Fish as Fertilizer by Mark L. Kuhlmann

Part 4—Does Salmon-Derived Nitrogen Affect the Terrestrial Ecosystem?

Instructions

After reading the background information for the table and figures, work with your neighbor (or a small group) to first describe and then interpret the data in the figures below. Use the Step 1–Step 2 approach described below.

Step 1: Describe the table as well as 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 presented in both the table and the figure.
Step 2: Try to interpret the data. What do they tell you about the effect of salmon on marine-derived nutrient levels in the stream ecosystem?

When you understand the table and the figure, try to answer the questions about them below. Be prepared to volunteer or be called on during our class discussion to explain the data or share your answer to a question.

Table 1 and Figure 10—Background

From: Helfield, J. M., and R. J. Naiman. 2001. Effects of salmon-derived nitrogen on riparian forest growth and implications for stream productivity. Ecology 82(9):2403–2409. Copyright by the Ecological Society of America. Used with permission.

Study sites were in the Kadashan and Indian River watersheds on Chichagof Island in SE Alaska. Each watershed contained one spawning and one reference site. Spawning sites were adjacent to reaches of stream where salmon spawned. Reference sites were adjacent to reaches without spawning, either because of a waterfall barrier or being located above the upstream extent of spawning. At each site, samples were taken along four 100-m transects extending laterally from the stream. Vegetation (leaves) of Sitka spruce and understory plants were analyzed for δ¹⁵N and carbon: nitrogen ratio. Basal area growth (increase in the size of the trunk at ground level) of Sitka spruce was determined from tree cores.

Table 1—Mean carbon: nitrogen (C:N) ratios and δ¹⁵N values in the foliage of riparian vegetation at spawning and reference sites. Sample sizes are shown in parentheses.

Figure 10—(A) Mean (± 1 standard error) annual basal area growth (mm²/year) of riparian Sitka spruce at spawning and reference sites. Two-factor ANOVA indicates a significant salmon effect (i.e., spawning vs. reference sites, P = 0.01), no significant effect of distance from the stream (i.e., within 25 m vs. beyond 25 m, P = 0.38), and a significant interaction effect of salmon and distance (P = 0.01). (B) Mean (± 1 standard error) annual basal area growth per unit area of riparian Sitka spruce at spawning and reference sites.

Questions

What is the significance of the carbon: nitrogen ratio?
What effect does salmon spawning have on the nutrient status of riparian plants?
Does MDN affect the growth of Sitka spruce?
The legend of Figure 10 mentions an interaction between salmon and distance. What is an interaction in this sense, and can you point it out graphically in Figure 10?
What is the difference between what is shown in Figure 10-A and what is shown in Figure 10-B?
Based on these results, what differences would you expect to find between the riparian forests of areas with salmon spawning and forests without salmon spawning?

Originally published at http://www.sciencecases.org/salmon_forest/case4.asp