fruiting structures:

It's been a fascinating natural history project -- for me anyway. And I think my students will enjoy it too. I've gained a lot of insight regarding the additional tools and resources we might want to put in place to make the project really work well. I've realized that it might be really helpful to have quantitative light sensors in front of and behind the aquarium to measure optical loss. It might be hard to calibrate that, however. Having a temperature sensor seems pretty critical. Those are cheap. I also want to look into making adaptors to use cell phones as microscopes: if you could just press those up to the bottle to take pictures it might be really helpful for collecting imagery and trying to identify what we're seeing inside.