Fish as Fertilizer by Mark L. Kuhlmann
Part 2A—Do Marine-Derived Nutrients Affect Stream Organisms?
Instructions—First Jigsaw Group
Read the background information for your figure carefully, then work with your group to understand the figure or figures in the section you are assigned. Use the Step 1–Step 2 approach described below.
Step 1: Describe the graph and what it shows. Make sure you understand how the figure is set up, what the axes show, and what information is depicted. Carefully describe the overall patterns in the data.
Step 2: Try to interpret the data.
When all of the members of your group understand the figure, work together to answer the accompanying questions; they will help guide you as you interpret the graphs and make conclusions. Write down anything your group still doesn’t understand (ask for help from your instructor if needed).
Next, prepare to help the rest of the class understand what you just learned. Think about how you can best explain the graph (its elements and what it shows) and your conclusions to other students who are seeing it for the first time. Be sure everyone in your group is ready to explain your figure(s).
Figure 3—Background
From: Wipfli, M.S., J. Hudson, and J. Caouette. 1998. Influence of salmon carcasses on stream productivity: Response of biofilm and benthic macroinvertebrates in southeastern Alaska, U.S.A. Canadian Journal of Fisheries and Aquatic Science 55: 1503–1511. Used with permission.
To determine the patterns of biofilm (microalgae, bacteria, and fungi attached to rocks) and macroinvertebrate abundances in a natural stream, rocks were sampled at two sites on Margaret Creek near Ketchikan, Alaska, during the fall spawning season. (Macroinvertebrates are animals without backbones that are large enough to see without magnification; juvenile insects are the most common in fresh water.) The carcass-enriched site was a salmon spawning area (75,000 salmon spawn in the creek annually) with abundant salmon carcasses. The control site was upstream of salmon spawning and contained no salmon carcasses. Biofilm (sometimes called epilithic organic matter, or EOM) abundance was measured as ash-free dry mass (AFDM): a sample is dried, weighed, oxidized (incinerated) in an oven, and reweighed. The difference in weights is AFDM, a measure of the organic material in the sample.
Figure 3—Mean (± 1 standard error) biofilm AFDM (A) and benthic macroinvertebrate densities (B) on stone surfaces within Margaret Creek, comparing upstream control (no salmon carcasses—open bars) and downstream (with salmon carcasses—shaded bars) areas. In (B), data for the two most abundant subgroups of macroinvertebrates are also shown. Chironomidae = midge larvae (Order Diptera); Baetis = a genus of mayflies (Order Ephemeroptera).
Questions
Does the presence of salmon carcasses affect the abundance of biofilm or macroinvertebrates? Support your answer by referring to specific data in the figures.
Based on these results, what effect do you predict salmon carcasses would have further up the stream food chain (e.g., on predators of macroinvertebrates such as fish)?
Originally published at http://www.sciencecases.org/salmon_forest/case2a.asp
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