4
Lipids in Freshwater Zooplankton:
Selected Ecological and
Physiological Aspects
Michael T. Arts
4.1. Introduction
The high-energy density of lipids relative to proteins or carbohydrates coupled
with the small body size of most planktonic invertebrates makes lipids the energy
storage biomolecules of choice for freshwater and marine zooplankters. Zooplankton lipids often comprise a significant portion of their dry weight, with
estimates ranging as high as 60-65% (Arts et aI., 1993; Cavaletto et aI., 1989).
Furthermore, lipid concentrations are often reported as a percentage of dry weight,
but the dry weight of zooplankton includes a significant contribution of less
digestible chitin. Therefore, the contribution of lipid to the digestible portion of
zooplankton is substantially higher than suggested from estimates of zooplankton
lipid calculated simply as a function of total dry weight. The most variable
component of zooplankton lipid is usually the energy reserve component. This
class of lipid is dominated by the triacylglycerols (TAG) and, to a lesser extent in
fresh waters, the wax esters. Most of the variability in individual lipid content
reflects changes in TAG levels because this class of lipids most closely tracks
seasonal changes in factors such as food supply and temperature, which are
known to influence the deposition or loss of lipids (Arts et aI., 1993). Phospholipids are a more refractory lipid class because they are an important constituent of
membrane lipid.
Zooplankton lipid is primarily dietary in origin, and current research based on
3H 2 0 and 14C-acetate incorporation rates (Goulden and Place, 1990) suggests that
very little de novo synthesis of lipid occurs. Thus, the amount and type of lipid
contained within zooplankton tissues are often a good reflection of their recent
feeding success and selectivity. Various other external factors such as temperature
and salinity and internal factors such as reproductive condition will also influence
zooplankton lipid content. Dynamic interactions between these factors produce
the temporal patterns in the zooplankton lipid content, and these patterns profoundly affect the flow of energy within the ecosystems of lakes and wetlands.
The role of zooplankton as gatherers and repositories of lipid and their intermediate position in the trophic chain make them a critical link between the lower and
higher trophic levels.
71
Lipids in Freshwater Zooplankton:
Selected Ecological and
Physiological Aspects
Michael T. Arts
4.1. Introduction
The high-energy density of lipids relative to proteins or carbohydrates coupled
with the small body size of most planktonic invertebrates makes lipids the energy
storage biomolecules of choice for freshwater and marine zooplankters. Zooplankton lipids often comprise a significant portion of their dry weight, with
estimates ranging as high as 60-65% (Arts et aI., 1993; Cavaletto et aI., 1989).
Furthermore, lipid concentrations are often reported as a percentage of dry weight,
but the dry weight of zooplankton includes a significant contribution of less
digestible chitin. Therefore, the contribution of lipid to the digestible portion of
zooplankton is substantially higher than suggested from estimates of zooplankton
lipid calculated simply as a function of total dry weight. The most variable
component of zooplankton lipid is usually the energy reserve component. This
class of lipid is dominated by the triacylglycerols (TAG) and, to a lesser extent in
fresh waters, the wax esters. Most of the variability in individual lipid content
reflects changes in TAG levels because this class of lipids most closely tracks
seasonal changes in factors such as food supply and temperature, which are
known to influence the deposition or loss of lipids (Arts et aI., 1993). Phospholipids are a more refractory lipid class because they are an important constituent of
membrane lipid.
Zooplankton lipid is primarily dietary in origin, and current research based on
3H 2 0 and 14C-acetate incorporation rates (Goulden and Place, 1990) suggests that
very little de novo synthesis of lipid occurs. Thus, the amount and type of lipid
contained within zooplankton tissues are often a good reflection of their recent
feeding success and selectivity. Various other external factors such as temperature
and salinity and internal factors such as reproductive condition will also influence
zooplankton lipid content. Dynamic interactions between these factors produce
the temporal patterns in the zooplankton lipid content, and these patterns profoundly affect the flow of energy within the ecosystems of lakes and wetlands.
The role of zooplankton as gatherers and repositories of lipid and their intermediate position in the trophic chain make them a critical link between the lower and
higher trophic levels.
71