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

In response to North American and Western European reductions in atmospheric emissions of SO,, research efforts are now being focused on the recovery of aquatic ecosystems from acidification. Improvements in water quality have been hampered by drought-induced acidification events, but the biological consequences of such events have not been described. We present evidence of biotic recovery in Swan Lake near Sudbury, Canada, in response to water quality improvement, then demonstrate the damaging impacts of a 1988 re-acidification event. Changes in species composition, richness, diversity, and multivariate indices were assessed from 1977 to 1997 for phytoplankton, from 1977 to 1990 for rotifers, and from 1977 to 1998 for crustacean zooplankton. While there was some evidence of recovery in the plankton during the 1980s, recovery was incomplete at the time of the re-acidification event. We suspect that the severity of past acidification, ongoing water quality problems, and biological resistance to colonization restricted recovery. The response of each taxonomic group to re-acidification varied. The recovery of both phytoplankton and rotifer communities was impaired by the re-acidification event; both phytoplankton and rotifers reverted to a damaged state, with the effect on phytoplankton lasting seven years. The recovering crustacean zooplankton community was not obviously influenced by the re-acidification event, probably because most acid-sensitive taxa had not recolonized the lake at the time of re-acidification. There was, however, an unexpected response of the crustacean zooplankton to re-acidification. While phytoplankton 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 the lake's re-acidification. Specifically we hypothesize that a massive hatching event of zooplankton resting eggs was triggered by increases in light, temperature, or oxygen concentrations at the sediment-water interface and/or desiccation of littoral sediments during the drought.