Experimental Approaches to Sediment Communities
345
Lawton (1990) has proposed that the upper bound of the constraint space
will vary with system productivity and there is some evidence for this from
freshwater studies (Cyr et al. 1997a). Exploring this proposition in marine
systems is difficult, because few have been sufficiently well documented to
permit the representation of a constraint space under different production
scenarios. One such system is the Ythan food web (Fig. 16.2). In their analysis,
Leaper and Raffaelli (1999) plotted the densities of 92 taxa, ranging in body
size from a few micrograms to several kilograms (Fig. 16.2). They concluded
that, whilst the general form of the regression relationship between body size
and density reported for other types of systems was similar for the Ythan, the
slope of the main trend line was sensitive to taxonomic resolution (Leaper
and Raffaelli 1999). Also, the shape of the Ythan constraint space differed
slightly from that proposed by Brown (1995; Fig. 16.2). More importantly, this
approach was shown to be insensitive to changes in the body size/abundance
plot brought about by whole-system enrichment. The large-scale changes in
the ecology of the estuary due to nutrient enrichment (a three-fold increase in
nitrogen), described in the preceding section, were not visually obvious as a
shift in the position of upper bound (Lawton 1990), because of the use of a
log-log plot, whilst the statistical tools available for such comparisons are not
sufficiently well developed for complex polygons (Scharf et al. 1998; Leaper
and Raffaelli 1999). Thus, whilst the constraint space provides a useful
conceptual model for thinking about body size-density relationships, it may
have limited application for detecting environmental perturbations, such as
enrichment.
16.3.2 Biomass Size Spectra
Plots of the abundance of different sized organisms in sediments (biomass
spectra) provide a useful alternative to traditional species-abundance plots
for representing benthic assemblages. Biomass spectra have the added
advantage that organism size has functional implications for tropho-dynamics (e. g. Gerlach et al. 1985; Sprules and Munawar 1986; Boudreau et al.
1991). Schwinghamer (1981) pioneered the description of body-size spectra
from marine sediments (Fig. 16.3), but there have been few subsequent
studies, probably due to the large effort involved. From the relatively few
studies carried out to date, it seems that benthic biomass spectra are conservative in shape, with a biomass trough at a body size of around 0.5-1 mm,
separating meiofaunal and macrofaunal taxa (Schwinghamer 1981, 1985,
1988). Other authors have found evidence for this biomass trough less convincing (Duplesia and Hargrave 1996; Ramsay et al. 1997; Leaper et al. 2000)
or only convincing for numbers (Le. not biomass) spectra (Raffaelli et al.
2000b). If the trough does occur, it may reflect a discontinuity in the physical
345
Lawton (1990) has proposed that the upper bound of the constraint space
will vary with system productivity and there is some evidence for this from
freshwater studies (Cyr et al. 1997a). Exploring this proposition in marine
systems is difficult, because few have been sufficiently well documented to
permit the representation of a constraint space under different production
scenarios. One such system is the Ythan food web (Fig. 16.2). In their analysis,
Leaper and Raffaelli (1999) plotted the densities of 92 taxa, ranging in body
size from a few micrograms to several kilograms (Fig. 16.2). They concluded
that, whilst the general form of the regression relationship between body size
and density reported for other types of systems was similar for the Ythan, the
slope of the main trend line was sensitive to taxonomic resolution (Leaper
and Raffaelli 1999). Also, the shape of the Ythan constraint space differed
slightly from that proposed by Brown (1995; Fig. 16.2). More importantly, this
approach was shown to be insensitive to changes in the body size/abundance
plot brought about by whole-system enrichment. The large-scale changes in
the ecology of the estuary due to nutrient enrichment (a three-fold increase in
nitrogen), described in the preceding section, were not visually obvious as a
shift in the position of upper bound (Lawton 1990), because of the use of a
log-log plot, whilst the statistical tools available for such comparisons are not
sufficiently well developed for complex polygons (Scharf et al. 1998; Leaper
and Raffaelli 1999). Thus, whilst the constraint space provides a useful
conceptual model for thinking about body size-density relationships, it may
have limited application for detecting environmental perturbations, such as
enrichment.
16.3.2 Biomass Size Spectra
Plots of the abundance of different sized organisms in sediments (biomass
spectra) provide a useful alternative to traditional species-abundance plots
for representing benthic assemblages. Biomass spectra have the added
advantage that organism size has functional implications for tropho-dynamics (e. g. Gerlach et al. 1985; Sprules and Munawar 1986; Boudreau et al.
1991). Schwinghamer (1981) pioneered the description of body-size spectra
from marine sediments (Fig. 16.3), but there have been few subsequent
studies, probably due to the large effort involved. From the relatively few
studies carried out to date, it seems that benthic biomass spectra are conservative in shape, with a biomass trough at a body size of around 0.5-1 mm,
separating meiofaunal and macrofaunal taxa (Schwinghamer 1981, 1985,
1988). Other authors have found evidence for this biomass trough less convincing (Duplesia and Hargrave 1996; Ramsay et al. 1997; Leaper et al. 2000)
or only convincing for numbers (Le. not biomass) spectra (Raffaelli et al.
2000b). If the trough does occur, it may reflect a discontinuity in the physical