Fish as Fertilizer by Mark L. Kuhlmann
Part 2B—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 4—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.
The researchers conducted an experiment in 36 artificial flow-through stream channels with natural substrate. Each artificial stream was set up to include a deeper pool habitat and a shallow “riffle” habitat. Macroinvertebrates colonized by drifting with the inflowing water. (Macroinvertebrates are animals without backbones that are large enough to see without magnification; juvenile insects are the most common in fresh water.) In half of the channels, one salmon carcass was placed in the upstream end. To sample macroinvertebrates, small-mesh nets were placed over the outflow and the substrate (rocks and gravel on the bottom) in one section of the artificial stream was agitated.
Biofilm was sampled from an unglazed clay tile at the downstream end of the artificial stream. 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 4—Mean (± 1 standard error) biofilm AFDM (ash-free dry mass) (A) and macroinvertebrate densities in pool (B) and riffle (C) habitats in artificial stream channels, comparing carcass-enriched (solid circles) with control (open circle) treatments over the course of the 87-day experiment.
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/case2b.asp
Copyright © 1999–2010 by the National Center for Case Study Teaching in Science. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.
