178
Approaching Planktonic Food Webs: Competition, Coexistence, and Chaos
6.3 Differential Nutrient Recycling and Resource Supply
Ratios
Up to this point, we have only considered systems with one limiting
resource. Even if phosphorus is generally identified as the element most
likely to become limiting to phytoplankton growth in lakes, algal biomass
contains many other essential elements that also must be present in sufficient quantities for the maintenance of vital cellular functions. Differential
requirements for essential elements like silicon and nitrogen among the
major phytoplankton groups are thought to be important in determining
major events in phytoplankton succession (cf. steps 11, 12, and 13 in the
PEG model of plankton succession; Sommer et al. 1986).
The resource competition model of Tilman (1980) has generally been
successful in predicting the outcome of competition experiments between
diatom species in gradients of Si:P supply ratios (summarized in Table 3.1
in Sommer 1989b). This predictive success implies a strong tradeoff
between competitive abilities for Si and P in different diatom species, thus
excluding the possible existence of a "superspecies" with superior competitive abilities for both elements. Based on the variability in "optimum"
N:P ratios, introduced by Rhee and Gotham (1980), in different phytoplankton species, a similar tradeoff between competitive ability for Nand P
has been proposed. The "optimum" N:P ratio is defined as the ratio
between the subsistence quotas (Q1 for N- and P-limited growth according
to the Droop model [Eq. (3.2)]. Although the "optimum" N:P ratio has
been interpreted as the N:P supply ratio leading to equal N- and P-limitation, this interpretation has been criticized by several authors, as summarized by Turpin (1988). As shown in Section 3.7, the optimum N:P supply
ratio will actually be growth rate-dependent, unless the asymptotic growth
rate parameters (Ji) of the Droop model are identical for N- and P-limited
growth [which will be strictly true only if the storage capacities (Q''l'Q1 for
Nand P are equal; cf. Eq. (3.3)].
Zooplankton grazers release the remains of ingested diatom frustules
mostly as particulate silicon (Paasche 1980). The limited recycling of Si by
grazers will therefore decrease the Si:P supply ratio to the disadvantage of
diatoms in the presence of herbivores (Sommer 1988b). In contrast, both N
and P are found to be recycled by herbivores in forms easily available to
algae (Lehman 1984). Since herbivores need both Nand P for their production of new biomass, the N:P ratio of recycled nutrients from grazing
zooplankton will be determined by the N:P ratio of the food in relation to
the requirements of the grazer (Sterner 1990; Andersen and Hessen 1991).
If the interplay between N:P ratios of food items and recycled nutrients
leads to a fluctuating N:P supply ratio, then zooplankton grazing could,
according to the theory of Tilman (1982), potentially mediate the coexistence of more than two phytoplankton species on two limiting resources,
Approaching Planktonic Food Webs: Competition, Coexistence, and Chaos
6.3 Differential Nutrient Recycling and Resource Supply
Ratios
Up to this point, we have only considered systems with one limiting
resource. Even if phosphorus is generally identified as the element most
likely to become limiting to phytoplankton growth in lakes, algal biomass
contains many other essential elements that also must be present in sufficient quantities for the maintenance of vital cellular functions. Differential
requirements for essential elements like silicon and nitrogen among the
major phytoplankton groups are thought to be important in determining
major events in phytoplankton succession (cf. steps 11, 12, and 13 in the
PEG model of plankton succession; Sommer et al. 1986).
The resource competition model of Tilman (1980) has generally been
successful in predicting the outcome of competition experiments between
diatom species in gradients of Si:P supply ratios (summarized in Table 3.1
in Sommer 1989b). This predictive success implies a strong tradeoff
between competitive abilities for Si and P in different diatom species, thus
excluding the possible existence of a "superspecies" with superior competitive abilities for both elements. Based on the variability in "optimum"
N:P ratios, introduced by Rhee and Gotham (1980), in different phytoplankton species, a similar tradeoff between competitive ability for Nand P
has been proposed. The "optimum" N:P ratio is defined as the ratio
between the subsistence quotas (Q1 for N- and P-limited growth according
to the Droop model [Eq. (3.2)]. Although the "optimum" N:P ratio has
been interpreted as the N:P supply ratio leading to equal N- and P-limitation, this interpretation has been criticized by several authors, as summarized by Turpin (1988). As shown in Section 3.7, the optimum N:P supply
ratio will actually be growth rate-dependent, unless the asymptotic growth
rate parameters (Ji) of the Droop model are identical for N- and P-limited
growth [which will be strictly true only if the storage capacities (Q''l'Q1 for
Nand P are equal; cf. Eq. (3.3)].
Zooplankton grazers release the remains of ingested diatom frustules
mostly as particulate silicon (Paasche 1980). The limited recycling of Si by
grazers will therefore decrease the Si:P supply ratio to the disadvantage of
diatoms in the presence of herbivores (Sommer 1988b). In contrast, both N
and P are found to be recycled by herbivores in forms easily available to
algae (Lehman 1984). Since herbivores need both Nand P for their production of new biomass, the N:P ratio of recycled nutrients from grazing
zooplankton will be determined by the N:P ratio of the food in relation to
the requirements of the grazer (Sterner 1990; Andersen and Hessen 1991).
If the interplay between N:P ratios of food items and recycled nutrients
leads to a fluctuating N:P supply ratio, then zooplankton grazing could,
according to the theory of Tilman (1982), potentially mediate the coexistence of more than two phytoplankton species on two limiting resources,