Understanding the factors that affect biological recovery from environmental stressors such as acidification is an important challenge in ecology. Here we report on zooplankton community recovery following the experimental acidification of Little Rock Lake, Wisconsin, USA. One decade following cessation of acid additions to the northern basin of Little Rock Lake (LRL), recovery of the zooplankton community was complete. Approximately 40% of zooplankton species in the lake exhibited a recovery lag in which biological recovery to reference basin levels was delayed by 1-6 yr after pH recovered to the level at which the species originally responded. Delays in recovery such as those we observed in LRL may be attributable to "biological resistance" wherein establishment of viable populations of key acid-sensitive species following water quality improvements is prevented by other components of the community that thrived during acidification. Indeed, we observed that the recovery of species that thrived during acidification tended to precede recovery of species that declined during acidification. In addition, correspondence analysis indicated that the zooplankton community followed different pathways during acidification and recovery, suggesting that there is substantial hysteresis in zooplankton recovery from acidification. By providing an example of a relatively rapid recovery from short-term acidification, zooplankton community recovery from experimental acidification in LRL generally reinforces the positive outlook for recovery reported for other acidified lakes.

A single stress, acidification with sulfuric acid, was applied to Little Rock Lake in a whole-ecosystem manipulation. We documented a wide range of responses to the acidification, including increases in the concentrations of various chemicals, shifts in microbial processes and a major increase in water clarity to UV-B radiation. Each of these changes could in itself be considered as a separate ecosystem stress that is distinct from the intended manipulation. Acidification in Little Rock Lake was accompanied by a number of substantial changes in the occurrence of organisms. A series of detailed investigations indicates that the mechanisms underlying these organismal changes are varied but cannot usually be tied to the direct effects of acidification. Overall, our results demonstrate how multiple stresses can arise from a single agent operating on an ecosystem and suggest that singly operating stresses may actually be quite rare.Human activities have generated a wide variety of stresses, operating from local to regional scales, that have had major effects on ecosystems (Daily 1997). Substantial scientific efforts have evaluated the effects of many of these AcknowledgmentsWe thank

Follow-up studies after whole-ecosystem-stress experiments can provide important insights into the recovery process itself and into basic ecosystem properties. We report here on zooplankton community recovery during the first 5 years following the experimental acidification of Little Rock Lake, Wisconsin, U.S.A. Acidity in the lake's treatment basin returned quickly to near pre-manipulation levels. Zooplankton population shifts, however, did not support our hypothesis that species that had increased in abundance with acidification would persist and resist the return of the pre-manipulation community. The three species that had proliferated most dramatically under low pH conditions-Daphnia catawba, Tropocyclops extensus, and Keratella taurocephala , returned close to their originally low, pre-acidification population levels during the early stages of acid recovery. Some species that had been reduced during low pH conditions, such as Diaptomus minutus and Daphnia dubia , did not recover to pre-manipulation levels. Overall, the zooplankton community in the treatment basin exhibits little similarity to that in the reference basin, a condition quite differ-ent from that which had occurred prior to the imposition of acid stress.

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Detecting and understanding long-term changes in zooplankton populations and communities requires sound sampling strategies and reliable estimates of annual abundance. We use a long-term, highly resolved data set from four north-temperate lakes to investigate the sampling requirements necessary to precisely quantify common zooplankton species and taxonomic aggregates as well as the influence that a reduced sampling regime has on our ability to investigate changes in community structure. Sample sizes required for estimating mean annual abundance (precision of 20%) ranged from 4-7 samples per year for total crustacean zooplankton abundance to as many as 34-86 samples per year for individual rotifer species, depending on the lake. Despite these often onerous sampling requirements for some taxa, patterns of population abundance were typically quite similar when compared using reduced (five samples per year) and extended (14 samples per year on average) data sets. The same 3-fold reduction in sampling produced taxa-specific shifts in community structure ranging between 6% to 20% for two-dimensional solutions and between 9% to 21% in three dimensions, depending on the lake. Although community structure was not significantly different between reduced and extended sampling scenarios, we found that the correlations between taxa and ordination axes could change and that these differences increased with the dimensionality of the ordination. Although individual investigators need to determine acceptable boundaries of sampling precision based on the objectives of their study, it appears that enumerating 5-10 sample events per year may adequately document long-term changes in populations and communities on an annual basis in many lakes.