196
Y. Olsen
lipid deposition are important factors to consider to understand lipid and EFA
dynamics of both freshwater or marine species.
8.2.8. Evaluation of Ecological Effects of Essential
Fatty Acids
Without doubt, aquatic animals must be supplied with specific EFAs of both the
0)3 and the 0)6 families to reproduce and grow. It also appears that the fatty acids
with longest chain length and number of double bonds, such as AA, DHA, and
EPA, are more essential than the C18 moieties and that most animals are able to
catabolize the longer moieties within both families into the shorter C 18 moieties.
Our quantitative understanding of animals' EFA requirements and metabolism,
including their flexibility to modify EFAs through anabolic reactions, is insufficient. This lack of knowledge also applies to the general range of variation in
physiological requirements between species of zooplankton and fish, both with
regard to the absolute EFA amounts and the relative composition of EFAs.
I suggest that the actual and potential ecological impacts of EFA deficiency
must be evaluated on the basis of a sound knowledge of the physiological function
of EFA in animals and the general physiological symptoms of EFA deficiency
mentioned above. One should not evaluate EFA deficiency on the basis of the
general idea that EFAs are just other types of elemental nutrients such as carbon
(or energy), phosphorus, or nitrogen. This is because animals may suffer from
EFA deficiency, which is expected to result in a reduced rate of reproduction, even
though phosphorus or nitrogen strictly limit the biomass of the population. In
other words, EFA may affect the intrinsic growth and reproduction rates of animal
populations but never the carrying capacity of the system, which must be controlled by a biologically conservative factor (e.g., phosphorus cannot be produced
by organisms). The most obvious effect of EFA deficiency on zooplankton and
fish communities is therefore likely to be an alteration of community structure
(i.e., species composition).
The relevance ofEFA deficiency in natural communities will, however, depend
on
• Whether the EFA supplied in the food is clearly in excess relative to carbon,
energy, and other elemental supplies
• Whether there are pronounced differences in EFA requirements between species of zooplankton and fish
Regarding the first question, we know that phytoplankton species are characterized by having few dominant fatty acids in relatively high concentrations
compared with animals (Table 8.1, Napolitano, this volume). We also know that
algae generally contain high levels of EFA, although there are some green algae
and cyanobacteria that appear to have low contents (Ahlgren et aI., 1992; Olsen,
1989; Pohl, 1982). This implies that situations in which one phytoplankton species is entirely dominant for long periods of time represent a potential threat of
EFA deficiency for herbivorous zooplankton, especially if the phytoplankton
Y. Olsen
lipid deposition are important factors to consider to understand lipid and EFA
dynamics of both freshwater or marine species.
8.2.8. Evaluation of Ecological Effects of Essential
Fatty Acids
Without doubt, aquatic animals must be supplied with specific EFAs of both the
0)3 and the 0)6 families to reproduce and grow. It also appears that the fatty acids
with longest chain length and number of double bonds, such as AA, DHA, and
EPA, are more essential than the C18 moieties and that most animals are able to
catabolize the longer moieties within both families into the shorter C 18 moieties.
Our quantitative understanding of animals' EFA requirements and metabolism,
including their flexibility to modify EFAs through anabolic reactions, is insufficient. This lack of knowledge also applies to the general range of variation in
physiological requirements between species of zooplankton and fish, both with
regard to the absolute EFA amounts and the relative composition of EFAs.
I suggest that the actual and potential ecological impacts of EFA deficiency
must be evaluated on the basis of a sound knowledge of the physiological function
of EFA in animals and the general physiological symptoms of EFA deficiency
mentioned above. One should not evaluate EFA deficiency on the basis of the
general idea that EFAs are just other types of elemental nutrients such as carbon
(or energy), phosphorus, or nitrogen. This is because animals may suffer from
EFA deficiency, which is expected to result in a reduced rate of reproduction, even
though phosphorus or nitrogen strictly limit the biomass of the population. In
other words, EFA may affect the intrinsic growth and reproduction rates of animal
populations but never the carrying capacity of the system, which must be controlled by a biologically conservative factor (e.g., phosphorus cannot be produced
by organisms). The most obvious effect of EFA deficiency on zooplankton and
fish communities is therefore likely to be an alteration of community structure
(i.e., species composition).
The relevance ofEFA deficiency in natural communities will, however, depend
on
• Whether the EFA supplied in the food is clearly in excess relative to carbon,
energy, and other elemental supplies
• Whether there are pronounced differences in EFA requirements between species of zooplankton and fish
Regarding the first question, we know that phytoplankton species are characterized by having few dominant fatty acids in relatively high concentrations
compared with animals (Table 8.1, Napolitano, this volume). We also know that
algae generally contain high levels of EFA, although there are some green algae
and cyanobacteria that appear to have low contents (Ahlgren et aI., 1992; Olsen,
1989; Pohl, 1982). This implies that situations in which one phytoplankton species is entirely dominant for long periods of time represent a potential threat of
EFA deficiency for herbivorous zooplankton, especially if the phytoplankton