124
J.E Cavaletto and W.S. Gardner
More extensive FA analysis on benthic macroinvertebrates is necessary to
develop a complete understanding of seasonal lipid and food web dynamics in
freshwater environments and how FAs may alter development and reproduction.
As in the case of Mysis, it is unclear whether their maturation time is more
dependent on food quantity or quality. An examination of FA profiles of mysids
and their potential food items in lakes with different mysid maturation times could
be beneficial in understanding why life cycle length varies in Mysis. For example,
improved growth of juvenile scallops was correlated to the occurrenCe of the
essential FA 22:6003 in their diet, rather than merely a diet abundant in lipids
(Parrish et aI. 1995). This essential FA is common to the Cryptophyceae. Seasonal
changes in algal composition and the essential FAs that they may provide could
contribute to the maturation time of Mysis either by direct consumption or indirectly through zooplankton prey.
Long-term monitoring of total lipid content in certain benthic invertebrates
could provide additional insight into ecosystem changes. Diporeia and other
amphipods may be ideal organisms for this approach. In Lake Michigan, seasonal
lipid content of Diporeia has been analyzed over several years at the same site. In
1984, the yearly mean lipid levels in Diporeia were 33.7% ± 3.3 (SE) of dry
mass, in 1988 the levels declined to 28.2% ± 1.0 (SE) of dry mass, and in 1989
levels were 25.7% ± 1.2 (SE) of dry mass (Cavaletto et aI., 1996; Gardner et aI.,
1985a). Maximum lipid levels in Diporeia reached 46% of dry mass in 1984, and
in 1988 maximum levels were only 30% of dry mass. The decline in total lipid
could reveal an increase in Diporeia population size causing intraspecific competition for food or reflect a decline in the food supply to the benthos. Diporeia
densities at this site have remained relatively stable through the 1980s (T. Nalepa,
personal communication), indicating that food availability to Diporeia may have
changed from the mid- to late 1980s. However, additional data are necessary to
reveal true trends in the lipid levels of Diporeia.
Lipid research on freshwater benthic macroinvertebrates has been focused
primarily on organisms from temperate to Arctic regions. Little or no information
is available for benthic invertebrates in subtropical and tropical lakes. Following
results from tropical zooplankton (Lee and Hirota, 1973), lipid energy reserves
may not be as important to subtropical/tropical as for temperate and Arctic benthic
organisms because the former may have a more continuous food supply. However,
it is possible that lipid variation may still occur for reasons other than climatic
temperature variation (e.g., high- and low-precipitation periods). It would be
interesting to document and compare lipid dynamics in invertebrates from
temperate/Arctic regions to those from subtropical/tropical regions. In addition,
FA profiles may be interesting to compare in benthic organisms from these two
contrasting environments. For instance, to improve membrane fluidity, the degree
of un saturation of membrane FAs appears to be inversely related to temperature
(Sargent, 1976). For this reason, FA unsaturation may be less in membrane lipids
of benthic species from subtropical/tropical than in those from temperate/Arctic
environments.
J.E Cavaletto and W.S. Gardner
More extensive FA analysis on benthic macroinvertebrates is necessary to
develop a complete understanding of seasonal lipid and food web dynamics in
freshwater environments and how FAs may alter development and reproduction.
As in the case of Mysis, it is unclear whether their maturation time is more
dependent on food quantity or quality. An examination of FA profiles of mysids
and their potential food items in lakes with different mysid maturation times could
be beneficial in understanding why life cycle length varies in Mysis. For example,
improved growth of juvenile scallops was correlated to the occurrenCe of the
essential FA 22:6003 in their diet, rather than merely a diet abundant in lipids
(Parrish et aI. 1995). This essential FA is common to the Cryptophyceae. Seasonal
changes in algal composition and the essential FAs that they may provide could
contribute to the maturation time of Mysis either by direct consumption or indirectly through zooplankton prey.
Long-term monitoring of total lipid content in certain benthic invertebrates
could provide additional insight into ecosystem changes. Diporeia and other
amphipods may be ideal organisms for this approach. In Lake Michigan, seasonal
lipid content of Diporeia has been analyzed over several years at the same site. In
1984, the yearly mean lipid levels in Diporeia were 33.7% ± 3.3 (SE) of dry
mass, in 1988 the levels declined to 28.2% ± 1.0 (SE) of dry mass, and in 1989
levels were 25.7% ± 1.2 (SE) of dry mass (Cavaletto et aI., 1996; Gardner et aI.,
1985a). Maximum lipid levels in Diporeia reached 46% of dry mass in 1984, and
in 1988 maximum levels were only 30% of dry mass. The decline in total lipid
could reveal an increase in Diporeia population size causing intraspecific competition for food or reflect a decline in the food supply to the benthos. Diporeia
densities at this site have remained relatively stable through the 1980s (T. Nalepa,
personal communication), indicating that food availability to Diporeia may have
changed from the mid- to late 1980s. However, additional data are necessary to
reveal true trends in the lipid levels of Diporeia.
Lipid research on freshwater benthic macroinvertebrates has been focused
primarily on organisms from temperate to Arctic regions. Little or no information
is available for benthic invertebrates in subtropical and tropical lakes. Following
results from tropical zooplankton (Lee and Hirota, 1973), lipid energy reserves
may not be as important to subtropical/tropical as for temperate and Arctic benthic
organisms because the former may have a more continuous food supply. However,
it is possible that lipid variation may still occur for reasons other than climatic
temperature variation (e.g., high- and low-precipitation periods). It would be
interesting to document and compare lipid dynamics in invertebrates from
temperate/Arctic regions to those from subtropical/tropical regions. In addition,
FA profiles may be interesting to compare in benthic organisms from these two
contrasting environments. For instance, to improve membrane fluidity, the degree
of un saturation of membrane FAs appears to be inversely related to temperature
(Sargent, 1976). For this reason, FA unsaturation may be less in membrane lipids
of benthic species from subtropical/tropical than in those from temperate/Arctic
environments.