Random Collecting (of a freshwater plankton sample from Lake Superior)

Random Collecting

by Howard Webb (St. Louis, MO, USA)

Introduction

I would like to get out into Lake Superior and explore for daphnia, but the size and conditions on the lake are prohibitive, both for my equipment and budget. Lake Superior is the largest freshwater body of water in the world. Not only is this a big lake, it is the deepest of the Great Lakes. Half of the volume of this lake would fill-up the rest of the Great Lakes along the US-Canadian border. While on vacation this year, the family and I were in Michigan and Canada and visited the eastern shores of Lake Superior. However, short of renting or chartering a boat, there were no good opportunities to collect daphnia. As an alternative, I decided to try sampling from St. Mary's River, which drains Lake Superior into Lake Huron. There is a canal that runs from the river through the city of Saulte Ste Marie which feeds the electrical power plant. Late one evening, I had a few minutes to try sampling in his canal, from one of the bridges.

Conditions

Much of the St. Mary's river is blocked by locks and dams, with the excess water used by hydro-electric power plants on both the US and Canadian shores. As such, the power canal in Saulte Ste Marie carries a considerable volume. Since the flow is controlled, the bridges over the canal are fairly close to the water, and it was easy to lower a plankton net into the current. On the down side, the current was so fast, that the net soon started skipping along the surface when it reached the end of the line. I had to use a technique of dropping the net and line straight down and letting the net sink while drifting. When the net rose to the surface, I would pull it in and start over. There was only time for six or eight of these manouvers before I had to quit.

Images

There was only one organism found in the net, though I was amazed to find anything (at least in one piece). It appears to be a Holopedium gibberum. Normally these have a gelatinous covering, but under the collecting conditions, it could easily have been washed off. I based this identification on:

Humped carapace
The body and legs being exposed, not covered by the carapace
2nd antenna cylindrical, not branched


Holopedium gibberum 40x, dark field Click image to view 1 meg file	Holopedium gibberum composite image Click image to view 1.3meg file

Technical Details

Environmental Conditions:
Water temperature: Unknown
Depth: guessing about 4 meters
Secci visibility: Lake Superior is incredibly clear, with visibility greater than 7 meters.
Location: lat: 46.49254, long: -84.33250.
Microscope: Bauch & Lomb monocular, 10x ocular, 4x, 10x and 40x objectives.
Camera: Canon A70
Software: Photoshop Elements

References

Center for Freshwater Biology: Image based key

Central Michigan University

Comments to the author Howard Webb are welcomed.

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Published in the November 2005 edition of Micscape Magazine.

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