EFFECTIVENESS OF PHYTOPLANKTON CONTROL BY LARGE-BODIED AND SMALL-BODIED ZOOPLANKTON

Employing in situ enclosures containing inocula of the lake zooplankton (mainly Daphnia galeata, Daphnia cucullata and Bosmina spp.) from a moderately eutrophic Lake Ros (Northern Poland) or large-bodied Daphina magna, the following observations on succession of phytoplankton were made: 1) whereas D. magna could control the density of all the photoplankton size classes, the lake zooplankton could not suppress the large-sized phytoplankters or net phytoplankton; 2) the lake zooplankton was able to control the density of small algae (< 50 μm), but its effect on large algae may be opposite: a promotion of net phytoplankton growth by removing small-sized algae which can out-compete net phytoplankton for limited PO4-P resources (<5 μg P l-1). Since efficiency of phytoplankton density control by D. magna decreased with an increase in net phytoplankton abundance, biomanipulation could not be successful without introducing or maintaining a high population of large-bodied cladoceran species before high densities of large algae would make the control of phytoplankton inefficient. © 1990 Kluwer Academic Publishers.