RESPONSE OF A PHYTOPLANKTON COMMUNITY TO CONTROLLED PARTIAL RECOVERY FROM EXPERIMENTAL ACIDIFICATION

Lake 223 was experimentally acidified with H2SO4 from 1976 to 1989. The pH was reduced from 6.7 (1974) to 5.0 (1981) and held there for 3 yr (1 981-83). Beginning in 1984 the pH was allowed to increase at a controlled rate by reduction of acid additions, resulting in a two-step recovery of 4 yr at pH approximately 5.5 and 2 yr at pH approximately 5.8. During the first 6 yr of early recovery, species diversity of the phytoplankton community increased linearly with pH and the number of common species increased. Total phytoplankton biomass remained elevated above preacidification estimates or increased. Dinoflagellates and cyanophytes remained codominant with little evidence of other taxonomic groups increasing.