Life in the Mississippi1
by Howard Webb (St. Louis, MO, USA)
For some time I have been wanting to investigate daphnia beyond the reaches of dipping a pole over the bank of a pond; in particular I have been curious about the spread of Daphnia lumholtzi, a non-native cladocera. An opportunity opened up in mid-October, and I went canoeing on the Mississippi River to see what I could find. Some studies indicated that the population of D. lumholtzi drops below detectable levels by November, so I had uncertain expectations for this late in the season.
Daphnia lumholtzi is an exotic "water flea", originally from Africa or Australia; and possibly brought into the United States via the tropical fish trade. There has been concern that its distinctive spines may give it a defensive advantage (against predators) over native cladocera, possibly changing some of the dynamics of local food chains.
In 1995, Daphnia lumholtzi were first found in the Illinois River. Since the confluence of the Illinois River and the Mississippi is only a few miles above where I was sampling; D. lumholtzi were likely to have floated into the basin, if not having set up a permanent population.
Compared to earlier experiments with the plankton net (on Simpson Lake), the results were quite good. For the short time I was actually towing the net, I collected over 100 cladocera. The surprise was the distribution of species; there were about a half dozen Daphnia mendota2, and the same number of Moina, the rest were all D. lumholtzi. The diverse sizes of the D. lumholtzi indicated a dynamic population, not just a few late season adults.
I used my plankton net; making both vertical tows (letting it drop to the bottom, then pulling it straight up), and pulling it along behind the canoe while paddling around. There was not enough current to anchor in one place and catch the flow of the river.
The first images are of live specimens, taken the same day as collected. To avoid injuring the daphnia, no cover slip was used. This is a trade off between a natural looking specimen, and better optics and images. The displayed images have been reduced and sharpened, click on any image to see the full size (file size is of the full sized image).
10x bright field, best mode
100x bright field, best mode
100x bright field, best mode
The limitations of my old Kodak are beginning to show. On the bright field images, the automatic light adjustment makes it appear as if the condenser iris is closed down too far. Visually this is not so, but the shadow is an artifact of the camera trying to create a uniform grey level (and not having controls to override the settings).
Boating on the Mississippi can be quite dangerous, especially in a small craft. Commercial barge-tows ply the main channel, and significant currents exist in many places.
I deliberately chose a location known as the "Alton basin", what is essentially a lake behind one of the river's major lock and dams (Alton lock and dam, #26). At this point the lock is 20km downstream, and the river is 1.4km wide. There is virtually no current in this part of the river, and by staying among the islands, and out of the main channel; all commercial, and most recreational traffic can be avoided. The canoe has an outrigger which greatly increases the stability, and we wear life-vests at all times. I also picked a day with no wind and good visibility.
Water temperature: 18 C.
Depth: 5 meters
Secci visibility: 0.5 meters
Location: UTM 15 728459E 4313696N (mile 214 on the river)
Microscope: Bauch & Lomb monocular, 10x ocular, 4x, 10x and 40x objectives.
Camera: Kodak 3200 Digital, best mode 1152x864
Software: Photoshop Elements
1Apologies to Mark Twain and his book Life on the Mississippi
2 There is some debate if this is Daphnia mendota, or a hybrid: Daphnia mendota galeata
Other articles on D. lumholtzi
Illinois Natural History Survey
INHS Doc #2068
Comments to the author,
Webb, are welcomed.
Published in the January 2004 edition of Micscape Magazine.
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