110
J.F. Cavaletto and W.S. Gardner
brates in the lake is in part responsible for their seasonal lipid content, this chapter
is organized according to habitats. Furthermore, research gaps in lipid dynamics
of freshwater benthic invertebrates are explored.
6.2. Results and Discussion
6.2.1. Slope and Profundal Zones
6.2.1.1. Crustacea. Amphipoda
The benthic amphipods Diporeia spp. are the most abundant benthic invertebrates
in the slope and profundal regions of the upper Laurentian Great Lakes (Nalepa,
1987); they also live in many other glacial lakes from temperate to subarctic zones
(Moore, 1979; Dadswell, 1974). In North America, Diporeia spp. were formerly
known as one species, Pontoporeia hoyi; however, recent changes in the taxonomy of these amphipods have identified at least two and perhaps as many as eight
species (Bousfield, 1989). Until the taxonomy of these benthic amphipods is
resolved, we will refer to the species formerly known as Pontoporeia hoyi as
Diporeia.
Diporeia inhabits the upper 2 cm of sediment and thoroughly mix this upper
sediment layer during burrowing and feeding (Robbins et aI., 1979). In addition,
Diporeia prefers feeding in fine silty sediments over coarse ones (Mozley and
Howmiller, 1977). The slope region (30-90 m) of the Great Lakes, with its cold
temperatures (4-8°C) and an adequate food supply, supports the highest Diporeia
densities throughout the year (Evans et aI., 1990; Nalepa, 1987; 10hannsson et aI.,
1985; Winne II and White, 1984). Diporeia is by far the most dominant organism
in profundal zones (>90 m); however, overall densities are lower than those
found in the slope region, likely because of diminished food supplies (Nalepa,
1991; Evans et aI., 1990).
Diporeia lipid levels vary seasonally, mainly because of changes in TAG, the
lipid reserve (Cavaletto et a!., 1996; Gardner et aI., 1985b). A seasonal increase in
Diporeia lipid reserves occurs sometime following a spring bloom of large
diatoms that settle to the bottom before the lake stratifies. However, various lag
times occur between the diatom bloom event and accumulation of lipid reserves in
the animals that appears to depend on the lake and depth (Fig. 6.1). Evidence of
this diatom sedimentation has been shown in the slope region of Lake Michigan
by examining alkane composition and biogenic silica fluxes in material collected
in a sediment trap located just below the thermocline at a 45-m-deep site in
Lake Michigan (Gardner et aI.. 1989). The ratio of aquatic plankton markers
(short alkane length; C 17 ) to terrestrial markers (long alkane length; C 29 ) indicated a large input of plankton/autochthonous material in the spring (late AprilMay), whereas the ratio during other times of the year indicated a terrestrial!
allochthonous input. In addition, high biogenic silica flux in early spring also
supported the occurrence of this event (Johengen et a!., 1994; Gardner et a!.,
J.F. Cavaletto and W.S. Gardner
brates in the lake is in part responsible for their seasonal lipid content, this chapter
is organized according to habitats. Furthermore, research gaps in lipid dynamics
of freshwater benthic invertebrates are explored.
6.2. Results and Discussion
6.2.1. Slope and Profundal Zones
6.2.1.1. Crustacea. Amphipoda
The benthic amphipods Diporeia spp. are the most abundant benthic invertebrates
in the slope and profundal regions of the upper Laurentian Great Lakes (Nalepa,
1987); they also live in many other glacial lakes from temperate to subarctic zones
(Moore, 1979; Dadswell, 1974). In North America, Diporeia spp. were formerly
known as one species, Pontoporeia hoyi; however, recent changes in the taxonomy of these amphipods have identified at least two and perhaps as many as eight
species (Bousfield, 1989). Until the taxonomy of these benthic amphipods is
resolved, we will refer to the species formerly known as Pontoporeia hoyi as
Diporeia.
Diporeia inhabits the upper 2 cm of sediment and thoroughly mix this upper
sediment layer during burrowing and feeding (Robbins et aI., 1979). In addition,
Diporeia prefers feeding in fine silty sediments over coarse ones (Mozley and
Howmiller, 1977). The slope region (30-90 m) of the Great Lakes, with its cold
temperatures (4-8°C) and an adequate food supply, supports the highest Diporeia
densities throughout the year (Evans et aI., 1990; Nalepa, 1987; 10hannsson et aI.,
1985; Winne II and White, 1984). Diporeia is by far the most dominant organism
in profundal zones (>90 m); however, overall densities are lower than those
found in the slope region, likely because of diminished food supplies (Nalepa,
1991; Evans et aI., 1990).
Diporeia lipid levels vary seasonally, mainly because of changes in TAG, the
lipid reserve (Cavaletto et a!., 1996; Gardner et aI., 1985b). A seasonal increase in
Diporeia lipid reserves occurs sometime following a spring bloom of large
diatoms that settle to the bottom before the lake stratifies. However, various lag
times occur between the diatom bloom event and accumulation of lipid reserves in
the animals that appears to depend on the lake and depth (Fig. 6.1). Evidence of
this diatom sedimentation has been shown in the slope region of Lake Michigan
by examining alkane composition and biogenic silica fluxes in material collected
in a sediment trap located just below the thermocline at a 45-m-deep site in
Lake Michigan (Gardner et aI.. 1989). The ratio of aquatic plankton markers
(short alkane length; C 17 ) to terrestrial markers (long alkane length; C 29 ) indicated a large input of plankton/autochthonous material in the spring (late AprilMay), whereas the ratio during other times of the year indicated a terrestrial!
allochthonous input. In addition, high biogenic silica flux in early spring also
supported the occurrence of this event (Johengen et a!., 1994; Gardner et a!.,