Document: THO-3-74-7

Hysteresis during recovery following a whole-lake acidification experiment: Biology lags substantially behind chemistry.

FROST, T.M.*, P.K.MONTZ, T.K.KRATZ and A.R.IVES

University of Wisconsin, Madison, WI 53706 USA ¹

Abstract:
Acidity in many ecosystems throughout North America and Europe has been decreasing as rates of sulfate emissions and deposition have declined. We present evidence here, however, that recovery by a zooplankton community lagged substantially behind chemical recovery following a whole-lake acidification experiment. The treatment basin of Little Rock Lake in northern Wisconsin, USA was acidified from its original pH of 6.1 in three, 2-year stages to 5.6, 5.2, and 4.7. It has been allowed to recover without intervention since the end of 1990. By 1996, the annual average surface pH value in the Treatment Basin was more than 6 and was fairly close to the Reference Basin pH of 6.2. In contrast, a similarity index comparing the zooplankton communities in the Treatment and Reference basin remained below 0.4 in 1996, substantially less than the values of more than 0.7 that occurred throughout most of the experiment's baseline year in 1984. This lack of similarity occurred despite the presence of the same zooplankton species in both basins. Our results suggest that recovery by biological communities can be delayed for at least several years after pH recovery in acidified systems.

Keywords: Acidification, Recovery, Stress, Zooplankton

This abstract is being presented at: 8:15 AM in session:
Oral Session #54: Lake Ecology.