5. Lipid Dietary Dependencies in Zooplankton
105
and Henry, 1985), and daphniids appear to depend on the diet for TAG (Goulden
and Place, 1990). In the 1988 experiment, the lipid-ovary index of D. catawba
increased from 0.3 to 2 as triacylglycerol was assimilated (Table 5.2). By contrast,
the fatty acid microcapsules without TAG produced no discernible change in the
lipid status of animals in these experiments (data not shown). Although the fatty
acid microcapsules were introduced at a lower concentration than the triacylglycerol microcapsules (25 . 10 3 • ml- I versus 100 . 10 3 • ml- I), this experiment and a later laboratory experiment (Goulden, unpublished data) suggest that
visible lipid accumulation depends in large part on the composition of the lipid
classes in the diet, particularly the presence and amount of triacylglyceroI.
The analysis of the algal diet during the October experiment from Lake Waynewood, when the Daphnia responded strongly to protein supplementation, established two important facts regarding the conditions during that experiment: first,
that the concentration of readily ingested algae was not very high, despite the high
algal biomass in the lake; and second, that these edible algae were not strongly
limited by nitrogen or phosphorus. Circulation of the water column to 6-8 m at
that time meant that nutrients that had accumulated in the anoxic hypolimnion
during the summer were being advected into the epilimnion. At the same time,
light availability was decreasing (only 1 % of surface irradiance penetrated to 3 m,
the middle of the mixed layer).
The edible fraction of algae amounted to 0.36 • 10 6 f.Lm 3 • ml- i of lake water,
equivalent to about 80 ng C . ml- i (Reynolds, 1984). This was well below 400 ng
C . ml- I, the level that saturates assimilation rate (Lampert, 1977) and maximum
clutch size (Goulden et aI., 1982) of similar-sized daphniids.
EFA deficiencies were not identified in this study, but we suspect that such
deficiencies, especially of the polyunsaturated EFAs, can occur wherever bacteria
and single-celled coccoid cyanobacteria form a major part of the diet of zooplankton. Recent studies by Mtiller-Navarra (1995) do suggest a more widespread
occurrence of limitation by EPA. Bacteria and coccoid cyanobacteria are deficient
in polyunsaturated fatty acids (Murata and Nishida, 1987; Wood, 1974; Erwin,
1973).
Lipids in lake seston of <50-f.Lm diameter can be 30-50% of dry weight,
depending on the time of the year (Kreeger et aI., 1997). This may suggest that
lake seston (assuming that the seston is primarily algae) reflects nutrient limitation
of the phytoplankton because nutrient limitation often stimulates lipid accumulation. However, these measurements need to be repeated in other lakes and during
different years to characterize what is happening in the phytoplankton and seston
of lakes. If these values do typify the composition of the edible fraction of the
seston consumed and digested by zooplankton, it would indicate that adequate
lipid is present for zooplankton reproduction and survival. Further, this suggests
that a dependency on dietary lipids is not a risky strategy for zooplankton.
Zooplankton populations are often found to be food-limited during the summer
months (Tessier, 1986; Lampert, 1985; Threlkeld, 1979). Under low food conditions, lipid reserves are crucial to the survival of Daphnia adults (Goulden and
Henry, 1985; Lampert and Bohrer, 1984) as well as embryos and neonates
105
and Henry, 1985), and daphniids appear to depend on the diet for TAG (Goulden
and Place, 1990). In the 1988 experiment, the lipid-ovary index of D. catawba
increased from 0.3 to 2 as triacylglycerol was assimilated (Table 5.2). By contrast,
the fatty acid microcapsules without TAG produced no discernible change in the
lipid status of animals in these experiments (data not shown). Although the fatty
acid microcapsules were introduced at a lower concentration than the triacylglycerol microcapsules (25 . 10 3 • ml- I versus 100 . 10 3 • ml- I), this experiment and a later laboratory experiment (Goulden, unpublished data) suggest that
visible lipid accumulation depends in large part on the composition of the lipid
classes in the diet, particularly the presence and amount of triacylglyceroI.
The analysis of the algal diet during the October experiment from Lake Waynewood, when the Daphnia responded strongly to protein supplementation, established two important facts regarding the conditions during that experiment: first,
that the concentration of readily ingested algae was not very high, despite the high
algal biomass in the lake; and second, that these edible algae were not strongly
limited by nitrogen or phosphorus. Circulation of the water column to 6-8 m at
that time meant that nutrients that had accumulated in the anoxic hypolimnion
during the summer were being advected into the epilimnion. At the same time,
light availability was decreasing (only 1 % of surface irradiance penetrated to 3 m,
the middle of the mixed layer).
The edible fraction of algae amounted to 0.36 • 10 6 f.Lm 3 • ml- i of lake water,
equivalent to about 80 ng C . ml- i (Reynolds, 1984). This was well below 400 ng
C . ml- I, the level that saturates assimilation rate (Lampert, 1977) and maximum
clutch size (Goulden et aI., 1982) of similar-sized daphniids.
EFA deficiencies were not identified in this study, but we suspect that such
deficiencies, especially of the polyunsaturated EFAs, can occur wherever bacteria
and single-celled coccoid cyanobacteria form a major part of the diet of zooplankton. Recent studies by Mtiller-Navarra (1995) do suggest a more widespread
occurrence of limitation by EPA. Bacteria and coccoid cyanobacteria are deficient
in polyunsaturated fatty acids (Murata and Nishida, 1987; Wood, 1974; Erwin,
1973).
Lipids in lake seston of <50-f.Lm diameter can be 30-50% of dry weight,
depending on the time of the year (Kreeger et aI., 1997). This may suggest that
lake seston (assuming that the seston is primarily algae) reflects nutrient limitation
of the phytoplankton because nutrient limitation often stimulates lipid accumulation. However, these measurements need to be repeated in other lakes and during
different years to characterize what is happening in the phytoplankton and seston
of lakes. If these values do typify the composition of the edible fraction of the
seston consumed and digested by zooplankton, it would indicate that adequate
lipid is present for zooplankton reproduction and survival. Further, this suggests
that a dependency on dietary lipids is not a risky strategy for zooplankton.
Zooplankton populations are often found to be food-limited during the summer
months (Tessier, 1986; Lampert, 1985; Threlkeld, 1979). Under low food conditions, lipid reserves are crucial to the survival of Daphnia adults (Goulden and
Henry, 1985; Lampert and Bohrer, 1984) as well as embryos and neonates