PARENT SESSION
Symposium 8: Ecosystem response and recovery: Assessing the 1991 Canada-US Air Quality Agreement
Organized by: RA Haeuber, R Birnbaum, and K Timoffee
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 517 B, Level 5, Palais des congrès de Montréal
Limits to ecosystem resilience in a boreal forest lake recovering from experimental acidification.
Turner, Michael*,1, Paterson, Michael1, Hesslein, Raymond1, Findlay, David1, Hendzel, Leonard1, McNicol, Donald2, 1 Fisheries and Oceans Canada, Winnipeg, Manitoba, Canada2 Environment Canada, Ottawa, Ontario, Canada
ABSTRACT- We examined that recovery from acidification of ecosystem structure and function would be rapid and proceed along a mirror image of the changes caused by acidification. To examine this hypothesis, a multidisciplinary team studied a small boreal forest lake (L302S) at the Experimental Lakes Area in northwestern Ontario, Canada. During a decade of sulfuric acidification, epilimnetic pH was lowered gradually from 6.8 to 4.5. During this acidification period remarkable structural and functional transformations of many ecosystem components were observed. We then allowed the lake pH to increase stepwise to 5.1, 5.8, 6.2 and 6.8 during the next decade. Despite some recovery with the increase in pH, symptoms of disturbance remained despite the relative simplicity of the experiment (e.g. there was no watershed acidification). Delays in recovery were common; i.e. recovery of biological properties was not monotonic with pH recovery. The greater delay in recovery of taxonomic richness for phytoplankton than for zooplankton was one of several surprises. The duration and features of recovery were also often difficult to predict because of constraints on recruitment of extirpated biota and food web restructuring even though sources for recolonization of extirpated species were nearbye. Predictions were also difficult because some features of recovery were indirect consequences of acidification caused by physical and chemical changes rather than pH alone. Climate variability was another force that influenced recovery through the dynamics of dissolved organic carbon. For these reasons use of pH alone is insufficient to characterize recovery of aquatic ecosystems from acidification. We further conclude that although the study lake displayed substantial resilience, it is probable that recovery of most boreal ecosystems from acidification will take longer than might have been hoped for. In many cases, recovery is unlikely to result in restoration of the pre-acidified ecosystems.
Key words: Recovery, Resilience, Acidification, Biodiversity