enclosures set up within vegetated and open water areas of a shallow lake, Jeppesen
et al.(2002) showed that where macrophytes were present, and fish densities
naturally low, zooplankton grazing was several-fold higher than phytoplankton
biomass and production, reflecting an intense predation pressure (Fig.7.1). The
opposite was true of the open water enclosures, where high fish densities resulted
in a substantially lower biomass of zooplankton (11% of that in the macrophyte
stand), and a more than five-fold higher phytoplankton biomass.
It is worth noting that in the macrophyte enclosures of Jeppesenet al.(2002)
the production of zooplankton appeared to be supported by a source other than
phytoplankton and bacterioplankton, possibly channelled through the micro-
bial food web or from periphyton. If this is true then the cascade seen in this lake
is merely an apparent one; zooplankton populations are maintained at higher
densities than phytoplankton alone can support and can, therefore, exert a
higher grazing pressure than would otherwise be expected. Similar suggestionsFigure 7.1Cascading trophic interactions within enclosures placed in the littoral
zone of a shallow lake (Stigsholm). Data illustrate carbon flow between trophic
levels in enclosures with and without submerged macrophytes. Boxes represent
biomass (mgCL^1 ) of different trophic components and arrows represent fluxes
(mgCL^1 d^1 ). COP¼copepods, largely cyclopoids, ROT¼rotifers, CLA¼cladocerans,
HNF¼heterotrophic nanoflagellates. PVI is the proportion of the volume inhabited by
plants. Originally published in Jeppesenet al.(2002).
BODY SIZE AND TROPHIC CASCADES IN LAKES 121