Grazers as Sources and Sinks for Nutrients:Conclusions, Limitations, and Speculations
209
input from primary to secondary producers. In an eutrophication gradient
of increasing nutrient loadings, this should lead to equilibrium zooplankton biomass increasing in proportion to primary production, without any
corresponding increase in phytoplankton biomass. Since algal P content is
intimately related to equilibrium growth rate, we would expect the nutritional value of algae as zooplankton food to increase in a gradient of nutrient loading rates. Zooplankton should therefore recycle an increasing
fraction of p contained in ingested food, and thus, function more as a
source than as a sink to the phytoplankton with increasing eutrophication.
Stoichiometric considerations of the phosphorus economy of grazers suggest that mineral nutrient limitation of secondary production is possible. The
model analysis in Section 5.3 nevertheless shows that an equilibrium situation
with grazer growth limited by food quality, but not food quantity, will be
dynamically unstable. The existence of such an unstable equilibrium point
would imply also the existence of a stable equilibrium point with zero grazer
biomass, and thus the possibility of deterministic extinction of the grazer
population. Since the likelihood of grazer extinction seems to increase with
nutrient loading, the consideration of predator stoichiometry leads to a
reinforcement of Rosenzweig's (1971) "paradox of enrichment".
Grazer extinction is possible when grazers are unable to attain positive
net growth, and thus unable to invade a phytoplankton community growing in equilibrium with dilution and sinking losses. This condition defines a
persistence boundary in terms of a critical water renewal rate, above which
grazer extinction becomes possible. With the set of parameters used in the
present models, the majority of proper lakes seem to have average dilution
rates well below this critical level. On the other hand, parameter sensitivity
analysis of the persistence boundary suggests that we should expect grazer
populations to be most likely to be driven to extinction by predatory
mortality in lakes with high dilution rates.
Since algal growth rates are constrained below an upper physiological
limit, there must also be an upper limit to the secondary production that
can be supported by a given algal biomass. Consequently, the equilibrium
level of zooplankton biomass in a simple two-level food chain cannot
increase indefinitely in proportion to nutrient loading, while maintaining
algal biomass compatible with food-limited grazer growth, but must
instead approach an asymptotic limit. This means that with increasing P
loading, an increasing fraction of total phosphorus will be available for
algal growth, and that algae will be able to depart further and further from
the equilibrium level if grazer biomass at any time is insufficient to control
their growth. As a result, there will be a critical loading level where persistent oscillations and alternating over- and undergrazing of algal biomass
becomes an alternative stationary state to the simple equilibrium point
with constant biomasses. In a sense, eutrophication can therefore be
regarded as a bifurcation parameter determining the qualitative dynamic
behavior of the system.
209
input from primary to secondary producers. In an eutrophication gradient
of increasing nutrient loadings, this should lead to equilibrium zooplankton biomass increasing in proportion to primary production, without any
corresponding increase in phytoplankton biomass. Since algal P content is
intimately related to equilibrium growth rate, we would expect the nutritional value of algae as zooplankton food to increase in a gradient of nutrient loading rates. Zooplankton should therefore recycle an increasing
fraction of p contained in ingested food, and thus, function more as a
source than as a sink to the phytoplankton with increasing eutrophication.
Stoichiometric considerations of the phosphorus economy of grazers suggest that mineral nutrient limitation of secondary production is possible. The
model analysis in Section 5.3 nevertheless shows that an equilibrium situation
with grazer growth limited by food quality, but not food quantity, will be
dynamically unstable. The existence of such an unstable equilibrium point
would imply also the existence of a stable equilibrium point with zero grazer
biomass, and thus the possibility of deterministic extinction of the grazer
population. Since the likelihood of grazer extinction seems to increase with
nutrient loading, the consideration of predator stoichiometry leads to a
reinforcement of Rosenzweig's (1971) "paradox of enrichment".
Grazer extinction is possible when grazers are unable to attain positive
net growth, and thus unable to invade a phytoplankton community growing in equilibrium with dilution and sinking losses. This condition defines a
persistence boundary in terms of a critical water renewal rate, above which
grazer extinction becomes possible. With the set of parameters used in the
present models, the majority of proper lakes seem to have average dilution
rates well below this critical level. On the other hand, parameter sensitivity
analysis of the persistence boundary suggests that we should expect grazer
populations to be most likely to be driven to extinction by predatory
mortality in lakes with high dilution rates.
Since algal growth rates are constrained below an upper physiological
limit, there must also be an upper limit to the secondary production that
can be supported by a given algal biomass. Consequently, the equilibrium
level of zooplankton biomass in a simple two-level food chain cannot
increase indefinitely in proportion to nutrient loading, while maintaining
algal biomass compatible with food-limited grazer growth, but must
instead approach an asymptotic limit. This means that with increasing P
loading, an increasing fraction of total phosphorus will be available for
algal growth, and that algae will be able to depart further and further from
the equilibrium level if grazer biomass at any time is insufficient to control
their growth. As a result, there will be a critical loading level where persistent oscillations and alternating over- and undergrazing of algal biomass
becomes an alternative stationary state to the simple equilibrium point
with constant biomasses. In a sense, eutrophication can therefore be
regarded as a bifurcation parameter determining the qualitative dynamic
behavior of the system.