EFFECT OF RAPID PH CHANGES ON PHYTOPLANKTON AND BACTERIOPLANKTON OF CLEAR AND HUMIC WATERS

The effect of rapid pH changes on the phyto- and bacterioplankton from four small forest lakes in southern Finland was studied in 6-day batc h culture experiments in spring and summer. In lake waters with the or iginal pH varying from 4.3 to 6.2, any decrease or increase in acidity decreased primary production, chlorophyll-a, phytoplankton biomass an d species richness, as well as community gross production and respirat ion. Similar trends were observed in dark fixation of inorganic carbon and densities of bacteria. No marked differences were found in the re sponse of organisms between clear and humic lakes. Most of the prevail ing algae, e.g. Isthmochloron trispinatum, Chrysococcus rufescens and Cryptomonas cf. marssonii, remained most abundant at the original pH. Few species, Glenodinium sp., Asterionella ralfsii and Chlamydomonas s pp., showed tolerance of acidity. Rapid changes in pH seem to affect t he metabolism of phytoplankton and bacteria very rapidly, and aquatic microorganisms seem to be closely adjusted to the prevailing acidity. Instead, major changes in community structure take a longer time, exte nding over the one week period used in these experiments.