Effects of Lake Acidification and Recovery on the Stability of Zooplankton Food Webs

Abstract

The effect of food web structure on community stability and resilience has rarely been examined using empirical data. Yet there is a practical application for such studies insofar as resistance stability determines the ability of a system to “absorb” anthropogenic stress and adjustment stability determines the reversibility of resulting damage. The stability of zooplankton food webs in 46 Precambrian Shield lakes was 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 evidenced by decreases in species richness, numbers of predatory and competitive links, directed connectance, predator generalization, and linkage density, identified by analysis of variance. Adjustment stability was demonstrated by changes in food web attributes in lakes with higher pH in 1990 than in the 1970s. Species richness, numbers of predatory and competitive links, linkage density, and predator generalization all increased relative to the 1970s values. Food web attributes of “recovering” lakes were statistically indistinguishable from those of lakes of similar pH that had not been more acidic in the 1970s. Similar trajectories of food web change were followed during environmental degradation and recovery. Planktonic food webs of anthropogenically acidified lakes may eventually recover to resemble their pre—acidification condition, given sufficient time without acidic inputs. Whether adjustment stability is a general feature of anthropogenically stressed systems remains to be determined.