EFFECTS OF LAKE ACIDIFICATION AND RECOVERY ON THE STABILITY OF ZOOPLANKTON FOOD WEBS

The effect of food web structure on community stability and resilience has rarely been examined using empirical data. Yet there is a practic al application for such studies insofar as resistance stability determ ines the ability of a system to ''absorb'' anthropogenic stress and ad justment stability determines the reversibility of resulting damage. T he stability of zooplankton food webs in 46 Precambrian Shield lakes w as examined using data collected in the 1970s, when pH ranged from 3.8 to 7.0, and in 1990, when pH had increased by up to two units in some lakes. Acidification overcame resistance stability at pH < 5.0, as ev idenced by decreases in species richness, numbers of predatory and com petitive links, directed connectance, predator generalization, and lin kage density, identified by analysis of variance. Adjustment stability was demonstrated by changes in food web attributes in lakes with high er pH in 1990 than in the 1970s. Species richness, numbers of predator y and competitive links, linkage density, and predator generalization all increased relative to the 1970s values. Food web attributes of ''r ecovering'' lakes were statistically indistinguishable from those of l akes of similar pH that had not been more acidic in the 1970s. Similar trajectories of food web change were followed during environmental de gradation and recovery. Planktonic food webs of anthropogenically acid ified lakes may eventually recover to resemble their pre-acidification condition, given sufficient time without acidic inputs. Whether adjus tment stability is a general feature of anthropogenically stressed sys tems remains to be determined.