Food Webs of the Great Rivers of the Central Basin: Application of Stable Isotopes in Bioassessment

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1 Food Webs of the Great Rivers of the Central Basin: Application of Stable Isotopes in Bioassessment Michael D. Delong Large River Studies Center Winona State University

2 What Do Food Webs Tell Us? Organic Matter Processing Primary Production Detritus Energy Flow Community Interactions

3 Food webs, therefore, function as integrators by linking a wide range of measures of ecosystem structure and function under a single descriptor The integrative nature of food webs is what makes them good candidates for assessment of ecosystem function

4 Wait, Why Use Ecosystem Function in Bioassessment? Structural measures (density/abundance) look at what is there Functional measures (productivity) describe how an ecosystem is working Functional measures, therefore, capture disruptions or deviations in the dynamics of an ecosystem Have actually been in use for some time

5 How Are Food Webs Constructed? Gut content analysis Detailed quantification of the stomach content of consumers Very labor intensive in the laboratory Measures what was eaten, not what is actually converted to biomass Can create a complex picture of interactions

6 Food Web Produced Using Gut Content Data (Caddisflies Only) Benke and Wallace (1997)

7 How Are Food Webs Constructed? Stable Isotopes Based on what is actually assimilated Field collection straightforward Laboratory procedures < gut contents Simplifies web by eliminating nonassimilated food

8 Isotopic Associations in Food Webs Carbon Isotopes (δ 13 C) Consumers within o / oo of food source (Keough et al. 1996) Nitrogen Isotopes (δ 15 N) Increases 2-3 o / oo between trophic levels in aquatic ecosystems (e.g., Hannson et al. 1997)

9 Initial Objective Develop food web models using stable isotopes to determine effects of 1993 flood on ecosystem function

10 Iowa Missouri Illinois Ohio River Mississippi River Missouri River 100 km

11 Initial Hypotheses Base of food web will change, causing change in overall trophic relationships Ratios of carbon and nitrogen isotopes in tissues of consumers would more closely resemble ratio of C 4 terrestrial plants (corn).

12 Organic Matter Sources Transported Organic Matter Coarse TOM (> 1 mm) Fine TOM ( µm) Ultrafine TOM (100 1 µm) Dissolved Nutrients* (< 1 µm) *includes DOC, DIN, DON Benthic Organic Matter Coarse BOM (>1 mm) Fine BOM ( µm) Benthic Algae Aquatic Macrophytes Terrestrial Leaf Litter (C 3 and C 4 vegetation)

13 Consumers Invertebrates Filterers Detritivores Grazers Predators Fish Invertivores Detritivores Zooplanktivores Piscivores

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17 Cut-off for C 3 plants

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19 Conclusions from Initial Study Trophic linkages did not shift in response to 1993 flood (Delong et al. 2001) Trophic dynamics of Upper Miss and lower Ohio very similar BUT lower Missouri trophic dynamics differ, as do isotopic ratios

20 Upper Mississippi River FTOM DOM Benthic Algae Invertebrate Primary Consumers Invertebrate Secondary Consumers Fish Invertivores Fish Piscivores Ohio River FTOM UTOM/DOM Benthic Algae Invertebrate Primary Consumers Invertebrate Secondary Consumers Fish Invertivores Fish Piscivores Missouri River C 3 Terrestrial Invertebrate Primary Consumers Invertebrate Secondary Consumers Fish Invertivores Fish Piscivores DOM

21 Mississippi Missouri Piscivores Insectivorous Fish Predaceous Inverts Detritivores Grazers Filterers

22 Quantitative Approaches What if you can only confidently narrow it down to two potential food sources? Mixing models % sourcea = [(δ 13 C consumer - δ 13 C sourceb )/(δ 13 C sourcea δ 13 C sourceb )] x 100 % sourceb = % sourcea

23 Quantitative Approaches Mixing models using both δ 13 C and δ 15 N Post (2002) dual isotope mixing models to determine trophic positions and maximum trophic position in food chains

24 Trophic Position = λ + (δ 15 N sc [(δ 15 N base1 )(α + δ 15 N base2 )(1 - α)]/ n ) where, α = (δ 13 C sc - δ 13 C base2 )/ (δ 13 C base1 - δ 13 C base2 )

25 Conventional Food Chain With Trophic Levels Level 5 Level 4 Level 3 Level 2 Level 1

26 Food Chain Using Mixing Model and Isotopic Ratios Position 5.8 Position 4.8 Position 4.1 Position 3.9 Position 3.1 Position 2 Position 1

27 Food Chain Using Mixing Model and Isotopic Ratios Position 5.8 Position 4.8 Position 4.1 Position 3.9 Position 3.1 Position 2 Position 1

28 Effect of Perturbation on Trophic Position Alter type or quantity of resources at base of chain, reducing average trophic position Reduced ecosystem health would lower maximum trophic position through Change in resources at base Loss of lower consumers

29 Summary Food webs represent an integration of critical biotic and abiotic factors in ecosystems Stable isotopes are an easy and cost-effective method for defining food webs Quantitative approaches can be used to create scaleable measures for ascertaining ecosystem health Not discussed techniques refined to better target basal food sources

30 Acknowledgments Thanks to all of the undergraduate students that have worked on these projects Research supported by National Science Foundation and Winona State University Jim Thorp, partner in crime, concepts and applications

31 Lazy Day on the Upper Mississippi

32 Isotope Fine ratios Transported of FTOM when Organic separated Matter using (100 Colloidal µm - 1 mm) Silica δ 15 N FTOM Living FTOM Composite FTOM Detritus δ 13 C