146
Nutrients, Algae and Herbivores - the Paradox of Enrichment Revisited
Species Substitutions, Tradeoffs, and Robustness. The parameter values in
Table 5.1 are deliberately chosen to resemble specific taxonomic groups of
phyto- and zooplankton (small, edible, fast-growing algae, like cryptomonads, and large cladoceran filter-feeders with high P demands, like
Daphnia). The sensitivity analysis in Fig. 5.15 indicates that the bifurcation
level P'L is quite sensitive to some of the model parameters, so that substitution of parameter values characteristic of other taxonomic or functional groups could potentially cause large changes in pOL'
In the discussion of interspecific differences among herbivorous
zooplankton in Section 4.7, it was argued that there appears to be a tradeoff
between high growth rate and low threshold food level for positive population growth, so that fast-growing species have high threshold levels, and
vice versa. If such a tradeoff exists, it should also imply a correlation
between the incipient limiting food level (C1 and the maximum ingestion
rate (/1, which again would imply that the decrease in pOL resulting from
substituting a grazer with lower C' (like a calanoid copepod instead of a
daphnid) would to some extent be counteracted by the corresponding
decrease in 1'.
The literature survey of interspecific differences among phytoplankton
in Section 3.4 indicated a tradeoff between maximum growth capacity and
the development of predator defence; species with large colony size, elongated cellular form, or other morphological structures that make them
more resistant to zooplankton grazing, have significantly lower maximum
growth rates than species without such adaptations. If predator defence is
limited to morphological adaptations that reduce grazer feeding efficiency,
without having any other negative effect on the grazer (excluding possible
biochemical defence mechanisms like endotoxins), the main effect on the
grazer should be to increase the incipient limiting food level (C1 such that
a higher algal biomass is needed to saturate the feeding response. This
should imply that the decrease in pOL resulting from substituting a grazingresistant alga with lower t1 would to some extent be counteracted by the
corresponding increase in C.
It thus appears that the presence of tradeoffs among competitive traits in
candidate phyto- and zooplankton species, would tend to make a loading
criterion based on the bifurcation input concentration more robust than if
the model parameters were completely independent. This gives hope of
developing a loading criterion with some generality beyond the simple twospecies target community of the model, consisting of a short, efficient food
chain from cryptomonads to daphnids.
The Loading Diagram. Thus far, the model response to changes in the
phosphorus loading has been studied by varying the input P concentration
PL' while keeping the dilution rate D at a fixed level. If we define the volumetric P loading rate as Lp = D PL [(~g P) r
l day"I], we see that the plankton
community can experience the same P supply rate for infinitely
Nutrients, Algae and Herbivores - the Paradox of Enrichment Revisited
Species Substitutions, Tradeoffs, and Robustness. The parameter values in
Table 5.1 are deliberately chosen to resemble specific taxonomic groups of
phyto- and zooplankton (small, edible, fast-growing algae, like cryptomonads, and large cladoceran filter-feeders with high P demands, like
Daphnia). The sensitivity analysis in Fig. 5.15 indicates that the bifurcation
level P'L is quite sensitive to some of the model parameters, so that substitution of parameter values characteristic of other taxonomic or functional groups could potentially cause large changes in pOL'
In the discussion of interspecific differences among herbivorous
zooplankton in Section 4.7, it was argued that there appears to be a tradeoff
between high growth rate and low threshold food level for positive population growth, so that fast-growing species have high threshold levels, and
vice versa. If such a tradeoff exists, it should also imply a correlation
between the incipient limiting food level (C1 and the maximum ingestion
rate (/1, which again would imply that the decrease in pOL resulting from
substituting a grazer with lower C' (like a calanoid copepod instead of a
daphnid) would to some extent be counteracted by the corresponding
decrease in 1'.
The literature survey of interspecific differences among phytoplankton
in Section 3.4 indicated a tradeoff between maximum growth capacity and
the development of predator defence; species with large colony size, elongated cellular form, or other morphological structures that make them
more resistant to zooplankton grazing, have significantly lower maximum
growth rates than species without such adaptations. If predator defence is
limited to morphological adaptations that reduce grazer feeding efficiency,
without having any other negative effect on the grazer (excluding possible
biochemical defence mechanisms like endotoxins), the main effect on the
grazer should be to increase the incipient limiting food level (C1 such that
a higher algal biomass is needed to saturate the feeding response. This
should imply that the decrease in pOL resulting from substituting a grazingresistant alga with lower t1 would to some extent be counteracted by the
corresponding increase in C.
It thus appears that the presence of tradeoffs among competitive traits in
candidate phyto- and zooplankton species, would tend to make a loading
criterion based on the bifurcation input concentration more robust than if
the model parameters were completely independent. This gives hope of
developing a loading criterion with some generality beyond the simple twospecies target community of the model, consisting of a short, efficient food
chain from cryptomonads to daphnids.
The Loading Diagram. Thus far, the model response to changes in the
phosphorus loading has been studied by varying the input P concentration
PL' while keeping the dilution rate D at a fixed level. If we define the volumetric P loading rate as Lp = D PL [(~g P) r
l day"I], we see that the plankton
community can experience the same P supply rate for infinitely