Location: Upper Peninsula of Michigan
Title: Food Web Control of Primary Production in Lakes
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 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
Carpenter, S.R., J.F. Kitchell, and J.R. Hodgson. 1985. Cascading trophic interactions and lake productivity. BioScience 35:643-39.
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.
Key Principles