Recovery of Crustacean Zooplankton Species Richness in Sudbury Area Lakes following Water Quality Improvements W. Keller Ontario Ministry of the Environment, 199 Larch Street, Sudbury, Ont. P3E 5P9, Canada and N. D. Yan Ontario Ministry sf the Environment, Dorset Research Centre, P.O. Box 39, Dorset, Ont. P0,4 IEO, Cdndda Keller, W., and Pd. D. Yan. 1991. Recovery of crustacean zooplankton species richness in Sudbury area [akes following water quality improvemer-~ts. Can. J. Fish. Aquat. Sci. 48: 1635-1 444. Temporal patterns in the species richness of crustacean zooplankton communities were assessed in eight Shedbury area lakes based on data collected between 1973 and 1986. Excluding a consistently nonacidic reference lake, the study lakes showed general reductions in acidity and trace metal concentrations during this period, related to reduced contaminant emissions from the Sudbury smelters. Despite water quality improvements, several of the study lakes continue to have low pH and elevated trace metal concentrations which have inhibited recovery of zooplankton species richness. However, in sakes with more favourable current pH and loever trace metal concentrations, substantial increases in the average species richness of crustacean plankton communities have occurred, apparently due to both invasion of new species and more frequent occurrence of existing species. These results clearly demonstrate that reductions in acid-forming emissions lead to both chemical and biological improvements in some aquatic systems. Les auteurs ont evaluk les regimes temporaux de la richesse des especes composant les communaut6s de crustaces nooplanctoniques de huit lacs de la region de Sudbury en se basant sur des donnees recueillies de 1973 2 1986. Sauf pour un lac de reference regulierement non acidifie, les lacs experimentahax ont montre des baisses generales de I'acidite et des teneurs en oligo486ments pendant cette periode, baisses liees A une reduction des emissions de polluants venant des fonderies de Sudbury. Malgre des ameliorations de la qualite de I'eau, plusieurs des lacs experimentaux montrent toujours un faible pH et des teneurs elevees en oligo-elements qui ont g$ne le retablis- sement de la diversite des especes de nooplancton. Par contre, dans les lacs de pH actuel plus approprie et i teneurs en oligo-6lements moins elevees, les auteurs ont note des augmentations importsntes de la diversite moyenne des espPces de crustaces planctoniques. Selon toute appsrence, ces augmentations seraient le resultat de I'arrivee de nouvelles espgces et de la presence plus frkquente des esp&cesd e j i sur place. Les resultats obtenus prouvent qu'une reduction des emissions acidogenes amPne des ameliorations de certains systPmes aquatiqkees autant au niveau biologique que chimique. Received january 29, 1990 Accepted March 13, 7991 (JA452) H istorically, severe degradation of lake water quality, related to atmospheric emissions of S and metals from local Cu and Ni smelting operations, has occurred in the Sudbury, Ontario (Canada), area (Gorham and Gordon 1960; Beamish and Harvey 197%; Billon et al. 1979; Keller et a1 . 1980). While many Sudbury area lakes still remain highly acidic and metal contaminated, general water quality improve- ments in the region have occurred in the last decade because of substantial reductions in S and metal emissions from the smelting industry (Lderte and Dillon 1984; Dillon et al. 1986; Hutchinson and Havas 1986; KeHHer and Bitblado 1986). Observed water quality changes have often been dramatic; however, the extent of biological responses to improved water quality in Sudbury area lakes is not known. Surveys have revealed positive responses in the rotifer community (Maclsaac et al. 1986) and chrysophyte assemblages (Dixit et al. 1989) of Swan Lake and Bake trout (Sakvelinus %aamaycush) and zoo- benthos in mitepine Lake (Gunn and Keller 1990), but the responses of other aquatic communities have not yet been described. Despite overall reductions in S deposition across Rep le 29 janvier 1990 Accept6 le 93 mars 1991 North America (Hedin et al. 1987; Dillon et ah. 1988a), doc- umentation of natural biological recovery from other areas is also very rare, restricted to observations by Kehso and Jeffries (1988) of limited reinvasion by fish in two Algoma, Ontario, lakes. Experimentally, partial recovery of the zooplankton community in artificially acidified Lake 223 was observed as the lake pH was allowed to increase from -5.0 to - 5.5 (Chang and Malley 1989). Documentation of posltive biological responses is essential to demonstrating the benefits and assess- ing the adequacy s f emission control strategies. In this paper we examine temporal changes in the numerical species richness (number of species in a defined number of individuals, after Hurlbert 1971) of crustacean plankton com- munities in seven Sudbury area lakes for evidence of recovery associated with water quality improvements, and we evaluate the factors responsible. Both direct chemical effects and indi- rect effects by altered predatory and competitive interactions may be expected to influence zooplankton communities (Brett 1989). Our measure of species richness is the average number sf species appearing in individual collections over the ice-free Can. 9. Fish. Aquat. 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