Fish as Fertilizer by Mark L. Kuhlmann

Part 1—Do Salmon Add Marine-Derived Nitrogen to the Stream Ecosystem?

Instructions

Turn to your neighbor (or with a small group) and work together to first describe and then interpret the data in the two figures. Use the Step 1–Step 2 approach described below to interpret the two figures.

• 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. What do they tell you about the effect of salmon on marine-derived nutrient levels in the stream ecosystem?

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

Figure 1—Background

From: Bilby, R.E., B.R. Fransen, and P.A. Bisson. 1996. Incorporation of nitrogen and carbon from spawning coho salmon into the trophic system of small streams: Evidence from stable isotopes. Canadian Journal of Fisheries and Aquatic Science 53:164–173. Used with permission.

Nitrogen and carbon stable isotope ratios were measured in aquatic organisms collected from a stream with spawning coho salmon (Grizzly Creek) and one lacking salmon (Stream 0372) because of an impassable waterfall. Apart from the presence or absence of salmon, the two streams are physically and ecologically similar. Both are small (4–6 m wide) with a pool-riffle morphology and similar riparian vegetation.

Samples at each site included epilithic organic matter (the film encrusting rocks in the stream composed of microbes such as bacteria, algae, and fungi), aquatic invertebrates, and cutthroat trout. Invertebrates were classified into 4 trophic (feeding) categories (shredders, grazers, collector-gatherers, and predators) but, because of seasonal scarcity of some types, only grazers (animals that scrape organic material from rocks) and predators are shown here. Samples were collected in the winter (just post-spawning) and early autumn (just before spawning) to examine seasonal variation in isotope ratios. Remember that, for both N and C, a higher δ value (less negative in the case of C) is indicative of marine-derived nutrients (MDN).

Figure 1—Difference in δ¹⁵N (A) and δ¹³C (B) values between Grizzly Creek (open bars—with salmon) and Stream 0372 (shaded bars—no salmon) for four trophic levels (epilithic organic matter (EOM), grazers, invertebrate predators, and cutthroat trout) during the winter (just post-spawning, carcasses present in Grizzly Creek) and early autumn (just pre-spawning, carcasses absent from all streams).

Figure 2—Background

From: Chaloner, D.T., K.M. Martin, M.S. Wipfli, P.H. Ostrom, and G.A. Lamberti. 2002. Marine carbon and nitrogen in southeastern Alaska stream food webs: Evidence from artificial and natural streams. Canadian Journal of Fisheries and Aquatic Sciences 59:1257–1265. Used with permission.

This study was designed to determine if marine-derived nutrients (nitrogen and carbon) from dying adult salmon ended up in stream fish, in this case juvenile coho salmon. The experiment was conducted in 36 artificial stream channels (250 cm long, 18 cm wide) gravity-fed with water from a nearby stream. The artificial streams were open to colonization by microbes and small animals (e.g., aquatic insect larvae) brought in by the water. Each artificial stream contained 3 juvenile coho salmon and 0–4 adult salmon carcasses, depending on the randomly-assigned treatment. After about 10 weeks in the artificial stream channels, a tissue sample was taken from each juvenile salmon for stable isotope analysis.

Figure 2—Mean δ¹⁵N (black square values on L vertical axis) and δ¹³C (black triangle values on R vertical axis) (± 1 standard error; n = 3) values for juvenile coho salmon after 10 weeks in artificial stream channels containing different numbers of salmon carcasses (horizontal axis). Different letters above the symbols indicate statistically significant differences between treatments (number of carcasses).

Questions

1. Do marine-derived nutrients (MDNs) from salmon get incorporated into the stream food web? If so, what parts (trophic levels or feeding categories)? Refer specifically to the data in the figures to support your answer.

2. (a) What is the significance of looking at both N and C stable isotope ratios? (Think about—or review, if you can’t remember them—how the biogeochemical cycles of these two elements differ, particularly in how they move within a food web.)

   (b) Compare the patterns of N and C in Figure 1. What does this tell us about how the MDN are entering the food web?

3. What do the seasonal changes in enrichment (Figure 1) tell us about the source of the MDN?

4. Both studies measured MDNs in fish. Aside from examining different species, does the experiment (Chaloner et al. 2002; Figure 2) tell you anything different than the observational study (Bilby et al. 1996; Figure 1)? If so, what?

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