78
M.T. Arts
4.4. Lipids as Indices of Stress
Zooplankton are often indirectly affected by stressors (e.g., toxicants) that affect
algae. These effects can be of a more acute nature, as when the abundance of
edible algae declines, or more chronic and subtle, as when the FA composition of
algae is affected by the presence of the stressor (Laura et aI., 1996; Mooney et aI.,
1995; Guckert et aI., 1992). Zooplankton are also directly susceptible to a wide
variety of stressors. In either situation, a strong case can be made that lipid levels
and/or FA composition can and will respond in several ways to presence of
stressors (see below). Stressors, in this regard, are not limited to anthropogenically derived stressors; natural stressors such as starvation or exposure to
suboptimal food, salinity, nutrient deficiency, and/or exposure to cyanobacterial
toxins may also influence lipid levels and FA composition. It is therefore critical
to realize, particularly in field studies, that observed changes in lipid concentrations or FA compositions will integrate the effects of both anthropogenic and
natural stressors. This integration will also include a genetic component because
an organism's response to stressors and its ability to secure food, reproduce, and
so on will also influenced by the phenotypic expression of its genes. Finally, lipids
will play a prominent role in sequestering nonpolar contaminants (Arts et aI.,
1996, 1995), and this will have a direct effect on bioaccumulation and depuration
rates (see Landrum and Fisher, this volume).
4.4.1. Ratio of Storage to Membrane Lipids
Because storage lipids (i.e., TAG and wax esters) are the most variable of all the
lipid classes and the most responsive to factors that affect the organisms' ability to
obtain and process food, exposure to contaminants often results in a reduction in
an organism's storage lipid content. For example, Capuzzo et al. (1984) demonstrated a decline in storage lipids in American lobster when they were exposed to
petroleum hydrocarbons. It is often useful to standardize the observed changes in
storage lipids against the less variable and primarily "structural" lipid classes
(phospholipids and sterols) as proposed by Fraser (1989). Using this approach,
Guckert et aI., (1992) showed that periphyton exposed to municipal effluent had
lower endogenous energy reserves (i.e., a decreased storage to membrane lipid
ratio) than unexposed periphyton. Similarly, Himbeault (1995) showed that periphyton collected on either rocks or artificial substrates had a reduced TAG-tophospholipid ratio immediately downstream of a municipal water treatment plant.
Although this approach has not yet been widely applied in studies involving
zooplankton, the study of HimbeauIt (1995) clearly demonstrated that the TAGto-phospholipid ratio in the mayfly Baetis tricaudatus was reduced when the
animals were exposed to water containing I and 10% municipal effluent.
4.4.2. Maternal Lipid Investment
Zooplankton allocate lipids to their eggs to provide their offspring with fuel for
growth and development and to offset the potential of starvation during the early
M.T. Arts
4.4. Lipids as Indices of Stress
Zooplankton are often indirectly affected by stressors (e.g., toxicants) that affect
algae. These effects can be of a more acute nature, as when the abundance of
edible algae declines, or more chronic and subtle, as when the FA composition of
algae is affected by the presence of the stressor (Laura et aI., 1996; Mooney et aI.,
1995; Guckert et aI., 1992). Zooplankton are also directly susceptible to a wide
variety of stressors. In either situation, a strong case can be made that lipid levels
and/or FA composition can and will respond in several ways to presence of
stressors (see below). Stressors, in this regard, are not limited to anthropogenically derived stressors; natural stressors such as starvation or exposure to
suboptimal food, salinity, nutrient deficiency, and/or exposure to cyanobacterial
toxins may also influence lipid levels and FA composition. It is therefore critical
to realize, particularly in field studies, that observed changes in lipid concentrations or FA compositions will integrate the effects of both anthropogenic and
natural stressors. This integration will also include a genetic component because
an organism's response to stressors and its ability to secure food, reproduce, and
so on will also influenced by the phenotypic expression of its genes. Finally, lipids
will play a prominent role in sequestering nonpolar contaminants (Arts et aI.,
1996, 1995), and this will have a direct effect on bioaccumulation and depuration
rates (see Landrum and Fisher, this volume).
4.4.1. Ratio of Storage to Membrane Lipids
Because storage lipids (i.e., TAG and wax esters) are the most variable of all the
lipid classes and the most responsive to factors that affect the organisms' ability to
obtain and process food, exposure to contaminants often results in a reduction in
an organism's storage lipid content. For example, Capuzzo et al. (1984) demonstrated a decline in storage lipids in American lobster when they were exposed to
petroleum hydrocarbons. It is often useful to standardize the observed changes in
storage lipids against the less variable and primarily "structural" lipid classes
(phospholipids and sterols) as proposed by Fraser (1989). Using this approach,
Guckert et aI., (1992) showed that periphyton exposed to municipal effluent had
lower endogenous energy reserves (i.e., a decreased storage to membrane lipid
ratio) than unexposed periphyton. Similarly, Himbeault (1995) showed that periphyton collected on either rocks or artificial substrates had a reduced TAG-tophospholipid ratio immediately downstream of a municipal water treatment plant.
Although this approach has not yet been widely applied in studies involving
zooplankton, the study of HimbeauIt (1995) clearly demonstrated that the TAGto-phospholipid ratio in the mayfly Baetis tricaudatus was reduced when the
animals were exposed to water containing I and 10% municipal effluent.
4.4.2. Maternal Lipid Investment
Zooplankton allocate lipids to their eggs to provide their offspring with fuel for
growth and development and to offset the potential of starvation during the early