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Nutrients. Algae and Herbivores - the Paradox of Enrichment Revisited
Whether a two-dimensional Lotka-Volterra system is attracted to a
stable equilibrium or to a limit cycle depends on the details of the predatorprey interaction. Stability studies have shown that the presence of a carrying capacity for the prey population tends to dampen the oscillations, while
the presence of a saturating predator response tends to destabilize the
system. Rosenzweig (1971) has shown by graphical arguments that when
the prey carrying capacity is increased, the stabilizing effect of prey density
dependence will weaken. The result is the famous paradox of enrichment
which states that a prey-predator system can be driven from a stable equilibrium to a qualitatively different limit cycling behavior by an enrichment
affecting the potential yield of prey in the system.
Rosenzweig (1971) interpreted this result as a warning that "man must
be careful in attempting to enrich ecosystems in order to increase its food
yield. There is a real chance that such activity may result in a decimation of
the food species that are wanted in greater abundance." From their experiences with whole-lake fertilization experiments made in order to increase
fish yield, McAllister et al. (1972) found Rosenzweig's interpretation of the
paradox of enrichment to be overly pessimistic, and suggested that future
research should be directed toward how the possible existence of critical
levels of enrichment could be utilized in practical ecosystem management.
Apparently, little progress has been made in this direction since then.
After the introduction of the concept of lake restoration by biomanipulation (Shapiro et al. 1975), a variant of the paradox of enrichment turns up
in a new context. The purpose of a biomanipulation is to improve water
quality by reducing the predation pressure on the zooplankton so that a
stable equilibrium with grazer-controlled algal biomass is established.
Benndorf (1987) observed that in the lakes where successful biomanipulation had been reported, the nutrient loading was either moderate or
unknown, and that in the few highly eutrophic lakes where biomanipulation had been attempted, it apparently had failed to improve water quality.
This observation has a clear resemblance to the paradox of enrichment: it
seems that when the carrying capacity of the algae is above some critical
level, in terms of nutrient loading, the establishment of a stable steady state
with grazer-controlled algal biomass becomes unlikely.
The total amount of nutrients available for plankton growth is determined by the balance between nutrient inputs to and nutrient losses from
the pelagic zone. The nutrient loss processes are to a large extent mediated
by the plankton organisms themselves through sinking and mortality, as
discussed in Chapter 2. As all members of the plankton need essential
nutrients to grow, the net amount of nutrients available for algal growth is
determined by the partitioning of nutrients among the community members. The actual yield of algal biomass at a given level of algal-bound nutrient depends on the growth rate of the algae, as discussed in Chapter 3. This
means that the algal carrying capacity in terms of the potential yield of
algal biomass becomes a dynamic entity related to both the nutrient supply
Nutrients. Algae and Herbivores - the Paradox of Enrichment Revisited
Whether a two-dimensional Lotka-Volterra system is attracted to a
stable equilibrium or to a limit cycle depends on the details of the predatorprey interaction. Stability studies have shown that the presence of a carrying capacity for the prey population tends to dampen the oscillations, while
the presence of a saturating predator response tends to destabilize the
system. Rosenzweig (1971) has shown by graphical arguments that when
the prey carrying capacity is increased, the stabilizing effect of prey density
dependence will weaken. The result is the famous paradox of enrichment
which states that a prey-predator system can be driven from a stable equilibrium to a qualitatively different limit cycling behavior by an enrichment
affecting the potential yield of prey in the system.
Rosenzweig (1971) interpreted this result as a warning that "man must
be careful in attempting to enrich ecosystems in order to increase its food
yield. There is a real chance that such activity may result in a decimation of
the food species that are wanted in greater abundance." From their experiences with whole-lake fertilization experiments made in order to increase
fish yield, McAllister et al. (1972) found Rosenzweig's interpretation of the
paradox of enrichment to be overly pessimistic, and suggested that future
research should be directed toward how the possible existence of critical
levels of enrichment could be utilized in practical ecosystem management.
Apparently, little progress has been made in this direction since then.
After the introduction of the concept of lake restoration by biomanipulation (Shapiro et al. 1975), a variant of the paradox of enrichment turns up
in a new context. The purpose of a biomanipulation is to improve water
quality by reducing the predation pressure on the zooplankton so that a
stable equilibrium with grazer-controlled algal biomass is established.
Benndorf (1987) observed that in the lakes where successful biomanipulation had been reported, the nutrient loading was either moderate or
unknown, and that in the few highly eutrophic lakes where biomanipulation had been attempted, it apparently had failed to improve water quality.
This observation has a clear resemblance to the paradox of enrichment: it
seems that when the carrying capacity of the algae is above some critical
level, in terms of nutrient loading, the establishment of a stable steady state
with grazer-controlled algal biomass becomes unlikely.
The total amount of nutrients available for plankton growth is determined by the balance between nutrient inputs to and nutrient losses from
the pelagic zone. The nutrient loss processes are to a large extent mediated
by the plankton organisms themselves through sinking and mortality, as
discussed in Chapter 2. As all members of the plankton need essential
nutrients to grow, the net amount of nutrients available for algal growth is
determined by the partitioning of nutrients among the community members. The actual yield of algal biomass at a given level of algal-bound nutrient depends on the growth rate of the algae, as discussed in Chapter 3. This
means that the algal carrying capacity in terms of the potential yield of
algal biomass becomes a dynamic entity related to both the nutrient supply