4. Lipids in Freshwater Zooplankton
81
starting to emerge (Hessen et aI., 1997). In addition, although zooplankton are
capable of a wide variety of adaptive responses to minimize their exposure to UVB (e.g., active swimming, vertical migration), studies on the effects of UV-B
radiation on specific life stages (eggs, larvae, juveniles) are still in their infancy
(Siebeck and Bahm, 1994).
Zooplankton obtain lipids largely from their diet; therefore, indirect effects of
UV-B on the algal, cyanobacterial, and bacterial food of zooplankton are perhaps
the most likely mechanism whereby UV-B radiation may influence zooplankton
lipids. Although numerous studies have demonstrated overall reductions in gross
photosynthetic rates (reviewed in Hader, 1996), few studies have examined the
effects of UV-B radiation on carbon allocation in freshwater algae (but see Arts
and Rai, 1997). Furthermore, in two different studies, exposure to UV-B radiation
(at fluxes below that required to inhibit photosynthetic carbon assimilation)
differentially altered the FA composition by reducing the concentrations of polyunsaturated fatty acids (PUFA) of marine algae (Goes et aI., 1994; Wang and Chai,
1994).
In larvae of marine and, presumably, freshwater fish, there is a large demand for
long-chain PUFA during early development. Fish must obtain these EFAs through
their diet (see Olsen, this volume). Deficits in EFAs, in particular 22:6(03, have
been shown to affect the number of rods in the photoreceptor population of the
eyes of herring (Bell and Dick, 1993) and to hamper the feeding of herring under
low light intensities (Bell et aI., 1995). Thus, any reductions in algal PUFA as a
result of exposure to elevated levels of UVR could have profound consequences
for larval feeding success. In addition, it is tempting to speculate that visual acuity
in the zooplankton will similarly be affected by the concentration ofPUFA in their
diet. The compound eye of Daphnia, for example, is important in the maintenance
of position in the water column (vertical migration) and for predator avoidance.
In the above contexts, more studies are urgently required on the effects of UVR
on carbon allocation in algae, including the effects of UVR on FA profiles as well
as how these changes in carbon allocation patterns and FA profiles influence both
zooplankton and larval fish survivorship, growth, and fecundity.
4.6. Research Needs and Suggested Future Directions
Many of the authors in this book have commented on the apparent lack of information on lipid dynamics in freshwater compared with marine ecosystems. Although this certainly has been true, it is also apparent that freshwater lipid researchers have made significant strides in recent years. The advent of microgravimetric extraction and quantification (Gardner et aI., 1985; Cavalleto and
Gardner, this volume), the Iatroscan TLC-FID analyzer, and advances in gas
chromatography techniques (Parrish, this volume) have greatly facilitated studies
on total lipid, lipid classes, and FAs, respectively, of zooplankton, other invertebrates, and algae. These types of studies have allowed researchers to begin to gain
an understanding of the role of various algal and cyanobacterial species as factors
81
starting to emerge (Hessen et aI., 1997). In addition, although zooplankton are
capable of a wide variety of adaptive responses to minimize their exposure to UVB (e.g., active swimming, vertical migration), studies on the effects of UV-B
radiation on specific life stages (eggs, larvae, juveniles) are still in their infancy
(Siebeck and Bahm, 1994).
Zooplankton obtain lipids largely from their diet; therefore, indirect effects of
UV-B on the algal, cyanobacterial, and bacterial food of zooplankton are perhaps
the most likely mechanism whereby UV-B radiation may influence zooplankton
lipids. Although numerous studies have demonstrated overall reductions in gross
photosynthetic rates (reviewed in Hader, 1996), few studies have examined the
effects of UV-B radiation on carbon allocation in freshwater algae (but see Arts
and Rai, 1997). Furthermore, in two different studies, exposure to UV-B radiation
(at fluxes below that required to inhibit photosynthetic carbon assimilation)
differentially altered the FA composition by reducing the concentrations of polyunsaturated fatty acids (PUFA) of marine algae (Goes et aI., 1994; Wang and Chai,
1994).
In larvae of marine and, presumably, freshwater fish, there is a large demand for
long-chain PUFA during early development. Fish must obtain these EFAs through
their diet (see Olsen, this volume). Deficits in EFAs, in particular 22:6(03, have
been shown to affect the number of rods in the photoreceptor population of the
eyes of herring (Bell and Dick, 1993) and to hamper the feeding of herring under
low light intensities (Bell et aI., 1995). Thus, any reductions in algal PUFA as a
result of exposure to elevated levels of UVR could have profound consequences
for larval feeding success. In addition, it is tempting to speculate that visual acuity
in the zooplankton will similarly be affected by the concentration ofPUFA in their
diet. The compound eye of Daphnia, for example, is important in the maintenance
of position in the water column (vertical migration) and for predator avoidance.
In the above contexts, more studies are urgently required on the effects of UVR
on carbon allocation in algae, including the effects of UVR on FA profiles as well
as how these changes in carbon allocation patterns and FA profiles influence both
zooplankton and larval fish survivorship, growth, and fecundity.
4.6. Research Needs and Suggested Future Directions
Many of the authors in this book have commented on the apparent lack of information on lipid dynamics in freshwater compared with marine ecosystems. Although this certainly has been true, it is also apparent that freshwater lipid researchers have made significant strides in recent years. The advent of microgravimetric extraction and quantification (Gardner et aI., 1985; Cavalleto and
Gardner, this volume), the Iatroscan TLC-FID analyzer, and advances in gas
chromatography techniques (Parrish, this volume) have greatly facilitated studies
on total lipid, lipid classes, and FAs, respectively, of zooplankton, other invertebrates, and algae. These types of studies have allowed researchers to begin to gain
an understanding of the role of various algal and cyanobacterial species as factors