5. Lipid Dietary Dependencies in Zooplankton
97
laboratory cultures; maintained on ASM medium [Carmichael and Gorham,
1974] in a semicontinuous flow culture system and kept in a growth phase) served
as a "complete" diet to test for food limitation in the broad sense.
Similar-sized adult animals (1.2-1.4 mm for Daphnia catawba; 1.8-2.2 mm
for D. pulicaria; 1.7-1.9 mm for D. laevis) collected from the lake were selected
for the experiments to minimize size differences in egg number. Individual animals were transferred to each BOD bottle (one per bottle in the Waynewood
experiments; one or three per bottle at Lacawac). The bottles were then placed on
a plankton wheel and rotated at 1 rpm, in a room receiving low ambient light
during daytime and with temperatures similar to the lake's epilimnion (19°C
during the late June and July experiments and 17°C during mid-October). Animals were maintained on experimental diets for 6-7 d (two to three instars) with
medium changes (water and supplements) at 1-2-d intervals. Body size, clutch
size, and lipid-ovary index (Tessier and Goulden, 1982) were recorded for each
animal when the experiment was terminated. Clutch size in Daphnia responds
within a few days to changes in diet (Frank, 1960). Differences in clutch size were
tested by ANOVA, followed by linear contrasts with the control or by two-way
ANOVA using SYSTAT (Wilkinson, 1988) or SAS (Anonymous, 1985). Log
transformations were usually used to equalize variances. Among treatment
differences in lipid-ovary index were determined by using Fisher's Exact test
(Sokal and Rohlf, 1981).
5.2.3. Characterization of the Natural Algal Diet
At the time of the October supplementation experiment in Lake Waynewood, the
natural algal diet of the Daphnia was analyzed in terms of grazing selectivity, biovolume of food ingested, and nutrient limitations of the algae. A composite
water sample from 0-6 m depths in Lake Waynewood was screened (153 f1m) to
remove large zooplankton, distributed among 1-L transparent polyethylene
containers (Cubitainers), and treated by adding Daphnia (0 or 20 . L -I, mostly
D. laevis), nitrogen (0 or 30 f1mol . L -I NH 4 N0 3 ), and phosphorus (0 or 6
/-Lmol . L -I NaH 2 P0 4 ) in all combinations with four replicates. Not all treatment
results are included in the Results section. Containers were suspended at 2-m
depth in Lake Lacawac for 3 cloudy days, providing light equivalent to that at 1 m
in Lake Waynewood. They were shaken twice daily. On day 3, 40-ml subsamples
from the containers were incubated with NaHI4C0 3 (Ill Bq . ml- I ) in the laboratory under "cool-white" fluorescent illumination (photosynthetically active radiation at 150 /-Lmol . m- 2 • s -I) for 4 h to measure photosynthetic rate. A freshly
collected water sample was incubated along with the day 3 container samples
("lake" treatment). Aliquots were counted by liquid scintillation after being acidified and bubbled to remove unincorporated 14C0 2 . Parallel subsamples from all
containers and the lake treatment were incubated with much lower 14C activity
(2.6 Bq . ml- I ) for autoradiography of algae. Total inorganic carbon concentration was determined by gas chromatography (Stainton, 1973) and was uniform
across all containers (0.40 mM . L -I). Samples were preserved with acid Lugol's
solution at 1 % (vol/vol) for algal enumeration and autoradiography.
97
laboratory cultures; maintained on ASM medium [Carmichael and Gorham,
1974] in a semicontinuous flow culture system and kept in a growth phase) served
as a "complete" diet to test for food limitation in the broad sense.
Similar-sized adult animals (1.2-1.4 mm for Daphnia catawba; 1.8-2.2 mm
for D. pulicaria; 1.7-1.9 mm for D. laevis) collected from the lake were selected
for the experiments to minimize size differences in egg number. Individual animals were transferred to each BOD bottle (one per bottle in the Waynewood
experiments; one or three per bottle at Lacawac). The bottles were then placed on
a plankton wheel and rotated at 1 rpm, in a room receiving low ambient light
during daytime and with temperatures similar to the lake's epilimnion (19°C
during the late June and July experiments and 17°C during mid-October). Animals were maintained on experimental diets for 6-7 d (two to three instars) with
medium changes (water and supplements) at 1-2-d intervals. Body size, clutch
size, and lipid-ovary index (Tessier and Goulden, 1982) were recorded for each
animal when the experiment was terminated. Clutch size in Daphnia responds
within a few days to changes in diet (Frank, 1960). Differences in clutch size were
tested by ANOVA, followed by linear contrasts with the control or by two-way
ANOVA using SYSTAT (Wilkinson, 1988) or SAS (Anonymous, 1985). Log
transformations were usually used to equalize variances. Among treatment
differences in lipid-ovary index were determined by using Fisher's Exact test
(Sokal and Rohlf, 1981).
5.2.3. Characterization of the Natural Algal Diet
At the time of the October supplementation experiment in Lake Waynewood, the
natural algal diet of the Daphnia was analyzed in terms of grazing selectivity, biovolume of food ingested, and nutrient limitations of the algae. A composite
water sample from 0-6 m depths in Lake Waynewood was screened (153 f1m) to
remove large zooplankton, distributed among 1-L transparent polyethylene
containers (Cubitainers), and treated by adding Daphnia (0 or 20 . L -I, mostly
D. laevis), nitrogen (0 or 30 f1mol . L -I NH 4 N0 3 ), and phosphorus (0 or 6
/-Lmol . L -I NaH 2 P0 4 ) in all combinations with four replicates. Not all treatment
results are included in the Results section. Containers were suspended at 2-m
depth in Lake Lacawac for 3 cloudy days, providing light equivalent to that at 1 m
in Lake Waynewood. They were shaken twice daily. On day 3, 40-ml subsamples
from the containers were incubated with NaHI4C0 3 (Ill Bq . ml- I ) in the laboratory under "cool-white" fluorescent illumination (photosynthetically active radiation at 150 /-Lmol . m- 2 • s -I) for 4 h to measure photosynthetic rate. A freshly
collected water sample was incubated along with the day 3 container samples
("lake" treatment). Aliquots were counted by liquid scintillation after being acidified and bubbled to remove unincorporated 14C0 2 . Parallel subsamples from all
containers and the lake treatment were incubated with much lower 14C activity
(2.6 Bq . ml- I ) for autoradiography of algae. Total inorganic carbon concentration was determined by gas chromatography (Stainton, 1973) and was uniform
across all containers (0.40 mM . L -I). Samples were preserved with acid Lugol's
solution at 1 % (vol/vol) for algal enumeration and autoradiography.