2
Fatty Acids as Trophic and Chemical
Markers in Freshwater Ecosystems
Guillermo E. Napolitano
2.1. Introduction
The elucidation of trophic relationships and the identification of sources and sinks
of organic matter are important steps for understanding the dynamics of aquatic
ecosystems (Pimm et aI.. 1991). The trophic relationships between aquatic organisms can be investigated in a number of ways. from the inspection of gut contents
to the use of biochemical. immunological (Orisley and Boyle, 1985), and stable
isotope analyses (Peterson and Fry, 1987). Some lipid species (fatty acids, fatty
alcohols, hydrocarbons, and sterols) are limited to certain taxa, so if the lipid in
question is metabolically stable (or retains its basic structure after consumption),
it may be used to trace energy transfers through the food chain, thus helping to
define predator-prey relationships.
Certain lipid markers can also indicate sources and sinks of discrete pools of
reduced carbon in different compartments of the ecosystem, including the
dissolved and particulate organic matter in the water column and sediments.
Lipids and, in particular, fatty acids have been extensively used in marine and
lacustrine biogeochemistry (Meyers and Ishiwatari, 1993; Saliot et aI., 1991).
Methods for fatty acid extraction, quantification, and identification require
substantial training and expertise (see Parrish, this volume), but they are often
simpler than the methods required for the detailed analyses of other biomolecules.
Furthermore, the study of lipids of aquatic organisms (in particular, marine fish
oils) was a challenge and, at the same time, a catalyst for the rapid development
and improvement of capillary gas chromatography (OC), the method of choice for
the analysis of complex mixtures of fatty acids. Fatty acids in aquatic organisms
typically have a chain length of between 12 and 24 carbon atoms and a variable
number of ethylenic bonds. Fatty acids rarely occur in a free form in organisms;
instead they are esterified to glycerol and other alcohols (see Parrish, this volume). Their content in different aquatic species varies greatly but normally accounts for about 2-15% of the dry weight (Lechevalier and Lechevalier, 1988).
Early studies of the composition and origin of lipids in marine phytoplankton
(Chuecas and Riley, 1969; Ackman et aI., 1968), zooplankton (Bottino, 1974),
and fish (Ackman and Burgher, 1964) provided the necessary information on the
21
Fatty Acids as Trophic and Chemical
Markers in Freshwater Ecosystems
Guillermo E. Napolitano
2.1. Introduction
The elucidation of trophic relationships and the identification of sources and sinks
of organic matter are important steps for understanding the dynamics of aquatic
ecosystems (Pimm et aI.. 1991). The trophic relationships between aquatic organisms can be investigated in a number of ways. from the inspection of gut contents
to the use of biochemical. immunological (Orisley and Boyle, 1985), and stable
isotope analyses (Peterson and Fry, 1987). Some lipid species (fatty acids, fatty
alcohols, hydrocarbons, and sterols) are limited to certain taxa, so if the lipid in
question is metabolically stable (or retains its basic structure after consumption),
it may be used to trace energy transfers through the food chain, thus helping to
define predator-prey relationships.
Certain lipid markers can also indicate sources and sinks of discrete pools of
reduced carbon in different compartments of the ecosystem, including the
dissolved and particulate organic matter in the water column and sediments.
Lipids and, in particular, fatty acids have been extensively used in marine and
lacustrine biogeochemistry (Meyers and Ishiwatari, 1993; Saliot et aI., 1991).
Methods for fatty acid extraction, quantification, and identification require
substantial training and expertise (see Parrish, this volume), but they are often
simpler than the methods required for the detailed analyses of other biomolecules.
Furthermore, the study of lipids of aquatic organisms (in particular, marine fish
oils) was a challenge and, at the same time, a catalyst for the rapid development
and improvement of capillary gas chromatography (OC), the method of choice for
the analysis of complex mixtures of fatty acids. Fatty acids in aquatic organisms
typically have a chain length of between 12 and 24 carbon atoms and a variable
number of ethylenic bonds. Fatty acids rarely occur in a free form in organisms;
instead they are esterified to glycerol and other alcohols (see Parrish, this volume). Their content in different aquatic species varies greatly but normally accounts for about 2-15% of the dry weight (Lechevalier and Lechevalier, 1988).
Early studies of the composition and origin of lipids in marine phytoplankton
(Chuecas and Riley, 1969; Ackman et aI., 1968), zooplankton (Bottino, 1974),
and fish (Ackman and Burgher, 1964) provided the necessary information on the
21