82
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
while the same volume may contain sufficient food for several days of
growth for a neonate of the same species. Animals grown in semicontinuous culture will therefore not experience the same food concentration
throughout their lifetime and the adults will be victim to dramatic "feast
and famine" cycles as a result of the feeding regime.
Additional problems will result when the animals are cultured under a
light:dark cycle that permits continued growth of the food algae. In the
experimental setup of Lynch (1989), this gave a paradoxical extrapolation to
positive population growth at zero food concentration. A simple calculation
shows that the observed net secondary production exceeded the food ration
by nearly an order of magnitude at the lowest food level ("0.0154 f.1g C mr l ,,)
of Lynch (1989), probably indicating that, in addition to algal growth after
transfer, bacterial contamination also contributed to the food supply.
As a case study on Daphnia pulex growth in transfer culture, we can take
a series of life-table experiments conducted by Frank et al. (1957). They
chose to vary the number of animals per vessel, with all vessels receiving
the same food addition at 2-day intervals, a design which probably makes it
easier to maintain a well-defined food supply among treatments than in
experiments where animal density is constant and food concentration is
varied. The major weakness with this experimental protocol would nevertheless be the same as for other semicontinuous designs: adults will experience a much more severe food shortage between food replenishments than
juveniles.
Frank et al. (1957) reported Daphnia pulex body size as biovolume
calculated from carapace length and width measurements, instead of the
more common units of length or dry weight used in more recent accounts.
The average biovolumes of neonates, primiparous females, and the largest
females in Frank et al. (1957) were 0.2, 2.3, and 8.7 mm'. Comparing these
biovolumes with the corresponding carbon units in Table 4.1 indicate a
roughly linear relationship between biovolume and carbon mass, using a
conversion factor of 6 (f.1g C) mm-'.
In contrast to the flow-through system simulated in Fig. 4.8, changes in
food concentration needs to represented explicitly in a model of Daphnia
growth in transfer culture. If animals are cultured in the dark (as in the
experiments of Frank et al. 1957), the mass balance of the food compartment will have no growth term; the body growth model [Eq. (4.9)] therefore needs only to be augmented with the differential equation
C=-lnB,
(4.13)
describing the reduction in food concentration [C; (mg C) 1"1] through
consumption. In Eq. (4.13) the specific ingestion rate lis a function of body
size and food concentration [Eqs. (4.10), (4.11)], whereas n is the crowding
level as individuals mrl. If body size (B) is in units of f.1g C, as in Eq. (4.9),
the right hand side in Eq. (4.13) will have the correct dimension (1lS C) mr l
day"1 = (mg C) r l day"l. Figure 4.9 shows the results of solving Eqs. (4.9) and
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
while the same volume may contain sufficient food for several days of
growth for a neonate of the same species. Animals grown in semicontinuous culture will therefore not experience the same food concentration
throughout their lifetime and the adults will be victim to dramatic "feast
and famine" cycles as a result of the feeding regime.
Additional problems will result when the animals are cultured under a
light:dark cycle that permits continued growth of the food algae. In the
experimental setup of Lynch (1989), this gave a paradoxical extrapolation to
positive population growth at zero food concentration. A simple calculation
shows that the observed net secondary production exceeded the food ration
by nearly an order of magnitude at the lowest food level ("0.0154 f.1g C mr l ,,)
of Lynch (1989), probably indicating that, in addition to algal growth after
transfer, bacterial contamination also contributed to the food supply.
As a case study on Daphnia pulex growth in transfer culture, we can take
a series of life-table experiments conducted by Frank et al. (1957). They
chose to vary the number of animals per vessel, with all vessels receiving
the same food addition at 2-day intervals, a design which probably makes it
easier to maintain a well-defined food supply among treatments than in
experiments where animal density is constant and food concentration is
varied. The major weakness with this experimental protocol would nevertheless be the same as for other semicontinuous designs: adults will experience a much more severe food shortage between food replenishments than
juveniles.
Frank et al. (1957) reported Daphnia pulex body size as biovolume
calculated from carapace length and width measurements, instead of the
more common units of length or dry weight used in more recent accounts.
The average biovolumes of neonates, primiparous females, and the largest
females in Frank et al. (1957) were 0.2, 2.3, and 8.7 mm'. Comparing these
biovolumes with the corresponding carbon units in Table 4.1 indicate a
roughly linear relationship between biovolume and carbon mass, using a
conversion factor of 6 (f.1g C) mm-'.
In contrast to the flow-through system simulated in Fig. 4.8, changes in
food concentration needs to represented explicitly in a model of Daphnia
growth in transfer culture. If animals are cultured in the dark (as in the
experiments of Frank et al. 1957), the mass balance of the food compartment will have no growth term; the body growth model [Eq. (4.9)] therefore needs only to be augmented with the differential equation
C=-lnB,
(4.13)
describing the reduction in food concentration [C; (mg C) 1"1] through
consumption. In Eq. (4.13) the specific ingestion rate lis a function of body
size and food concentration [Eqs. (4.10), (4.11)], whereas n is the crowding
level as individuals mrl. If body size (B) is in units of f.1g C, as in Eq. (4.9),
the right hand side in Eq. (4.13) will have the correct dimension (1lS C) mr l
day"1 = (mg C) r l day"l. Figure 4.9 shows the results of solving Eqs. (4.9) and