Food webs and trophic cascades in lakes How

Food webs and trophic cascades in lakes

How to represent trophic relationships? (Paine 1980) 1. Connectedness Based on observations 2. Energy flow web Based on literature and measurements 3. Functional web based on controlled manipulations

1. Connectedness food web • Depicts observed feeding links between organisms. • Indicates ‘structure’ of system.

Food web of the North Atlantic • Simple food chain

Another food web of the North Atlantic • Complex food web

2. Energy flow food web • Trophic level – number of energy transfers between primary producers and consumer (trout). • Assumes discrete food chain, simple trophic structure

Energy flow • Trophic level concept useful for predicting contaminant levels • Bioaccumulation: the accumulation of substances that are not excreted and are subsequently passed from prey to predator 3 4 5 Estimated # of trophic levels

Energy flow: more complex pathways Use diet information to describe energy flow pathways Trophic position – trophic height of species. Considers diet and energy flow pathways. Species can fall between trophic levels. Omnivory – feeding on more than one trophic level

3. Functional food web • Depicts dynamically important interactions • Impact of predators on prey populations

Summary • Many different ways to view food webs. • Basis for food web ecology and ecosystem ecology. • Ecosystem management and many applied problems (contaminants, fisheries management) require understanding of complex species interrelationships. • Species do not exist in isolation – all linked through the food web and predator/prey relationships

Trophic Cascade Hypothesis 1. Nutrient supply limits algae biomass (chl-a) and production 2. Nutrients explain only some of the variability in algal biomass Does food web structure explain remaining variation in algae biomass and production? ? ? Concern with functional web: Impact of predators on prey

Trophic Cascade Hypothesis Piscivore biomass in relation to biomass (solid line) and production (dashed line) of vertebrate zooplanktivores, large herbivores, and phytoplankton.

(Top-down) trophic cascades • Nutrients and predation explain variation in primary production • Trophic levels are alternately limited by resources and predation

Conditions for Trophic Cascades 1. Consumers can control biomass of prey

Conditions for Trophic Cascades If Cpredator > Pprey Prey controlled by predator If Cpredator < Pprey Prey not controlled by predator P = production C = consumption Bioenergetic budget G=growth (~ production) C=consumption R=respiration F=feces E=excreta

Conditions for Trophic Cascades 2. Simple and wellknown food web

Conditions for Trophic Cascades 3. Sizeselective predation alewife From Brooks and Dodson 1965

Experimental tests of trophic cascades • Stream food webs • Small lake • Lake Mendota

Trophic cascades in California streams Number of trophic levels (3 or 4) determines abundance of predators, grazers, and algae! Algae Invertebrate grazers Invertebrate predators Fish

Trophic Cascade Hypothesis: Peter and Paul Lake, MI Piscivore lake Planktivore lake Peter and Paul Lake, MI

Food web biomanipulation of Lake Mendota (1976 -2001): Stocking piscivores to improve fishing and water clarity • Algal blooms a major problem • Piscivore populations were low • Biomanipulation had reduced algal levels in other lakes (Peter and Paul) • Lots of long-term data • Partnership between DNR and UW Madison

Lake Mendota pelagic food web • Food web of intermediate complexity • Simplification of reality • Captures major species and interactions • Lumps species into trophic groups

• Piscivore stocking 1987 -1999: – 2. 7 million walleye – 170, 000 northern pike From Lathrop et al. in press

Walleye fingerlings (1 + year old hatcheryraised)

Harvest regulations • Size limits • Daily bag limit changed 5 to 1 for pike 5 to 3 for walleye From Lathrop et al. in press

Total consumption estimated using bioenergetic model From Lathrop et al. in press

Impact of changing planktivore community on zooplankton?

Zooplankton response Small-bodied Daphnia galeata mendotae Large-bodied Daphnia pulicaria From Lathrop et al. in press

Impact on water clarity? ? Dotted = large-bodied zooplankton Solid = small-bodied zooplankton Impact on water clarity? ? From Lathrop et al. in press

Major cisco dieoff 1 year before stocking began From Lathrop et al. in press

From Lathrop et al. in press

From Lathrop et al. in press Must consider nutrient loading as well!

Biomanipulation of Lake Mendota: Summary • Success: Daphnia pulicaria have dominated for over a decade • Changes in fisheries management affected water quality. Links fields of limnology and fisheries. • Management considered the human dimension in an ecosystem context • Managers learned how food webs can be manipulated to improve water quality

Biomanipulation of Lake Mendota: Summary • Importance of reducing nutrient loading • Nutrients set the potential algal biomass, food web structure can affect realized biomass Planktivore systems Piscivore systems

Energy flow • Energy and nutrients are transferred up the food chain • Bioenergetic approach – calculate energetic budget for organism or population • I= ingestion • NU= lost through feces • A= assimilation • R= respiration • G= growth • B= biomass B

Energy flow • Link up bioenergetic budgets for multiple trophic levels Trophic level 1 Trophic level 2 Trophic level 3

Trophic cascade • Stream food webs in California Algae Invertebrate grazers Invertebrate predators Fish

Food web of Lake Mendota • Intermediate complexity • Simplification of reality • Capture the most species and interactions • Lump species into trophic groups

What about benthic-pelagic linkages? • Is the benthic food chain linked to the pelagic food chain? ? ? ?

Benthic-pelagic linkages • Fish get much of their energy from benthic food chains

Importance of benthic food webs • Reliance on benthic food chains varies among lakes • Decreases with lake area