1. Fish Stocking and Trophic Cascades: Modeling Phosphorus Cycling in Lakes Nathaniel Decker & Kate Merrick Environmental Systems Modeling Final Presentation 5/13/04

2. Background Target Audience & Teaching Objectives Ecology classes (Biol 120 & 416) Food webs, phosphorus cycling… Conceptual Model of Trophic Cascade Feedback Loops Introduction to P Cycling in Lakes Relevance to Real-world Scenarios

3. Trophic Cascades Effects of consumers propagate through lake food webs to influence primary production (Carpenter and Kitchell 1988) Top-down and bottom-up controls Example: Introduction of trout into previously fishless lakes changes nutrient cycling via cascading trophic interactions (Simon and Townsend 2003)

4. Plants Grazers Inorganic P + Fish + - Detritus + - + + -

5. Phosphorus Cycling in Lakes Phosphorus availability limits primary production Impact of fish on inorganic P has crucial effect on production Stocks and Flows in P Cycle Seasonal variability in lake sediment redox potential

6. Stocks and Flows in P Cycle Seasonal variability in lake sediment redox potential

7. Relevance Aerial Fish Stocking in Sierra Nevada Mountains Golden Trout Spawning

8. Methods Model design and formulations 2 nd order growth with Michaelis-Menton limitation 1 st order loss: living stocks to detritus, mineralization of detritus Heuristic interface

9. 2 nd order growth with Michaelis-Menton limitation 1 st order loss: living stocks to detritus, mineralization of detritus

10. Results: Heuristic Scenarios 1. Steady-state P cycling 2. Fish Introduction 3. Top-down control 4. Bottom-up control 5. Seasonal Variabilities 6. Fish Orgy!

11. Steady-state P cycle Fish introduction Inorganic P Plant P Detrital P Grazer P Fish P

12. Top-down Control Initial Scenario Increased top-down control: higher grazing rate Inorganic P Plant P Detrital P Grazer P Fish P

13. Bottom-Up Control Grazer stocking switch limits effect of fish reduction of grazing. Fish affect plant stocks by increasing the flow through detritus and inorganic P.

14. Seasonal Variability Dissolved Inorganic P (PO4) Complexed Inorganic P(FePO4) Plant P PAR Seasonal variability affects a lake with introduced fish

15. Fish Orgy?! Plant P with Initial Fish Stock at: 10 5 0.05 No Fish

16. Conclusions and Implications Trophic interactions link components of food webs via multiple mechanisms No stock is an island, every unit’s death affects me (in memoriam: John Donne) Modeling can simplify these interactions to provide conceptual understanding of intensely complex processes

17. Hypothetical Food Webs: Fish added to previously fishless lakes (Neo Martinez) a) All direct feeding links from trout are highlighted in color, with all other links and nodes blackened. b) All nodes and links two feeding links away from trout are shown in color, which includes the entire web. Red circles indicate trophic species that are lost in lakes where fish are present.

18. Conclusions and Implications Trophic interactions link components of food webs via multiple mechanisms No stock is an island, every unit’s death affects me (in memoriam: John Donne) Modeling can simplify these interactions to provide conceptual understanding of intensely complex processes Real-world implications of fish stocking and food webs Future modeling possibilities

19. Implications and Future Directions Stocking previously fishless lakes may set off trophic cascades that lead to algal blooms Decline in water quality due to decreased [O 2 ] Decline of the mountain yellow-legged frog due to the introduction of non- native fishes into naturally fishless lakes. (Knapp 1996) more than 80% of the Sierra Nevada’s naturally- fishless lakes now contain non-native trout