AN EXPERIMENTAL DEMONSTRATION OF TROPHIC INTERACTIONS AFFECTING WATER-QUALITY OF RICE-LAKE, ONTARIO (CANADA)

Eight cylindrical enclosures (3 m diameter, 2.7 m long, V = 20m(3)) we re installed in eutrophic Rice Lake (Ontario, Canada) in late spring o f 1987. Fish (yearling yellow perch (Perca flavescens) and macrophytes (Potamogeton crispus) presence and absence were set at the beginning of the experiment to yield four combinations of duplicate treatments. The purpose of the experiment was to determine if the phytoplankton, z ooplankton, macrophytes and fish species resident in the lake interact to influence water quality (major ions, phosphorus, algal densities a nd water clarity). The presence of fish was associated with: (1) decre ased biomass of total zooplankton, (2) decreased number of species in the zooplankton, (3) decreased average size of several zooplankton tax a, (4) higher total phosphorus concentrations, (5) higher phytoplankto n and chlorophyll alpha concentrations, (6) lower water clarity, (7) l ower potassium levels during macrophyte die-back, (8) lower pH and hig her conductivity in the presence of macrophytes. Biomass of large Daph nia species (but not total zooplankton) was highly correlated with the algal response (r(2) = 0.995) and was associated with reduced biomass of several algal taxa including some large forms (Mougeotia, Oedogoni um) and several colonial blue-green algae. However, no significant con trol of late summer growth of the bloom-forming blue-green alga Anabae na planctonica Brun. was achieved by the Daphnia presence-fish absence treatment. Release of phosphorus to the water column during the die-b ack of P. crispus was not an important phenomenon.