4. Lipids in Freshwater Zooplankton
83
4.6.3. Essential Fatty Acids
Recently, much interest has been engendered in the so-called EFAs, members of
the (003) and (006) series of FAs. For example, the studies of Milller-Navarra
(1995a,b) have suggested that Daphnia growth rates may be well correlated with
the sestonic content of eicosapentaenoic acid (20:5003). It is well known that fish
and, in fact, all vertebrate species studied to date have an absolute requirement for
certain of these PUFAs (Sargent et aI., 1995). In addition, there are mammalian
health implications because inadequate representation of these compounds in the
diet has been implicated by numerous clinical studies in a wide variety of pathological conditions or added "health-risk potentialities" (Craig-Schmidt et aI.,
1996; Osterud et aI., 1995; van Houwelingen et aI., 1995).
One of the first steps necessary to demonstrate whether EFAs playa major role
limiting the production of freshwater organisms is to document where and when
these compounds exist in the lake or wetland. This requires that careful inventories of the FA composition of both primary producers (Ahlgren et aI., 1990) and
higher-order consumers (Bell et aI., 1994; Hanson et aI., 1985) be conducted in a
variety of systems. This is a formidable task both because of the large numbers of
species and because of the seasonal variability in FA composition. Changes in FA
composition and biomass will also accompany changes in the different life stages
of an organism, and these changes will be at least partially explained by shifts in
diet as the animal matures. Relevant questions in this context might be, How does
the FA and, in particular, the EFA composition change in the same species across
trophic gradients or between freshwater and saline lakes at similar trophic levels?
At what times of the year are sestonic EFA concentrations greatest, and which
organisms appear most capable to capitalize on these nutritionally important
particles? Are there important "bottlenecks" in the production of EFAs in the
phytoplankton? How are the FAs of freshwater zooplankton modified by fish
following dietary lipid transfer (Desvilettes, 1994)?
Fish have an absolute EFA requirement of roughly 0.5-1.0% of the diet
(Sargent et aI., 1995). Similar sorts of estimates for freshwater zooplankton are
not yet available; however, the zooplankton may well be highly responsive to the
presence of food sources containing relatively high concentrations of EFAs (but
see Goulden et aI., this volume) or at least have evolved effective strategies to
store and conserve EFAs. If the appearance of EFAs in the seston follows more or
less predictable seasonal patterns, then peaks in feeding or population density
should occur when EFAs are most abundant or obtainable. By extension, if the
procurement of EFAs is a strong driving force in the evolution of vertebrate
communities, then fish might be expected to show some preference for zooplankton species rich in EFA.
4.6.4. Effects of Temperature Changes
Altering FA composition to affect changes in membrane fluidity is one of the
principal methods that organisms use to adapt and acclimate to changes in temperature (Gurr and Harwood, 1991). Thus, membrane FA composition plays a
83
4.6.3. Essential Fatty Acids
Recently, much interest has been engendered in the so-called EFAs, members of
the (003) and (006) series of FAs. For example, the studies of Milller-Navarra
(1995a,b) have suggested that Daphnia growth rates may be well correlated with
the sestonic content of eicosapentaenoic acid (20:5003). It is well known that fish
and, in fact, all vertebrate species studied to date have an absolute requirement for
certain of these PUFAs (Sargent et aI., 1995). In addition, there are mammalian
health implications because inadequate representation of these compounds in the
diet has been implicated by numerous clinical studies in a wide variety of pathological conditions or added "health-risk potentialities" (Craig-Schmidt et aI.,
1996; Osterud et aI., 1995; van Houwelingen et aI., 1995).
One of the first steps necessary to demonstrate whether EFAs playa major role
limiting the production of freshwater organisms is to document where and when
these compounds exist in the lake or wetland. This requires that careful inventories of the FA composition of both primary producers (Ahlgren et aI., 1990) and
higher-order consumers (Bell et aI., 1994; Hanson et aI., 1985) be conducted in a
variety of systems. This is a formidable task both because of the large numbers of
species and because of the seasonal variability in FA composition. Changes in FA
composition and biomass will also accompany changes in the different life stages
of an organism, and these changes will be at least partially explained by shifts in
diet as the animal matures. Relevant questions in this context might be, How does
the FA and, in particular, the EFA composition change in the same species across
trophic gradients or between freshwater and saline lakes at similar trophic levels?
At what times of the year are sestonic EFA concentrations greatest, and which
organisms appear most capable to capitalize on these nutritionally important
particles? Are there important "bottlenecks" in the production of EFAs in the
phytoplankton? How are the FAs of freshwater zooplankton modified by fish
following dietary lipid transfer (Desvilettes, 1994)?
Fish have an absolute EFA requirement of roughly 0.5-1.0% of the diet
(Sargent et aI., 1995). Similar sorts of estimates for freshwater zooplankton are
not yet available; however, the zooplankton may well be highly responsive to the
presence of food sources containing relatively high concentrations of EFAs (but
see Goulden et aI., this volume) or at least have evolved effective strategies to
store and conserve EFAs. If the appearance of EFAs in the seston follows more or
less predictable seasonal patterns, then peaks in feeding or population density
should occur when EFAs are most abundant or obtainable. By extension, if the
procurement of EFAs is a strong driving force in the evolution of vertebrate
communities, then fish might be expected to show some preference for zooplankton species rich in EFA.
4.6.4. Effects of Temperature Changes
Altering FA composition to affect changes in membrane fluidity is one of the
principal methods that organisms use to adapt and acclimate to changes in temperature (Gurr and Harwood, 1991). Thus, membrane FA composition plays a