The ability of submerged macrophytes to moderate the structure of food
webs with respect to lake eutrophication management has been
intensively studied in recent years. Many lake managers have adopted
the option of increasing macrophyte abundance in order to restore
eutrophic waters, with a view to improve water quality, increase water
transparency and reduce phytoplankton biomass. These studies are mostly
based upon the comparison of macrophyterich and macrophyte-free waters.
There is less quantitative information on how different levels of
macrophyte abundance, in particular very low levels, influence
ecosystem structure, or vice versa, within the same ecosystem. This
paper takes a multivariate statistical approach to examine ecosystem
function with low macrophyte abundance (ca. 5%) in the context of
nutrient-phytoplankton-zooplankton interaction in a eutrophic shallow
lake. It shows that with low levels of macrophyte coverage,
phytoplankton biomass was only limited by nutrient (TP and Si)
availability, whilst nutrient (Si) as well as grazing (zooplankton and
Daphnia) factors were both limiting phytoplankton levels in
macrophyte-free waters. With respect to interactions between Daphnia
and chlorophyll-a, the results suggest that low macrophyte cover does
not result in a reduction of phytoplankton biomass due to the presence
of Daphnia. Rather, low macrophyte cover provides a refuge that
maintains constant Daphnia predation, so reducing chlorophyll-a
fluctuation. These results add credence to the causal interpretation of
different interactions amongst nutrients, phytoplankton and zooplankton
at very low macrophyte densities. (C) 2002 Published by Elsevier
Science Ltd.