Assessing the health of a zooplankton community in a small Precambrian Shield lake during recovery from experimental acidification

Abstract

Acidified lakes recover chemically relatively quickly following the reduction or cessation of acidic inputs. Although fish, invertebrate, and phytoplankton communities are reported to begin to return to preacidification states in chemically-improving lakes, the process and extent of biological recovery are not well-documented. The experimental acidification of Precambrian Shield Lake 223 (27.4 ha surface area; 14.4 m maximum depth) in the Experimental Lakes Area in northwestern Ontario, provides an opportunity to compare the zooplankton community prior to acidification with that during progressive acidification and during chemical recovery. Acidified with sulfuric acid from pH 6.47 (ice-free season mean) in 1976 to pH 5.0 (1981 to 1983), Lake 223 has been allowed to recover in steps of pH 5.5 (1984 to 1987), pH 5.8 (1988 to 1990), and pH 6.11 (1991). Total zooplankton biomass showed no trend to increase or decrease during the acidification and recovery, but species composition changed. Compared with species composition at pH 6.13 early in acidification in 1977, the ‘recovering’ community at pH 6.11 in 1991 had the previously-dominant cladoceran species present in very low numbers and had two newly-appearing cladoceran species. The community had lost one species of calanoid and gained none and lost two species of cyclopoids and gained two. It appeared to lose four species of rotifiers and gain seven. In nearby unmanipulated reference Lake 239 (56.1 ha; 30.4 m), species shifts were recorded but they involved rarer species, not dominants as in Lake 223. Although the zooplankton community in 1991 is in a new state with respect to species composition, static measures of total community biomass, contribution to biomass by the four main taxonomic groups, per cent smilarity to the preacidification community (for crustaceans), and biomass of herbivores do not indicate impairment of community health. Lowered species diversity for both crustaceans and rotifers partially returned to preacidification levels. Nevertheless, the rotifer community in 1991 was more dissimilar to the preacidification community than was the crustacean community, and carnivore biomass appeared to be depressed in Lake 223. The Lake 223 zooplankton community at pH 6.11 in 1991 appears to be in a state of flux.