Environ. Sci. Technol. 1993, 27, 1621-1624
Lake Acidification: Effects on Crustacean Zooplankton Populations Karl E. Havens,. Norman D. Yan,t and Wendel Keller*
Department of Blologlcal Sciences and Water Resources Research Institute, Kent State University, Kent, Ohio 44242, Ontario Ministry of the Environment, Dorset, Ontario POA 1EO Canada, and Ontario Ministry of the Environment, Sudbury, Ontario P3E 5P9 Canada The ranked acid sensitivities of six common crustacean zooplankton taxa were determined from a multilake field survey in Ontario and from laboratory bioassays. The two approaches gave the same ranking (from most to least sensitive): Daphnia galeata mendotae, Daphnia retrocurua, and Skistodiaptomus oregonensis > Diaphanosoma birgei > Mesocyclops edax > Bosmina longirostris. This finding suggests that acidification has caused the widespread damage which has been documented for the zooplankton of Ontario and northeastern U.S.lakes.
Introduction Thousands of North American soft-water lakes have been acidified by the deposition of acid from the atmosphere (1-4). A scarcity of historical data prevents a quantification of the actual severity of biological damage. Models that link regional water quality with biodiversity (5) do suggest that many species have been lost from the acidified lakes. However, those models have a correlational not a causal basis. Because many variables beside acidity might be responsible for the correlations, the modeled species losses must be regarded as only hypotheses. To convert hypotheses to documentation, we must bridge the gap between correlation and cause. Recently, Keller et al. (6)presented a method; they compared results from acid toxicity tests with field distributional data. The latter showed that the crustacean zooplankter D. galeata mendotae declined in lakes of pH C6.0 and was virtually absent at pH C5.5. In toxicity tests, the median lethal pH level (LCso) was 5.85. The close agreement of lab and field results indicated that acidity is the major factor determining D. galeata mendotae distribution in softwater lakes. The present study extends that approach to other crustacean taxa important to acid-sensitive regions of North America and tests the hypothesis that field surveys and laboratory bioassays give the same ranked acid sensitivity of taxa. Methods Distributional data for six crustacean taxa (Bosmina longirostris, Daphnia galeata mendotae, Daphnia retrocurua, Diaphanosoma birgei, Skistodiaptomus oregonensis, and Mesocyclops edax) were obtained from a multi-lake survey in the south-central, Algoma, and Sudbury regions of Ontario. Animals were collected by net tows (76 pm mesh) in a single mid-summer visit to each lake. Further methodological details were given previously (6). To remove the influence of trace-metal contamination associated with the Sudbury smelters, Sudbury lakes with Cu concentrations 1 5 pg L-l were excluded from the data set, as were lakes with pH >7.5 or conductivity >50 NS. The latter was done to restrict t Ontario Ministry of
the Environment, Doreet. Ontario Ministry of the Environment, Sudbury.
0013-936X/93/0927-1621$04.00/0
0 1993 American Chemical Society
the analyses to acid-sensitive lakes. The final data set contained 305 lakes. The ranked acid sensitivities of the zooplankton taxa were determined as follows. For each taxon, we converted the abundance data to presencelabsence data by coding the presence to 2 and the absence to 1for each lake. Logit regression models (7, 8) were then generated to predict the probability of presence of each species from lake-water pH. From the model outputs, we determined the pH at which the probability of each taxon’s presence was reduced by 50% from that at pH 7.0. pH 7.0 was chosen because it represents the current median pH of acid-sensitive lakes in low-sulfur deposition areas of northwestern Ontario (2).
This overall approach was more appropriate than a statistical analysis based on the raw abundance data. Single mid-summer visits to lakes may not give accurate estimates of species abundance, but they have been found to provide adequate information regarding presence and absence (9). Toxicity tests were performed on three dates for each taxon, using Havas’s Mirror Lake Medium, a simulated soft-water lake solution (10). The pH was adjusted to 7.0, 6.5, 6.2, 6.0, 5.7, 5.5, 5.2, 5.0, 4.7, and 4.5 with ultrapure HzS04. These pH values cover a range which is typical for lakes in acid-sensitive regions of North America with high-sulfur deposition. Test animals were obtained from East Twin Lake, OH (pH >8,128 mg L-l CaC03,19-24 “ C on the collection dates), and were placed in groups of 10 into 50-mL vessels of the media. Adult animals were used in the bioassays. Exposures lasted 48 h (at 19-22 “C), and pH was readjusted at 12-h intervals (drift was CO.1 unit). At 48 h, the animals were observed at X30 magnification; those which lacked movement were classified as dead. Median lethal pH levels (Lc50) were calculated using probit analysis (11). Low control mortalities (averaging 6 % ) indicated that lack of an acclimation period and use of populations from an alkaline lake did not result in stress from transfer to the soft-water medium. Further, preliminary studies showed that mortalities in experimental vessels stabilized after 24 h (in pairwise comparisons of 24- and 48-h L C ~ pH O values for the various taxa, p values associated with t-tests always exceeded 0.20). This indicates that ionoregulatory membranes were acclimated to the test waters and that any protective effect of the source water (e.g., “tightening” of membrane channels by Ca ions) was gone. Hence, the results based on 48-h LC50 values are valid.
Results and Discussion The six crustacean taxa examined herein are important members of the Ontario zooplankton, occurring in 2786% of the lakes. There is great variation in their distribution along the pH gradient from 4.5 to 7.5 (Figure 1). Daphnia galeata mendotae abundance5 decline at pH 6.5 and are sharply reduced at pH below 6.2. The species is rare in lakes of pH below 5.9. Skistodiaptomus Environ. Sci. Technol., Vol. 27, No. 8. 1893
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Flgure 1. Relationshlps between lake-water pH and the abundance of six crustacean zooplankton taxa in 305 Ontario lakes. The hlstograms represent arithmetic average abundances for groups of 30 lakes, sorted by pH.
Table I. Summary of Lagit Regression Analyses: The Modeled Parameters, Their Diagnostics, and Use for Ranking Acid Sensitivity of Taxa.
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Table 11. Lethal pH Thresholds for Six Crustacean Taxa
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