72
M.T. Arts
There is currently a proliferation of interest in the role of essential fatty acids
(EFAs) in animal health (Hall, 1996) and in human health (Layne et aI., 1996;
Osterud et aI., 1995; Renaud et aI., 1995). Recently, researchers have begun to
examine the role of EFAs in structuring zooplankton (Hagen et aI., 1995; MiillerNavarra, 1995a; Norsker and Stottrup, 1994) and fish (Bell and Sargent, 1996;
Schwalme, 1994) populations and communities. Much research has also focused
on algal EFA content because variations in EFA concentration and composition at
this trophic level largely determine the amount and type of EFAs available for the
rest of the food chain. The concept has been exploited by the mariculture industry,
in which intense investigations centered around manipulating EFAs in feed stock
are well underway to guarantee EFA availability for a variety of cultured organisms (Reis et aI., 1996; and see Olsen, this volume).
The role of lipids as solvents for lipophilic contaminants is another reason to
examine seasonal patterns of lipids in zooplankton. The sensitivity of lipids to
factors that affect the feeding success of zooplankton makes them a useful stress
detector, but only if the other conditions known to affect lipid levels are also
carefully monitored and controlled. Thus, with the proper controls and ecological
insights, lipids can be an excellent indicator of stress, particularly chronic stress
(see Desy, 1996; Himbeault, 1995; Dauble et aI., 1985).
My primary goal in this chapter is to illustrate selected aspects of lipid studies
involving, primarily, zooplankton and to convince the reader of the usefulness of
incorporating some measure of lipid content/composition in future studies. I also
provide suggestions for what I believe to be important new research avenues in the
field of lipids and highlight areas of research that are undergoing rapid expansion.
Although the emphasis of this chapter is on freshwater zooplankton, lipid research
in marine systems is included. Additional information on the ecophysiology of
lipids in marine crustacean zooplankton can be found in Arts (1997). Although
concepts presented here derive from work on zooplankton, reference is also made
to other groups of organisms.
4.2. Usefulness of Areal Energy Reserve Estimates
Lipid levels in invertebrates are usually expressed as lipid content (i.e., the
amount of lipid per animal) or as lipid concentration (i.e., the amount of lipid per
unit mass of tissue). Lipid concentration has been variously measured as lipid/dry
weight, lipid/ash-free dry weight, lipid/lipid-free dry weight, and lipid/wet
weight, with no clear consensus as to which measure is most appropriate. Lipid/
lipid-free dry weight may be preferred because this measure suffers less from
problems of autocorrelation. Depending on the application, one expression may
be preferred over the other, although, in principle, if the weights of the organisms
are provided, conversion from one expression to the other is straightforward.
Expression of lipid as lipid content is useful when one desires to characterize
absolute differences in lipid, whereas the use of lipid concentration is more suited
to providing information on the relative change in lipid over the season or in
M.T. Arts
There is currently a proliferation of interest in the role of essential fatty acids
(EFAs) in animal health (Hall, 1996) and in human health (Layne et aI., 1996;
Osterud et aI., 1995; Renaud et aI., 1995). Recently, researchers have begun to
examine the role of EFAs in structuring zooplankton (Hagen et aI., 1995; MiillerNavarra, 1995a; Norsker and Stottrup, 1994) and fish (Bell and Sargent, 1996;
Schwalme, 1994) populations and communities. Much research has also focused
on algal EFA content because variations in EFA concentration and composition at
this trophic level largely determine the amount and type of EFAs available for the
rest of the food chain. The concept has been exploited by the mariculture industry,
in which intense investigations centered around manipulating EFAs in feed stock
are well underway to guarantee EFA availability for a variety of cultured organisms (Reis et aI., 1996; and see Olsen, this volume).
The role of lipids as solvents for lipophilic contaminants is another reason to
examine seasonal patterns of lipids in zooplankton. The sensitivity of lipids to
factors that affect the feeding success of zooplankton makes them a useful stress
detector, but only if the other conditions known to affect lipid levels are also
carefully monitored and controlled. Thus, with the proper controls and ecological
insights, lipids can be an excellent indicator of stress, particularly chronic stress
(see Desy, 1996; Himbeault, 1995; Dauble et aI., 1985).
My primary goal in this chapter is to illustrate selected aspects of lipid studies
involving, primarily, zooplankton and to convince the reader of the usefulness of
incorporating some measure of lipid content/composition in future studies. I also
provide suggestions for what I believe to be important new research avenues in the
field of lipids and highlight areas of research that are undergoing rapid expansion.
Although the emphasis of this chapter is on freshwater zooplankton, lipid research
in marine systems is included. Additional information on the ecophysiology of
lipids in marine crustacean zooplankton can be found in Arts (1997). Although
concepts presented here derive from work on zooplankton, reference is also made
to other groups of organisms.
4.2. Usefulness of Areal Energy Reserve Estimates
Lipid levels in invertebrates are usually expressed as lipid content (i.e., the
amount of lipid per animal) or as lipid concentration (i.e., the amount of lipid per
unit mass of tissue). Lipid concentration has been variously measured as lipid/dry
weight, lipid/ash-free dry weight, lipid/lipid-free dry weight, and lipid/wet
weight, with no clear consensus as to which measure is most appropriate. Lipid/
lipid-free dry weight may be preferred because this measure suffers less from
problems of autocorrelation. Depending on the application, one expression may
be preferred over the other, although, in principle, if the weights of the organisms
are provided, conversion from one expression to the other is straightforward.
Expression of lipid as lipid content is useful when one desires to characterize
absolute differences in lipid, whereas the use of lipid concentration is more suited
to providing information on the relative change in lipid over the season or in