92
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
Olsen et al. (1986b, Fig. 6) reported P released per C ingested (pI/) as
function of the P:C ratio of the food particles (Q). If we rescale these observations by the typical P content in Daphnia [0 = 30 (JIS P) (mg Cr1j Olsen et
al. 1986bj Andersen and Hessen 1991], the P recycling index pRO and the
relative P content of the food algae Q/O can be calculated. Although the
available data are limited (specially for an interval around Q/O = 1), Fig.
4.12 indicates that the intermediate P utilization strategy (n = 1) fits the
observations better than the other two (n = 0 or 00). When it is kept in mind
that the parameters s· and [(2 are estimated independently of the data, the
correspondence between observations and predictions must be considered
satisfactory.
Growth Limitation by Food Composition and/or Abundance. So far, we
have implicitly assumed that food abundance is sufficient to support a
positive carbon balance in the animals. Andersen and Hessen (1991)
showed that cladoceran zooplankton were able to maintain constant P:C
ratio even when starving, indicating that body P is lost in proportion to
respiratory C losses when the overall carbon balance is negative. If the food
carbon concentration is below the threshold level for positive individual
growth (C < C'1, so that the animals have a net loss of body C, it is reasonable to assume that growth (loss) is independent of food composition.
In other words, that growth is proportional to food P content only when
C < C" and Q < 0, otherwise growth is determined by the carbon balance
alone.
In the discussion above, we have considered only the maintenance costs
of respiration, and neglected the body material lost by molting. This
approximation will be valid if we assume that the molt has the same P:C
ratio as the rest of the body. While Andersen and Hessen (1991) found the
exoskeleton residues after persulfate digestion to contain no phosphorus,
this does not necessarily mean that the intact exoskeleton is P free. On the
contrary, Yan et al. (1989) indicate that the Daphnia carapace has a high P
content, while Scavia and McFarland (1982) found conspicuous peaks in
the P release rate from Daphnia magna at the time of molting, indicating
that the P contained in the old carapace cannot be completely reclaimed for
anabolic purposes.
If we assume that molting represents a proportional loss of body P and e,
the maintenance of a constant P:C ratio should have no effect on the allocation between growth, reproduction, and molting. Food composition then
will only affect the total amount of material available for produfing new
biomass as body tissues, exoskeleton, or eggs. This means that Eqs. (4.14),
(4.15) will be unmodified by the inclusion of food composition effects in
the individual growth model of Section 4.4, as will the submodel describing
feeding as function of body size and food concentration [Eq. (4.17)]. In
order to describe the carbon balance under suboptimal food composition,
Eq. (4.16) needs to be replaced by the following pair of equations:
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
Olsen et al. (1986b, Fig. 6) reported P released per C ingested (pI/) as
function of the P:C ratio of the food particles (Q). If we rescale these observations by the typical P content in Daphnia [0 = 30 (JIS P) (mg Cr1j Olsen et
al. 1986bj Andersen and Hessen 1991], the P recycling index pRO and the
relative P content of the food algae Q/O can be calculated. Although the
available data are limited (specially for an interval around Q/O = 1), Fig.
4.12 indicates that the intermediate P utilization strategy (n = 1) fits the
observations better than the other two (n = 0 or 00). When it is kept in mind
that the parameters s· and [(2 are estimated independently of the data, the
correspondence between observations and predictions must be considered
satisfactory.
Growth Limitation by Food Composition and/or Abundance. So far, we
have implicitly assumed that food abundance is sufficient to support a
positive carbon balance in the animals. Andersen and Hessen (1991)
showed that cladoceran zooplankton were able to maintain constant P:C
ratio even when starving, indicating that body P is lost in proportion to
respiratory C losses when the overall carbon balance is negative. If the food
carbon concentration is below the threshold level for positive individual
growth (C < C'1, so that the animals have a net loss of body C, it is reasonable to assume that growth (loss) is independent of food composition.
In other words, that growth is proportional to food P content only when
C < C" and Q < 0, otherwise growth is determined by the carbon balance
alone.
In the discussion above, we have considered only the maintenance costs
of respiration, and neglected the body material lost by molting. This
approximation will be valid if we assume that the molt has the same P:C
ratio as the rest of the body. While Andersen and Hessen (1991) found the
exoskeleton residues after persulfate digestion to contain no phosphorus,
this does not necessarily mean that the intact exoskeleton is P free. On the
contrary, Yan et al. (1989) indicate that the Daphnia carapace has a high P
content, while Scavia and McFarland (1982) found conspicuous peaks in
the P release rate from Daphnia magna at the time of molting, indicating
that the P contained in the old carapace cannot be completely reclaimed for
anabolic purposes.
If we assume that molting represents a proportional loss of body P and e,
the maintenance of a constant P:C ratio should have no effect on the allocation between growth, reproduction, and molting. Food composition then
will only affect the total amount of material available for produfing new
biomass as body tissues, exoskeleton, or eggs. This means that Eqs. (4.14),
(4.15) will be unmodified by the inclusion of food composition effects in
the individual growth model of Section 4.4, as will the submodel describing
feeding as function of body size and food concentration [Eq. (4.17)]. In
order to describe the carbon balance under suboptimal food composition,
Eq. (4.16) needs to be replaced by the following pair of equations: