88
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
CheckIey (1980) found that the gross growth efficiency (the fraction of
ingested food incorporated into gonadal or somatic tissue) of the marine
copepod Paracalanus was related to the N:C ratio of the food. Apparently,
the "excess" carbon in algal food with low N:C ratio was disposed of as increased feces production, indicating that major changes in food carbon
utilization can result from changes in the elemental composition of the
food.
Lehman and Naumoski (1985) reported large differences in phosphorus
assimilation and excretion among Daphnia pulex raised on P-limited and
P-sufficient algal food, suggesting that the phosphorus utilization of the
animals is also affected by the P content of their food. Olsen et a1. (1986b)
inferred from phosphorus mass-balance studies on Daphnia that there
must be a critical P:C ratio below which the animals are unable to sustain
their maximal growth rate, and showed that estimates of this critical food
phosphorus content were within the P:C ratios observed in mildly nutrientlimited algae. The idea of nutrient-limited zooplankton growth is further
supported by Sterner et al. (1993), who found dramatic reductions in
growth, reproduction, and survival in Daphnia pulex reared on P-limited
Scenedesmus acutus compared to animals given nutrient-sufficient cells of
the same food species.
Andersen and Hessen (1991) concluded from an investigation of C, N,
and P content in natural populations of several species of zooplankton that
interspecific variations in elemental composition were generally higher
than intraspecific variations, and that this pattern was preserved under experimental starvation and food enrichment, suggesting that zooplankton
are able to maintain their elemental ratios under changing food composition. This homeostatic view of zooplankton mineral nutrient economy is
also supported by Sterner (1990), who showed that, among several possible
alternatives, a model based on the assumption of constant elemental ratios
in the grazers gave the best fit to empirical data from Le Borgne (1982) on
Nand P release by marine zooplankton.
Growth Limitation by Food P Content. The maintenance of constant
elemental composition puts strong constraints on the fraction of food C
that can be utilized for production of new biomass when the food is P-deficient relative to the requirements of the animal. In Appendix A3 it is shown
under quite general conditions that the constraints from balanced growth
will be satisfied when the P loss rate from catabolic processes is described
by a family of power functions [Eq. (A3.11»), enabling different degrees of
catabolite reutilization to be represented by varying the exponent n. Different strategies for nutrient conservation give relationships between food P
content and food C assimilation efficiency (Fig. A3.3) which are confined
between the two extremes corresponding to constant excretion rate (n = 0),
and to zero excretion rate when maximal C assimilation is impossible
(n =
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
CheckIey (1980) found that the gross growth efficiency (the fraction of
ingested food incorporated into gonadal or somatic tissue) of the marine
copepod Paracalanus was related to the N:C ratio of the food. Apparently,
the "excess" carbon in algal food with low N:C ratio was disposed of as increased feces production, indicating that major changes in food carbon
utilization can result from changes in the elemental composition of the
food.
Lehman and Naumoski (1985) reported large differences in phosphorus
assimilation and excretion among Daphnia pulex raised on P-limited and
P-sufficient algal food, suggesting that the phosphorus utilization of the
animals is also affected by the P content of their food. Olsen et a1. (1986b)
inferred from phosphorus mass-balance studies on Daphnia that there
must be a critical P:C ratio below which the animals are unable to sustain
their maximal growth rate, and showed that estimates of this critical food
phosphorus content were within the P:C ratios observed in mildly nutrientlimited algae. The idea of nutrient-limited zooplankton growth is further
supported by Sterner et al. (1993), who found dramatic reductions in
growth, reproduction, and survival in Daphnia pulex reared on P-limited
Scenedesmus acutus compared to animals given nutrient-sufficient cells of
the same food species.
Andersen and Hessen (1991) concluded from an investigation of C, N,
and P content in natural populations of several species of zooplankton that
interspecific variations in elemental composition were generally higher
than intraspecific variations, and that this pattern was preserved under experimental starvation and food enrichment, suggesting that zooplankton
are able to maintain their elemental ratios under changing food composition. This homeostatic view of zooplankton mineral nutrient economy is
also supported by Sterner (1990), who showed that, among several possible
alternatives, a model based on the assumption of constant elemental ratios
in the grazers gave the best fit to empirical data from Le Borgne (1982) on
Nand P release by marine zooplankton.
Growth Limitation by Food P Content. The maintenance of constant
elemental composition puts strong constraints on the fraction of food C
that can be utilized for production of new biomass when the food is P-deficient relative to the requirements of the animal. In Appendix A3 it is shown
under quite general conditions that the constraints from balanced growth
will be satisfied when the P loss rate from catabolic processes is described
by a family of power functions [Eq. (A3.11»), enabling different degrees of
catabolite reutilization to be represented by varying the exponent n. Different strategies for nutrient conservation give relationships between food P
content and food C assimilation efficiency (Fig. A3.3) which are confined
between the two extremes corresponding to constant excretion rate (n = 0),
and to zero excretion rate when maximal C assimilation is impossible
(n =