Fish Stocking and Trophic Cascades: Modeling Phosphorus Cycling in Lakes

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
Grazers

Detritus
-
Plants

-

Inorganic P
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
• 2nd order growth with Michaelis-Menton limitation
• 1st order loss living stocks to detritus,
  mineralization of detritus
• Heuristic interface

9
2nd order growth with Michaelis-Menton
limitation1st 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
Inorganic P Plant P Detrital P Grazer P Fish P
Fish introduction
12
Top-down Control
Initial Scenario
Inorganic P Plant P Detrital P Grazer P Fish P
Increased top-down control higher grazing rate
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
Plant P PAR
Dissolved Inorganic P (PO4) Complexed Inorganic
P(FePO4)
Seasonal variability affects a lake with
introduced fish
15
Fish Orgy?!
16
Conclusions and Implications

• Trophic interactions link components of food webs
  via multiple mechanisms
• No stock is an island, every units 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 units 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 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 Nevadas
  naturally-fishless lakes now contain non-native
  trout