Fish Stocking and Trophic Cascades:

Modeling Phosphorus Cycling in Lakes

Nathaniel Decker & Kate MerrickEnvironmental Systems Modeling

Final Presentation 5/13/04

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

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)

Plants

Grazers

Inorganic P

+

Fish

+

-

Detritus

+

-

+

+-

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

Stocks and Flows in P Cycle

Seasonal variability in lake sediment redox potential

Relevance

Aerial Fish Stocking in Sierra Nevada Mountains

Golden Trout Spawning

Methods

• Model design and formulations• 2nd order growth with Michaelis-Menton limitation• 1st order loss: living stocks to detritus,

mineralization of detritus

• Heuristic interface

2nd order growth with Michaelis-Menton limitation1st order loss: living stocks to detritus, mineralization of detritus

Results: Heuristic Scenarios1. Steady-state P cycling

2. Fish Introduction

3. Top-down control

4. Bottom-up control

5. Seasonal Variabilities

6. Fish Orgy!

Steady-state P cycle

Fish introduction

Inorganic P

Plant P

Detrital P

Grazer P

Fish P

Top-down Control

Initial Scenario

Increased top-down control: higher grazing rate

Inorganic P

Plant P

Detrital P

Grazer P

Fish P

Bottom-Up ControlGrazer stocking switch limits effect of fish reduction of grazing .

Fish affect plant stocks by increasing the flow through detritus and inorganic P.

Seasonal VariabilityDissolved Inorganic P (PO4)Complexed Inorganic P(FePO4)

Plant PPAR

Seasonal variability affects a lake with introduced fish

Fish Orgy?!

Plant P with Initial Fish Stock at:

10

5

0.05

No Fish

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

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.

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

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 [O2]

• 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