Download Food Web Control of Primary Production in Lakes

Location: Upper Peninsula of Michigan
Title: 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 fish-eating
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).
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
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. Volunteer for a local community group involved in habitat restoration
using biomanipulation.
2. Write or e-mail your local politicians about water pollution cleanup in
your region.
3. Form a student group having an environmental protection mission.
4. 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/, http://www.aahe.org/service/srv-links.htm.
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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