Littoral microcrustacea in Lake 302S in the Experimental Lakes Area of Canada: acidification and recovery

1. The littoral microcrustacean community (Cladocera and Copepoda) was examined from 1989 to 1991 in a lake experimentally acidified to pH 4.5, and from 1992 to 1997 during the early stages of pH recovery. 2. Cladoceran abundance declined significantly from 1989 to 1991 (pH 4.5), but species richness did not change. Acantholeberis curvirostris, Simocephalus serrulatus, Latona spp. (Latona setifera, L. parviremis), and all species of chydorid Cladocera declined markedly in abundance while at PH 4.5. The abundance of cyclopoid copepods was low and Ceriodaphnia and calanoid copepods were absent. 3. Recovery of the community was subsequently monitored as PH was incrementally changed to a target PH 5.1 in 1992 and 1993, and to 5.8 in 1994-97. Species richness remained unchanged. Chydorid cladocerans remained at low abundance in 1992, and only Chydorus cf. brevilabris increased substantially from 1993 to 1996. Non-chydorid Cladocera increased in abundance in 1992, declined again in 1993, then gradually increased (mainly due to Ophryoxus gracilis) in 1994-96. All species declined in 1997 as minnows recolonized the lake. The calanoid copepod Leptodiaptomus minutus was present in low numbers in 1997. 4. The microcrustacean community in the littoral zone of Lake 302S has not yet shown consistent signs of recovery from acidification.