THE IMPACT OF PH ON INTERACTIONS AMONG PHYTOPLANKTON ALGAE, ZOOPLANKTON AND PERCH (PERCA-FLUVIATILIS) IN A SHALLOW, FERTILE LAKE

1. The combined effects of increasing pH (< 9-11) and increasing densi ties of perch (0, two, four per enclosure) have been investigated in p olyethylene enclosures of volume about 11001 in a shallow, fertile lak e in Cheshire. 2. Increasing PH decreased carbon dioxide concentration s, chlorophyll a concentrations and diatom biomass. It led to increase s in the proportions of Cryptophyta (to pH 10) and Chlorophyta (to PH 11). Although ample inocula were present, significant growths of cyano phytes were not recorded, contrary to expectation. 3. Increasing pH le d to increases in numbers of Daphnia hyalina particularly at pH 10 but collapse at pH 11. This persistence at PH 10, independent of perch nu mber, is attributed to reduced feeding of the fish, which survived at this pH. Other zooplankters (Ceriodaphnia spp., Bosmina longirostris, Polyphemus pediculus, Cyclops spp.) all declined with increasing pH. 4 . Increasing fish density resulted in an increase in chlorophyll a con centration and in the biomass of Chlorophyta but had no effect on cyan ophyte or cryptophyte biomass. 5. Increasing fish density led to decli nes in Daphnia in the untreated lake-water controls (pH < 9) and at pH 9, but had no effect at PH 10 where Daphnia hyalina was very abundant . Numbers of Ceriodaphnia spp. and Bosmina longirostris increased with increasing fish density, whilst numbers of Polyphemus pediculus and C yclops spp. were independent of it. 6. The results are discussed in th e light of contemporary knowledge of the factors resulting in large cy anophyte growths and of the factors controlling the stability of macro phyte-dominated and phytoplankton-dominated states in shallow lakes. T he interaction between pH and fish density and consequent non-linear r esponse of Daphnia hyalina is particularly notable because of the sign ificance of zooplankton grazing in shallow lakes.