4. Lipids in Freshwater Zooplankton
75
Second, a successful demonstration of cause and effect between a stressor and
the lipid energy reserves of the study organism depends on a knowledge of the
normal time course of lipid deposition/loss. This information is necessary to apply
proper statistical and logistical experimental design to chronic or acute ecotoxicological experiments in which the goal is to further our understanding of the
effects of anthropogenic stressors on lipid energetics.
Third, during food web manipulation experiments there is often a time lag
between the applied perturbation and the observed effect (Carpenter and Kitchell,
1992). Because lipids usually comprise the greatest fraction of energy-storing
biomolecules in freshwater zooplankton, information on their rates of accrual and
decline as they pass among trophic levels can lead to a better understanding of
food web dynamics. This is particularly true if one examines the trophic level
farthest removed from the perturbation. Thus, better estimates of the time required
for lipids to pass between trophic levels should enhance our ability to predict the
duration of time lags in food web studies.
Despite the importance of lipid energy reserves, their flux rates are seldom
measured. Gut fluorescence analyses provide information on recent feeding success in terms of pigment intake but do not provide explicit information on rates of
lipid acquisition (Ohman, 1988). Detailed ingestion studies (Pond et aI., 1995;
Bradshaw et aI., 1990) provide information on ingestion and assimilation efficiency and are an important component in the determination of the time course
and biochemical specifics of lipid acquisition in zooplankton.
Rapid transitions in small-scale local climatic events are potentially useful in
providing information on rates of lipid energy reserve deposition or loss. This is
because algal lipid reserves are known to be directly influenced by light and
temperature in the light saturated zone (Wainman et aI., this volume); lipid levels
generally increase with increasing light and temperature. For example, over the
course of a few days during a recent Group for Aquatic Productivity (GAP)
workshop, photosynthetically active radiation (PAR) was reduced to 50% below
that of a typical sunny day, air temperatures plummeted, and strong winds cooled
the epilimnetic surface waters by nearly 2°C (Fig. 4.2). Concomitant with these
physical events, concentrations of TAG in the phytoplankton declined, followed
by a corresponding TAG of adult male copepods (Diaptomus sieilis). Males were
chosen for these analyses because they obviate somewhat the need to be concerned with reproductive condition because, with females, greater care must be
taken to get representative samples of gravid versus nongravid individuals. After
only 2 d, there was an early indication of a decline in TAG levels in the copepods
in response to the decline in algal TAG, and certainly, after 4 d, the response was
clearly evident.
Bourdier and Amblard (1989) investigated the effect of starvation and diet
variability on the lipid composition and content of the calanoid copepod Aeanthodiaptomus dentieornis in controlled laboratory experiments. They demonstrated
that it took ~20 d following starvation to restore all the TAG of the copepods and
<20% of the wax esters. Further, they showed that the rate of lipid restoration
following starvation was dependent on the species of algae fed to the copepods.
75
Second, a successful demonstration of cause and effect between a stressor and
the lipid energy reserves of the study organism depends on a knowledge of the
normal time course of lipid deposition/loss. This information is necessary to apply
proper statistical and logistical experimental design to chronic or acute ecotoxicological experiments in which the goal is to further our understanding of the
effects of anthropogenic stressors on lipid energetics.
Third, during food web manipulation experiments there is often a time lag
between the applied perturbation and the observed effect (Carpenter and Kitchell,
1992). Because lipids usually comprise the greatest fraction of energy-storing
biomolecules in freshwater zooplankton, information on their rates of accrual and
decline as they pass among trophic levels can lead to a better understanding of
food web dynamics. This is particularly true if one examines the trophic level
farthest removed from the perturbation. Thus, better estimates of the time required
for lipids to pass between trophic levels should enhance our ability to predict the
duration of time lags in food web studies.
Despite the importance of lipid energy reserves, their flux rates are seldom
measured. Gut fluorescence analyses provide information on recent feeding success in terms of pigment intake but do not provide explicit information on rates of
lipid acquisition (Ohman, 1988). Detailed ingestion studies (Pond et aI., 1995;
Bradshaw et aI., 1990) provide information on ingestion and assimilation efficiency and are an important component in the determination of the time course
and biochemical specifics of lipid acquisition in zooplankton.
Rapid transitions in small-scale local climatic events are potentially useful in
providing information on rates of lipid energy reserve deposition or loss. This is
because algal lipid reserves are known to be directly influenced by light and
temperature in the light saturated zone (Wainman et aI., this volume); lipid levels
generally increase with increasing light and temperature. For example, over the
course of a few days during a recent Group for Aquatic Productivity (GAP)
workshop, photosynthetically active radiation (PAR) was reduced to 50% below
that of a typical sunny day, air temperatures plummeted, and strong winds cooled
the epilimnetic surface waters by nearly 2°C (Fig. 4.2). Concomitant with these
physical events, concentrations of TAG in the phytoplankton declined, followed
by a corresponding TAG of adult male copepods (Diaptomus sieilis). Males were
chosen for these analyses because they obviate somewhat the need to be concerned with reproductive condition because, with females, greater care must be
taken to get representative samples of gravid versus nongravid individuals. After
only 2 d, there was an early indication of a decline in TAG levels in the copepods
in response to the decline in algal TAG, and certainly, after 4 d, the response was
clearly evident.
Bourdier and Amblard (1989) investigated the effect of starvation and diet
variability on the lipid composition and content of the calanoid copepod Aeanthodiaptomus dentieornis in controlled laboratory experiments. They demonstrated
that it took ~20 d following starvation to restore all the TAG of the copepods and
<20% of the wax esters. Further, they showed that the rate of lipid restoration
following starvation was dependent on the species of algae fed to the copepods.