4. Lipids in Freshwater Zooplankton
73
response to stress. An additional way to express lipid levels is as a function of
surface area (Arts et aI., 1992). In this approach, lipid contents are determined
from stratified samples collected from the pelagic regions of lakes or wetlands.
The measured lipid contents (fLg per animal) are then multiplied by the areal
density of the organisms and the results expressed as areal lipid (total lipid) or
areal energy reserves (TAG plus wax esters, if they are present). Expression of
lipids in the form of areal energy reserve may offer some unique insights into
community structure and ecosystem functioning.
In saline Redberry Lake in central Saskatchewan, for example, three species of
zooplankton (Diaptomus sieilis, D. nevadensis, and Daphnia pulex) are dominant
(Arts et aI., 1993). Expression of total lipid contents on an individual versus an
areal basis provides a different representation of the seasonal lipid patterns in this
lake (Fig. 4.1). Areal lipid patterns are a function of the density of organisms as
well as the individual lipid contents so that population responses are readily
evident. For example, in terms of individual lipid, lipid levels of D. nevadensis
would appear to dwarf the contribution of the other two species in winter and
spring (Fig. 4.1). However, by virtue of the relatively high densities of D. sieilis, it
is clear that this species' contribution to overall zooplankton lipid levels on a
community basis is much higher. D. nevadensis, because of constraints imposed
by the size of its feeding basket, prefers copepodites and smaller Daphnia over
adult copepods as prey items (Arts et aI., 1993). Copepodites of D. sieilis are
typically most abundant in Redberry Lake from July to September, and this is
clearly reflected in the sharp increase in areal lipids of D. nevadensis observed
during these months (Fig. 4.1). During this period, areal lipids increase fivefold,
whereas individual lipids increase from a low of ",60 fLg per animal to a high of
'" 115 fLg per animal. Thus, the areal representation of lipids more clearly accentuates the strong seasonal dependence of D. nevadensis on appropriately sized prey
items (Fig. 4.1).
4.3. Time Course of Lipid Deposition/Loss
The flux of energy reserve lipids is a highly dynamic process and is influenced by
a variety of factors (see above). To measure the effects of these factors on lipid
energy reserves, some a priori estimate of the time required for an observable
change to be detected in the organism(s) under study is required. The time course
of lipid deposition and/or loss is an important issue for several reasons.
First, detailed data on phytoplankton community structure made synchronously
with measurements of zooplankton lipid reserves provide information about the
suitability of a particular algal species for herbivorous zooplankton (Arts et aI.,
1993, 1992). This is not a trivial association because most zooplankton-phytoplankton interactions cannot be modeled in the laboratory. This is partially because many of the algal species (and zooplankton) found in nature have either not
been cultured or are very difficult, expensive, or time-consuming to culture. In
addition, it is not usually possible in the laboratory to recreate the species assem-
73
response to stress. An additional way to express lipid levels is as a function of
surface area (Arts et aI., 1992). In this approach, lipid contents are determined
from stratified samples collected from the pelagic regions of lakes or wetlands.
The measured lipid contents (fLg per animal) are then multiplied by the areal
density of the organisms and the results expressed as areal lipid (total lipid) or
areal energy reserves (TAG plus wax esters, if they are present). Expression of
lipids in the form of areal energy reserve may offer some unique insights into
community structure and ecosystem functioning.
In saline Redberry Lake in central Saskatchewan, for example, three species of
zooplankton (Diaptomus sieilis, D. nevadensis, and Daphnia pulex) are dominant
(Arts et aI., 1993). Expression of total lipid contents on an individual versus an
areal basis provides a different representation of the seasonal lipid patterns in this
lake (Fig. 4.1). Areal lipid patterns are a function of the density of organisms as
well as the individual lipid contents so that population responses are readily
evident. For example, in terms of individual lipid, lipid levels of D. nevadensis
would appear to dwarf the contribution of the other two species in winter and
spring (Fig. 4.1). However, by virtue of the relatively high densities of D. sieilis, it
is clear that this species' contribution to overall zooplankton lipid levels on a
community basis is much higher. D. nevadensis, because of constraints imposed
by the size of its feeding basket, prefers copepodites and smaller Daphnia over
adult copepods as prey items (Arts et aI., 1993). Copepodites of D. sieilis are
typically most abundant in Redberry Lake from July to September, and this is
clearly reflected in the sharp increase in areal lipids of D. nevadensis observed
during these months (Fig. 4.1). During this period, areal lipids increase fivefold,
whereas individual lipids increase from a low of ",60 fLg per animal to a high of
'" 115 fLg per animal. Thus, the areal representation of lipids more clearly accentuates the strong seasonal dependence of D. nevadensis on appropriately sized prey
items (Fig. 4.1).
4.3. Time Course of Lipid Deposition/Loss
The flux of energy reserve lipids is a highly dynamic process and is influenced by
a variety of factors (see above). To measure the effects of these factors on lipid
energy reserves, some a priori estimate of the time required for an observable
change to be detected in the organism(s) under study is required. The time course
of lipid deposition and/or loss is an important issue for several reasons.
First, detailed data on phytoplankton community structure made synchronously
with measurements of zooplankton lipid reserves provide information about the
suitability of a particular algal species for herbivorous zooplankton (Arts et aI.,
1993, 1992). This is not a trivial association because most zooplankton-phytoplankton interactions cannot be modeled in the laboratory. This is partially because many of the algal species (and zooplankton) found in nature have either not
been cultured or are very difficult, expensive, or time-consuming to culture. In
addition, it is not usually possible in the laboratory to recreate the species assem-