MEPS 260:19-31 (2003)  -  doi:10.3354/meps260019

ABSTRACT: A natural planktonic community was incubated for 2 wk to study its response to different levels of pH, ranging from 8 to 9.5. A general increase in phytoplankton biomass was observed over time in the pH 8 to 9 incubations. In the pH 9.5 incubation, the phytoplankton biomass decreased close to detection limit during the first week; however, at the termination of the experiment, the initial biomass level was regained. In the pH 8 and 8.5 incubations, the diatoms Cerataulina pelagica, Cylindrotheca closterium and Leptocylindrus minimus became numerous, whereas in the pH 9 and 9.5 incubations, C. closterium solely made up the diatom biomass at the termination of the experiment. Photosynthetic dinoflagellates of the genus Ceratium, which were initially abundant, did not grow well in any of the incubations, probably due to the low nutrient concentrations. The protozooplankton biomass increased over time in the pH 8 to 9 incubations. In the pH 9.5 incubation, the protozooplankton biomass decreased close to detection limit during the first 3 d of the experiment and stayed at that level until the termination of the experiment. The biomass increase found in the pH 8 to 9 incubations was due to an increase in the number of ciliates, because the heterotrophic dinoflagellate number remained almost constant. Most protozooplankton species incubated at pH 9.5 died; however, the ciliate Myrionecta rubra survived at almost the same cell number as in the lower pH incubations. Overall a species succession occurred among both phototrophic and heterotrophic protists when pH approached 9. In the pH 9.5 incubation, the number of different protist taxa was reduced from 34 at the start of the experiment to 10 at the termination of the experiment. In conclusion, our study indicates that elevated pH (>9) in nature will affect the entire plankton community mainly by reducing the species richness and by favouring algal blooms due to loss of grazing.

KEY WORDS: High pH · Planktonic protists · Ciliates · Diatoms · Dinoflagellates · Cylindrotheca closterium · Copepods