Summary and Conclusions
203
"canopy" and "undergrowth" species exploiting spatially separated
resources. Such a scenario would be impossible to represent in spatially
homogeneous environment implicitly assumed in the present models.
In one of the most celebrated results of theoretical plankton ecology, it is
inferred that fluctuating resource ratios could support the coexistence of an
arbitrary number of species (Tilman 1982) on a small number of resources.
In Section 6.3 it is shown that for N and P as the two potentially limiting
nutrients, differential nutrient recycling by grazers will lead to diverging
resource ratios and thus to competitive exclusion, even when the species
are able to coexist in the absence of grazers.
The emergence of chaotic dynamics in the carbon cycling model of
Section 6.4 introduces a qualitatively new source of endogenous fluctuations in resource supply rates, which might conceivably give rise to a wider
set of temporal niches for different phytoplankton species, although the
question of whether prey-predator dynamics on aperiodic attractors really
can support more prey species than classical periodic orbits has not yet
been resolved.
Classical models of food-chain productivity (Hairston et al. 1960;
Oksanen et al. 1981) require that competition and predation can be considered as mutually exclusive; that is, a single trophic level is controlled either
from the trophic level below, by competition for limiting resources, or from
above, by predators on the next trophic level. Since there are no carnivore
levels in the models investigated in this Chapter, the zooplankton can obviously not be controlled by predation. Still, phytoplankton does not appear
to be strictly controlled by predation, since competitive exclusion leading
to species replacements in the algal community occurs as function of the
nutrient loading conditions. The model considered in Section 6.2 illustrates
that the ability of a selective grazer to tilt the balance between competing
phytoplankton species depends on both the exact nature of the competitive
tradeoffs between the prey species and the nutrient supply to the system.
A key feature of all the models presented in this work is that the availability of essential elements can potentially limit secondary production.
Both simulations and observations (Hessen et al. 1992) indicate that a
significant fraction of total nutrients can be contained in zooplankton biomass, and thus be unavailable to phytoplankton. The considerations of
stoichiometric constraints on grazer production and abundance suggest
that, in contrast to the view advocated by Harris (1986), heavy grazing and
high turnover rates of algal biomass do not necessarily preclude nutrient
limitation and, consequently, resource competition.
The bold assertion of Sommer (1989), cited in the introduction to this
Chapter, is only partly supported by the models presented here. Although
the interaction between predators and their prey can, under certain
assumptions, promote the coexistence of several algal species on a single
limiting resource, this conclusion does not appear to be structurally stable
under the influences of differential grazing or differential nutrient recy-
203
"canopy" and "undergrowth" species exploiting spatially separated
resources. Such a scenario would be impossible to represent in spatially
homogeneous environment implicitly assumed in the present models.
In one of the most celebrated results of theoretical plankton ecology, it is
inferred that fluctuating resource ratios could support the coexistence of an
arbitrary number of species (Tilman 1982) on a small number of resources.
In Section 6.3 it is shown that for N and P as the two potentially limiting
nutrients, differential nutrient recycling by grazers will lead to diverging
resource ratios and thus to competitive exclusion, even when the species
are able to coexist in the absence of grazers.
The emergence of chaotic dynamics in the carbon cycling model of
Section 6.4 introduces a qualitatively new source of endogenous fluctuations in resource supply rates, which might conceivably give rise to a wider
set of temporal niches for different phytoplankton species, although the
question of whether prey-predator dynamics on aperiodic attractors really
can support more prey species than classical periodic orbits has not yet
been resolved.
Classical models of food-chain productivity (Hairston et al. 1960;
Oksanen et al. 1981) require that competition and predation can be considered as mutually exclusive; that is, a single trophic level is controlled either
from the trophic level below, by competition for limiting resources, or from
above, by predators on the next trophic level. Since there are no carnivore
levels in the models investigated in this Chapter, the zooplankton can obviously not be controlled by predation. Still, phytoplankton does not appear
to be strictly controlled by predation, since competitive exclusion leading
to species replacements in the algal community occurs as function of the
nutrient loading conditions. The model considered in Section 6.2 illustrates
that the ability of a selective grazer to tilt the balance between competing
phytoplankton species depends on both the exact nature of the competitive
tradeoffs between the prey species and the nutrient supply to the system.
A key feature of all the models presented in this work is that the availability of essential elements can potentially limit secondary production.
Both simulations and observations (Hessen et al. 1992) indicate that a
significant fraction of total nutrients can be contained in zooplankton biomass, and thus be unavailable to phytoplankton. The considerations of
stoichiometric constraints on grazer production and abundance suggest
that, in contrast to the view advocated by Harris (1986), heavy grazing and
high turnover rates of algal biomass do not necessarily preclude nutrient
limitation and, consequently, resource competition.
The bold assertion of Sommer (1989), cited in the introduction to this
Chapter, is only partly supported by the models presented here. Although
the interaction between predators and their prey can, under certain
assumptions, promote the coexistence of several algal species on a single
limiting resource, this conclusion does not appear to be structurally stable
under the influences of differential grazing or differential nutrient recy-