In addition to clearing out the leaves, the fast flow in the creek created some interesting fluvial features. Example number one is this curious gravel bar that was not there a week ago. The gravel is quite coarse — 2-4 cm in diameter — but it’s extremely loose, which is typically of recently deposited sediment.
It seems likely that the sediment comes from beneath the fallen tree that cuts across the creek just upstream of the gravel bar. The tree restricts the stream flow, forcing the water to speed up, and when the water found it’s way through by cutting under the tree, it had enough energy to excavate a hole under the tree and deposit the resulting sediment just a meter or so away.
Constriction of the stream by the fallen tree focused flow beneath it, digging out sediment and depositing just downstream on the gravel bar.
The rapid, snow-melt driven, flow in the creek has receded a little, but it managed to clear out most of the dead leaves that have carpeted the stream bed since the fall. Now that the rocky bottom is exposed, hopefully, we’ll be able to see some more of the benthic fauna that’s been invisible for the last few months.
Washing out the dead leaves has exposed the rocks and rapids.
Clear ice in January, 2013: Ice thick enough to support a person’s weight, but clear enough to see through. Creating this beautiful shock pattern took a lot of time and concerted effort.
In January, the creek froze over after a few days of below zero (Celcius) temperatures. However, we’d missed the snow that usually accompanies a drop in temperature (it had hit to the south of us), so the ice froze clear and firm. Since they’re used to seeing the whiter ice — on lakes and ice rinks, for example — my students wondered why the ice was so clear.
In February, we got both the snow and cold, which incorporated air bubbles into the ice, resulting in a very different, cloudier ice.
Snow incorporated into the ice makes it cloudier and, I think, somewhat ethereal. Note the same fallen walnut tree trunk as in the picture above.
It was quite neat to see the contrast, and made me wonder about the ecological impact. After all, clear ice lets sunlight through much more efficiently than cloudy ice.
While not quite as dramatic as drilling through four kilometers of ice to find signs of live in an Antarctic lake that’s been isolated from the rest of the world for over 100,000 years, we observed filamentous algae blooming under the clear ice on the creek earlier this winter.
Algae under the microscope (40x magnification).
I collected some of the algae and put it into the fish tank; I was hoping I could use it when we looked at plant cells because aquatic plants tend to have larger cells that are easier to see under the microscope.
However, one of my students is keeping tadpoles (also from the creek) in the fish tank. She noticed that the tadpoles were hanging out on top of the algae, and the algae was disappearing. Well, at least we’d solved her problem about what to feed the tadpoles.
Unknown microbe hanging out in the algae.
While the filamentous algae might not be as good as the Egeria densa for plant-cell microscopy, it does host quite a number of other microbes that are fascinating to look at.
Ecological Role of Algae
Based on these observations, ecologically the filamentous algae does not just provide habitat for protists and other microbes, it also appears to be a significant source of food for larger animals, like the tadpoles, and probably also the small fish that live in the creek. Bubbles trapped under the ice. With all the algae growing in the water, and the clear ice, these bubbles may well be made of oxygen.
Therefore, I’d hypothesize that in the winter, when the fish disappear, and most of animal life is quite subdued, the algae blooms because it’s not being grazed on nearly as much (see the picture above). When the weather warms, however, it’s the turn of the algae to repressed.
It would be interesting to have a student monitor the algae growth, and the fish/tadpole population, over the course of the school year to see if the relationship is more than just coincidence.
P.S. After our last snowfall, the melting snow has put a lot of water into the creek, and all the algae appears to have been washed away.
Trivia night let out at midnight. The ground was still covered with a smooth layer of snow. The meltwater from the daytime sun had refrozen to make the snow crisp on top, like a cold crème brûlée. The moon was close to full. I had my camera.
The moon reflected in a pool partially covered by ice.
I spent about twenty minutes traipsing through the woods along the banks of the creek. Not having a tripod made it impossible to get long-exposure without setting the camera down somewhere stable, so I ended up lying prone on the snow. Whilst my jacket and sweater made my top half well insulated, there was just a single layers of broadcloth separating my legs from the snow. I didn’t have a problem with the cold, but my body heat melted the snow, and I got wet.
I’ve collected a set of aquatic plants for our fish tank for the middle school students to be able to look at their cells under the microscope. A few are from the store, like the Eregia densa I’ve used in the past, but we’ve also grabbed some algae from the creek, and Mr. Woodbury brought in some algae specifically for our two resident tadpoles.
I was checking out at the creek algae under the microscope when I came across these two microbes. They both were motile and seemed to be surrounded by cilia, but I really don’t know what they are.
Surface tension supports the water strider on the surface of the still water of the creek.
Down at the creek the water striders are out. They can stand, walk and jump on the surface of the water without penetrating the surface because of the force of surface tension that causes water molecules to stick together — it’s the same cohesive force that make water droplets stick to your skin. I got a decent set of photos to illustrate surface tension.
The water striders still create ripples on the surface of the water, even though they never break the surface.
The green canopy that over hangs the creek allows for some nice photographs.
Crayfish camouflaged against the rocks in the creek.
One of my favorite things about the Fulton School campus is the little creek that runs along the boundary. It’s small, dynamic, and teeming with life.
The crayfish are out in force at the moment. Some of the high-schoolers collected one last fall and it survived the winter in our fish tank (also populated with fish from the creek).
They are quite fascinating to observe; wandering around the sandy bed as if they own the place; aggressive with their pincers occasionally; but then darting backward amazingly fast if they feel threatened.
The one in our tank has just molted a second time, so now we have two almost perfect exoskeletons sitting around the science lab.
Crayfish exoskeleton. From pincers to tail the skeleton is approximately 10cm long.
