92
C.E. Goulden et al.
deficiencies should ultimately enable us to better explain changes in food quality
and whether they affect feeding behavior (e.g., omnivory, excess feeding) that
could have substantial impacts on ecosystem structure and processes.
The primary food of most zooplankton is algae. It is well established that algal
biochemistry is diverse and changeable. Fundamental differences exist among the
various taxonomic groups of prokaryotic and eukaryotic algae (e.g., differences in
fatty acid compositions among algal phyla) (Wood, 1974; Erwin, 1973; also see
chapters that discuss variability of lipids in aquatic systems by Wainman et aI.,
this volume; Napolitano and Cicerone, this volume; Olsen, this volume). In a
review of laboratory studies demonstrating poor survival and reproduction of
freshwater herbivores on single algal species diets, Provasoli et al. (1959) argued
that such differences in biochemistry of apparently edible algae were major determinants of food quality and potentially induced severe dietary deficiencies in
herbivores (see Olsen, this volume). Algal biochemistry also changes depending
on growth conditions. Limitations by light or by inorganic nutrients in the medium
affect biosynthetic pathways. Under growth-limiting conditions, certain algal species accumulate large amounts of energy-rich compounds composed largely of
saturated fatty acids or carbohydrate storage products. Biosynthesis of proteins
and polyunsaturated fatty acids usually declines, and these changes are evident in
the biochemical composition of the algae (Thompson et aI., 1996, 1992; Siron et
a!., 1989; Shifrin and Chisholm, 1981). These biochemical differences in the algae
have been demonstrated in laboratory experiments to affect feeding rates and food
selectivity (Butler et a!., 1989; Cowles et aI.. 1988; Houde and Roman, 1987).
growth, and reproduction of zooplankton (Kiorboe, 1989; Scott, 1980).
We assume that similar effects on growth and reproduction should occur in
natural populations of zooplankton in both freshwater and marine habitats. However, tests offood quality effects are very difficult to design and perform in natural
habitats. Recently, MUller-Navarra (1995) has found evidence indicating a polyunsaturate fatty acid limitation in natural populations of Daphnia. This approach
is promising.
Defining food quality is complex because it can be affected by the presence of
toxic compounds, chelators, nutrients, and morphological characters that interfere
with ingestion or digestion. For the purposes of this study, we refer to a resource
as low in quality if the animal is unable to maximize growth and reproduction, per
unit of resource carbon mass, by selectively modifying its rate of ingestion of food
species or its assimilation of ingested foods. Poor food quality therefore is determined by the presence of deficiencies in the biochemical composition of the food
relative to the consumer's requirements. A deficiency should be detectable by
supplementing the diet with specific compounds that will increase a consumer's
rate of growth and reproduction.
The basis of our interests in lipids as limiting nutrients in daphniids has been
stimulated by two studies previously performed by Goulden and Place (1993,
1990). Adult daphniids accumulate lipid preferentially, relative to other biochemical components of the diet (Goulden and Place, 1993). This was determined by
measuring total lipids accumulated in adult animals during a single instar. The
C.E. Goulden et al.
deficiencies should ultimately enable us to better explain changes in food quality
and whether they affect feeding behavior (e.g., omnivory, excess feeding) that
could have substantial impacts on ecosystem structure and processes.
The primary food of most zooplankton is algae. It is well established that algal
biochemistry is diverse and changeable. Fundamental differences exist among the
various taxonomic groups of prokaryotic and eukaryotic algae (e.g., differences in
fatty acid compositions among algal phyla) (Wood, 1974; Erwin, 1973; also see
chapters that discuss variability of lipids in aquatic systems by Wainman et aI.,
this volume; Napolitano and Cicerone, this volume; Olsen, this volume). In a
review of laboratory studies demonstrating poor survival and reproduction of
freshwater herbivores on single algal species diets, Provasoli et al. (1959) argued
that such differences in biochemistry of apparently edible algae were major determinants of food quality and potentially induced severe dietary deficiencies in
herbivores (see Olsen, this volume). Algal biochemistry also changes depending
on growth conditions. Limitations by light or by inorganic nutrients in the medium
affect biosynthetic pathways. Under growth-limiting conditions, certain algal species accumulate large amounts of energy-rich compounds composed largely of
saturated fatty acids or carbohydrate storage products. Biosynthesis of proteins
and polyunsaturated fatty acids usually declines, and these changes are evident in
the biochemical composition of the algae (Thompson et aI., 1996, 1992; Siron et
a!., 1989; Shifrin and Chisholm, 1981). These biochemical differences in the algae
have been demonstrated in laboratory experiments to affect feeding rates and food
selectivity (Butler et a!., 1989; Cowles et aI.. 1988; Houde and Roman, 1987).
growth, and reproduction of zooplankton (Kiorboe, 1989; Scott, 1980).
We assume that similar effects on growth and reproduction should occur in
natural populations of zooplankton in both freshwater and marine habitats. However, tests offood quality effects are very difficult to design and perform in natural
habitats. Recently, MUller-Navarra (1995) has found evidence indicating a polyunsaturate fatty acid limitation in natural populations of Daphnia. This approach
is promising.
Defining food quality is complex because it can be affected by the presence of
toxic compounds, chelators, nutrients, and morphological characters that interfere
with ingestion or digestion. For the purposes of this study, we refer to a resource
as low in quality if the animal is unable to maximize growth and reproduction, per
unit of resource carbon mass, by selectively modifying its rate of ingestion of food
species or its assimilation of ingested foods. Poor food quality therefore is determined by the presence of deficiencies in the biochemical composition of the food
relative to the consumer's requirements. A deficiency should be detectable by
supplementing the diet with specific compounds that will increase a consumer's
rate of growth and reproduction.
The basis of our interests in lipids as limiting nutrients in daphniids has been
stimulated by two studies previously performed by Goulden and Place (1993,
1990). Adult daphniids accumulate lipid preferentially, relative to other biochemical components of the diet (Goulden and Place, 1993). This was determined by
measuring total lipids accumulated in adult animals during a single instar. The