The influence of drought-induced acidification on the recovery of planktonin Swan Lake (Canada)

In response to North American and Western European reductions in atmospheri c emissions of SO,, research efforts are now being focused on the recovery of aquatic ecosystems from acidification. Improvements in water quality hav e been hampered by drought-induced acidification events, but the biological consequences of such events have not been described. We present evidence o f biotic recovery in Swan Lake near Sudbury, Canada, in response to water q uality improvement, then demonstrate the damaging impacts of a 1988 re-acid ification event. Changes in species composition, richness, diversity, and m ultivariate indices were assessed from 1977 to 1997 for phytoplankton, from 1977 to 1990 for rotifers, and from 1977 to 1998 for crustacean zooplankto n. While there was some evidence of recovery in the plankton during the 198 0s, recovery was incomplete at the time of the re-acidification event. We s uspect that the severity of past acidification, ongoing water quality probl ems, and biological resistance to colonization restricted recovery. The res ponse of each taxonomic group to re-acidification varied. The recovery of b oth phytoplankton and rotifer communities was impaired by the re-acidificat ion event; both phytoplankton and rotifers reverted to a damaged state, wit h the effect on phytoplankton lasting seven years. The recovering crustacea n zooplankton community was not obviously influenced by the re-acidificatio n event, probably because most acid-sensitive taxa had not recolonized the lake at the time of re-acidification. There was, however, an unexpected res ponse of the crustacean zooplankton to re-acidification. While phytoplankto n and rotifer richness decreased, crustacean richness increased even though lake pH fell from near 6 to 4.5. We hypothesize that the; explanation is a complex interaction among chemical and physical changes associated with th e lake's re-acidification. Specifically we hypothesize that a massive hatch ing event of zooplankton resting eggs was triggered by increases in light, temperature, or oxygen concentrations at the sediment-water interface and/o r desiccation of littoral sediments during the drought.