Looking at flow microhabitats using fluorescein and chalk

By Jessica Bolker, University of New Hampshire

Type of Resource Laboratory activity 
Topic Other 
Taxa Widely applicable 
Organizational Level Ecological 
Estimated time to do activity half an hour (if stream is nearby), plus retrieving chalk a day later 
Background required/level introductory 
Role of activity in your course Get students into the field, and give them a chance to see the effects of boundary layers, local flow differences, and induced currents in real, local habitats. It’s easy, hands-on, outdoors, fun, and directly connected to lecture: a useful first lab at the beginning of the course. It can be done during a class period if one has a campus stream, or alternatively students can be sent home with the materials and tasked to find and examine a local stream of their choosing. 
What students might learn from this course or activity Fluorescein: Watching the dispersal of fluorescein dye powerfully illustrates the local idiosyncrasies of flow, especially around and upstream of rocks, branches, and other objects. It also provides a dramatic, if qualitative, comparison of general flow velocities in different (even adjacent) parts of a stream. Traces of dye squirted on or near the substrate remain in place for a surprisingly long time, thanks to the boundary layer (a concept I find hard to lecture about convincingly, and thus especially important to demonstrate). Chalk: Depending on flow and elapsed time, erosion can be dramatic…especially, and for many students counterintuitively, on the downstream side. This is a nice illustration of the effects of induced vortices, which I discuss in the context of the feeding strategies of black fly larvae. Using chalk and fluorescein together (e.g. in the same spot in the stream) helps students think about how different but complementary techniques can yield a more complete picture of what’s going on in a given situation.  
Special tools, equipment or software needed Fluorescein (also known as uranine) is available in powdered form (often gratis from a colleague in limnology or oceanography who has huge quantities on hand). Even if you can’t scam some for free, it is very inexpensive, if you buy the right kind (check catalogs for the cheapest): the stuff certified for injecting into humans to mark retinal vessels, etc. is pricey, but for stream use, the crudest form will do fine. Fluorescein is stable, non-toxic, and highly water-soluble. A small quantity (around 1 gram) dissolved in 10 ml of water makes a highly concentrated stock solution, which you can then dilute further: the stuff is visible down to about 1 part in 3 million. (The exact dilution is not important – try for a deep orange color.) Each student might be equipped with a small plastic jar of solution and a 10-ml pipette or syringe, the latter with a bit of catheter tubing on the needle.

Chalk: use the fat kind (about 1 by 4 inches) that is sold in supermarket toy aisles for kids’ sidewalk art, or (by the gross) for marking boxcars in freight yards (Dixon Ticonderoga #88809 Crayon Chalk (white), Dixon Ticonderoga Co., Heathrow, FL 32746). Chalk varies, so definitely do a test-run with the exact kind you’ll give to students. The Dixon Ticonderoga variety works very well. Each student should have at least 2 sticks for comparing different stream sites (and probably a back-up, also). If chalk is sent home, it’s nice to put it in 50-ml screw cap tubes for transport so it doesn’t break – this is especially useful for bringing much-skinnier eroded chalk back to class for show and tell.
Safety precautions, possible permissions necessary Don't fall in the river. 
Miscellaneous advice - pitfalls to avoid Fluorescein – students should squirt dye directly into water, especially right near substrate or obstacle surfaces, rather than dropping it in from above the surface of the stream. Make sure to try a variety of areas and regions of flow. Chalk – make sure to label upstream/downstream sides of stick before putting it in place. Make sure to anchor securely by pushing it at least an inch down into the substrate. (Different anchoring methods would need to be devised for rock or cement surfaces.) Give students extra chalk, as some pieces will likely erode completely, be swept away, or mysteriously disappear. This also lets students examine different time ranges as well as different areas of flow.

Instead of a write-up of this exercise, I asked students to post their observations on the class’s online discussion board, where they could also read and reply to their colleagues’ postings. This gave all of us an overview of everyone’s results.
Frequently asked questions by students Q: Why can’t I see any flow patterns (the water just all turned orange)? A: Squirt dye directly into the water, don’t drop it in from above the surface. Make sure to try squirting some directly onto the substrate (define “substrate” if necessary!). Q: My chalk looks exactly the same as yesterday – what should I do? A: Record that datum, and check it again tomorrow. Q: My chalk disappeared – what should I do now? A: Here are another couple of pieces of chalk…try again, and make sure chalk is firmly anchored. Perhaps try an area with lower flow. 
Description Two simple ways to visualize localized stream flow and its effects. 
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