Box 7.3 Trophic pyramids and turnover rate in primary producers
A cascade appears to explain the loss of aquatic plants in shallow lakes.
Across a range of eutrophic to hypertrophic lakes, increased fish biomass
leads to a reduction in the density of grazing invertebrates on the surfaces of
submerged plants, an increase in periphyton on the plant surfaces and, due
to competition with periphyton, a reduction in plant biomass (Jones & Sayer,
2003 ). Data from 17 shallow lakes across this gradient were used to deter-
mine the distribution of biomass by trophic height. All lakes are of similar
depth (circa 1 m). Total biomass per m^2 was measured, calculated from size
measurements, or estimated from mean annual chlorophyll concentrations
(Jones & Sayer2003; Beresford, Davidson, Jones, Sayer & Perrow, unpublished
data). Species were allocated to discrete trophic levels according to known or
published descriptions of diet, such that 1¼primary producers,
2 ¼herbivorous/detritivorous invertebrates, 3¼predatory invertebrates,
4 ¼invertivorous fish, 5¼piscivorous fish (both invertivorous and piscivo-
rous fish feed omnivorously in these systems (Jones & Waldron,2003) but
were allocated to higher trophic levels than predatory invertebrates follow-
ing Jonssonet al.(2005). Inclusion of predatory invertebrates in the same
trophic level as fish had no significant effect on the analysis). Primary pro-
ducers were classified according to slow or fast turnover of biomass (macro-
phytes and macroalgae versus microalgae (periphyton and phytoplankton))
and the proportion of the biomass of these two groups determined. By
ranking the lakes by the proportion of slow to fast turnover of primary
producers it is possible to see a change in the trophic pyramid across this
gradient, such that relatively more biomass appears to be present within the
upper trophic levels where the turnover of primary producers is high
(Fig. 7.4). A triangular distribution of biomass within the trophic pyramid is
indicative of the presence of long loops with weak links, inferring stability
(Neutelet al., 2002), and a square or inverted pyramid of biomass is indicative
of a trophic cascade. The slope of the side of the biomass pyramid has been
suggested as a possible indication of the stability of the system (Raffaelli,
2002 ).
The slope of biomass within the trophic pyramid (determined by regres-
sion) indicates that this change in food-web structure with the rate of turn-
over of primary producer biomass is significant (Fig.7.5), suggesting that the
occurrence of a trophic cascade is related to high biomass turnover in pri-
mary producers. In such circumstances there is considerable passage of
organic matter through the web, to the upper trophic levels where it
becomes bound in relatively long-lived individuals. In the lakes with slow
turnover of primary producers the biomass is bound at the bottom of the web
BODY SIZE AND TROPHIC CASCADES IN LAKES 129