Experimental acidification of Little Rock Lake,Wisconsin: The first four years of chemical and biological recovery

Little Rock Lake was experimentally acidified in 1984–1990 during which sulfuric acid was added to one basin, decreasing pH from 6.1 to 5.6, 5.1 and 4.7. The lake has been allowed to recover without manipulation since autumn 1990. By the third year of recovery, ～40% of the change necessary to return to pre-acidification values of pH, acid neutralizing capacity (ANC), sulfate (SO₄ ^2?) and calcium (Ca²⁺) had occurred. During recovery years 1–2, ANC was closely predicted by models based on acidification phase observations, but recovery during years 3–4 was slower than predicted. A possible explanation for the slowed recovery is acidification of the upper 0–5 cm of sediment, which acts as a sink for the ANC generated via SO₄ ^2? reduction, the primary recovery mechanism. Trends for Zooplankton did not follow pH recovery very closely. Species diminished by acidification (e.g. Keralella cochlearis, Daphnia dubia) have not recovered, but species that dominated the community at pH 4.7 (e.g. K. taurocephala, D. catawba) have not maintained high populations. The time required for the Zooplankton community to recover to pre-manipulation conditions is uncertain. Delays also have been observed for the mayfly species Caenis, which had disappeared at pH 4.7. In contrast, reproductive success of largemouth bass (Micropterus salmonides) mirrored that observed during acidification; egg hatch and survival of young-of-the-year to autumn recurred when pH exceeded response levels documented during acidification. Overall, recovery has not closely followed the pattern predicted by acidification responses.