Fish as Fertilizer by Mark L. Kuhlmann

Part 3—How Does Marine-Derived Nitrogen Get to Stream-Side Terrestrial Ecosystems?

Instructions

After reading the background information for the figure, work with a small group to first describe and then interpret the data in the figures below. 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.

Figures 6, 7, and 8—Background

From: Mathewson, D., M. Hocking, and T. Reimchen. 2003. Nitrogen uptake in riparian plant communities across a sharp ecological boundary of salmon density. BMC Ecology 3:4 (Figure 6) and Hocking, M., and T. Reimchen. 2002. Salmon-derived nitrogen in terrestrial invertebrates from coniferous forests of the Pacific Northwest. BMC Ecology 2:4 (Figures 7 and 8). Figures used in accordance with BioMed Central Open Access license agreement (http://www.biomedcentral.com/info/about/license).

Samples of vegetation and terrestrial macroinvertebrates were collected from two salmon-bearing watersheds, the Claste and Neekas Rivers, in coastal British Columbia. A waterfall, 1 km upstream on the Claste River and 2.1 km upstream on the Neekas River, blocks further upstream salmon migration in both rivers.

Foliar (leaf) samples were collected within 15 m of the river, from 50 m below to 50 m above the waterfall. Species collected were deerfern (Blechnum spicant), false azalea (Menziesii ferruginea), devil's club (Oplopanax horridus), salmonberry (Rubus spectabilis), Alaskan blueberry (Vaccinium alaskaense), red huckleberry (V. parvifolium) and western hemlock (Tsuga heterophylla). At least 9 individuals of each species were sampled at each site. Terrestrial macroinvertebrates were collected using pitfall trap arrays and hand collecting along the rivers above and below the falls, up to 100 m from the stream.

Both plant and invertebrate samples were analyzed for stable nitrogen isotopes (δ¹⁵N). Invertebrate samples were also analyzed for stable carbon isotopes (δ¹³C). For both elements, an elevated heavy isotope ratio is indicative of marine-derived nutrients (MDN). The researchers looked at both N and C in the invertebrates to try to figure out whether the animals were getting MDN directly from salmon carcasses or by consuming MDN-enriched plants.

Figure 6—δ¹⁵N values in riparian vegetation collected immediately below and above waterfall barriers to salmon at Claste and Neekas Rivers, British Columbia, Canada. t-test results: *P < 0.05; **P < 0.01.

Figure 7—δ¹⁵N values in four trophic groupings of litter-based invertebrates collected immediately below and above waterfall barriers to salmon at Claste and Neekas Rivers, British Columbia, Canada. Invertebrates are ranked (left to right) based on increasing consumption of animal protein. t-test results: **P < 0.01; ***P < 0.001.

Figure 8—δ¹³C values in four trophic groupings of litter-based invertebrates collected immediately below and above waterfall barriers to salmon at Claste and Neekas Rivers, British Columbia, Canada. Invertebrates are ranked (left to right) based on increasing consumption of animal protein. t-test results: *P < 0.05.

Questions for Figures 6, 7, and 8

1. Do salmon-delivered (marine-derived) nutrients get into the food web of the riparian community along salmon spawning streams? If so, what nutrients (N or C) and what part or parts of the community (e.g., plants, root feeders, etc.)? Support your answer by referring to specific data in the figures.

2. In the invertebrates, is there a difference between the effects of salmon on δ¹⁵N and δ¹³C? What does this tell us about how MDN are getting to the invertebrates?

Figure 9—Background

From: Hilderbrand, G.V., T.A. Hanley, C.G. Robbins, and C. Schwartz. 1999. Role of brown bears (Ursus arctos) in the flow of marine nitrogen into a terrestrial ecosystem. Oecologia 121: 546–550. Panel A originally appeared as Figure 1a, page 548; panel B originally appeared as Figure 2, page 548. Used with permission of the author and Springer Science and Business Media.

Bears consume large quantities of salmon during spawning runs. Some of the nutrients from the consumed salmon, especially nitrogen, will be excreted a short time later in wastes (urine and feces). This study examined the effects of bears on the redistribution of salmon-derived nitrogen into the riparian forest.

Study sites were located on the Kenai Peninsula, Alaska. Mystery Creek had runs of several salmon species and abundant brown bears. Russian River had abundant salmon but few bears (because of the presence of human anglers). Cooper Creek had few salmon or bears.

Spruce needles were collected along 2 transects/site running perpendicular to the stream and analyzed for δ¹⁵N signature. The spatial distribution of bears was measured using position data from radio- or satellite-collar tracking of the movement patterns of 59 female bears. Bear spatial distributions was assumed to be directly proportional to N deposition in wastes (because who is going to measure it directly?).

Figure 9—Spatial patterns of adult female brown bear locations (mean ± 1 standard error) (A) and δ¹⁵N signatures of spruce needles (mean ± 1 standard error) (A and B) in relation to distance from a stream. Note the difference in the scale of the horizontal axis (Distance) in the two graphs. Bears were only present at Mystery Creek; salmon were not present at Cooper Creek. No spruce were encountered beyond 400 m from Cooper Creek and 500 m at Russian River.

Questions for Figure 9

1. What effect does salmon spawning have on the level of MDN in riparian plants? Support your answer by referring to specific data in the figures.

2. What effect do bears have on the delivery of MDN to riparian plants—i.e., are bears a significant vector for the movement of MDN into the terrestrial ecosystem? Support your answer by referring to specific data in the figures above.

3. Do the effects of salmon and bears (on MDNs) appear to be independent of one another? Explain.

4. In addition to waste products, what are some other ways that bears might deliver MDN to riparian ecosystems?

5. Other than by bears, make one or two hypotheses about how the MDNs are being transported from the stream to the terrestrial ecosystem. How could you test these hypotheses?

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