Download Food Web Control of Primary Production in Lakes

Environmental Case Study
Upper Peninsula of Michigan
Food Web Control of Primary Production in Lakes
Introduction
Too often, aquatic ecosystems become green in late summer as the result of eutrophication. This
excessive primary production is the opportunistic response of nuisance planktonic algae to increased
nutrient inputs. Eutrophication of lakes, expressed as excessive algal production, is generally driven by
excess loading of phosphorus from nonpoint terrestrial sources.
Ecologists recently determined that primary production in both terrestrial and aquatic ecosystems
can be influenced by food web structure. This trophic impact of top-down control (trophic cascade
hypothesis) of primary production was proposed by Stephen Carpenter, James Kitchell, and James
Hodgson (1985) for aquatic ecosystems. They studied a suite of four lakes in Michigan's Upper Peninsula
for the last seventeen years in the formulation and support of their hypothesis.
Biomanipulation
Biomanipulation of the food web can increase the abundance of large zooplanktonic herbivores,
which in turn controls algal biomass. The researchers proposed that while nutrient input (bottom-up
controls) may determine potential primary production in a lake, it is the relative predominance of fisheating fish (piscivores) and plankton-eating fish (zooplanktivores) that significantly alters the rate of
primary production by ultimately increasing the rate of herbivory (grazing) on the algal production.
Figure 1. Predicted effects of piscivores on planktivore, herbivore, and phytoplankton
biomass and production (data from Carpenter, Kitchell, and Hodgson 1985).
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Environmental Case Study
Upper Peninsula of Michigan
Food Web Control of Primary Production in Lakes
In lakes, the trophic cascade hypothesis predicts that the manipulation of top predators (e.g.,
largemouth bass) propagates changes in primary production. Specifically, an increase in piscivore
biomass causes a concomitant decrease in zooplanktivore biomass, resulting in increased biomass and
herbivory by zooplankton and an ultimate reduction in primary production (Figure 1). Because bass
predation can virtually eliminate minnows from the food chain in small lakes, effective zooplanktivory is
greatly reduced. Large zooplankton, the preferred prey of size-selective minnows, can now dominate the
zooplankton assemblage in the absence of zooplanktivory. Since large zooplankton (e.g., Daphnia spp.)
are much more efficient herbivores than smaller species (e.g., Bosmina spp.), planktonic algae are more
effectively grazed out of the water column, resulting in a reduction in algal biomass and primary
production, and improved water quality.
Carpenter and his research associates (1995) tested their ideas and demonstrated that the
results in their study lakes supported the trophic cascade hypothesis. The responses were due clearly to
the food web manipulations.
Carpenter and associates further tested the hypothesis by directly fertilizing lakes (bottom-up
controls) that had contrasting food webs. Algal biomass accumulated in one of their study lakes with an
abundance of zooplantivorous fishes and small species of zooplankton grazers. In another study lake
dominated by piscivorous fish and large-bodied grazers, primary production was contained below
predicted models of nutrient loading, with the exception of nuisance (inedible) bluegreen algae. It was
clearly demonstrated that plankton grazers are able to maintain primary production at levels significantly
below what is predicted of a phosphorus load that could produce eutrophication in these lakes. Thus,
water quality may be improved over a wide range of nutrient loading by the biomanipulation of the food
web.
References
Literature
1. Carpenter, S.R., D.L. Christensen, J.J. Cole, K.L. Cottingham, X.He, J.R. Hodgson, J.F. Kitchell,
S.E. Knight, M.L. Pace, D.M. Post, D.E. Schindler, and N. Voichick. 1995. Biological control of
eutrophication in lakes. Environmental Science and Technology 29:784-86.
2. Carpenter, S.R., J.F. Kitchell, and J.R. Hodgson. 1985. Cascading trophic interactions and lake
productivity. BioScience 35:643-39
Websites
1. Broads Authority – Barton Broad Project at http://www.broadsauthority.gov.uk/broads/pages/barton6.html
Key Principles
1. Top-down Control of Eutrophication
2. Biomanipulation of a Food Web
3. Improving Water Quality
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Environmental Case Study
Upper Peninsula of Michigan
Food Web Control of Primary Production in Lakes
Ethical Considerations
1. What are the benefits of artificially manipulating an aquatic environment?
2. What are the environmental consequences artificially manipulating an aquatic environment?
3. What is the value of biomanipulation in managing other types of environments?
Civic Engagement & Service Opportunities
1.
2.
3.
4.
Volunteer for a local community group involved in habitat restoration using biomanipulation.
Write or e-mail your local politicians about water pollution cleanup in your region.
Form a student group having an environmental protection mission.
Set up a public forum at your school discussing possible biomanipulation applications for your
area.
Learn more about community service as part of your educational enrichment by visiting the following
websites: http://www.learnandserve.org/, http://www.servicelearning.org/.
Author
James R. Hodgson
Departments of Biology and Environmental Science
St. Norbert College
De Pere, WI, 54115
[email protected]
Edited & Revised in 2005 by
Dr. Brian Shmaefsky
Professor of Biology & Service Learning Coordinator
Kingwood College
20,000 Kingwood Drive, HSB 202V
Kingwood, TX 77339
[email protected]
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