Environ. Sci. Technol. 1990, 24, 1259-1261
Inferred Effects of Lake Acidification on Daphnia galeafa mendotae Wendei Keiler,*st Norman D. Yan,' Keith E. Holtte,ti* and J. Roger Pitbiadoll Ontario Ministry of the Environment, Sudbury, Ontario, Canada P3E 5P9
Introduction Large numbers of Canadian Shield lakes have been acidified by the atmospheric deposition of anthropogenic sulfur (1). Biological damage attributable to acidification occurs at all levels of aquatic food webs (2, 3); however, documentation of this damage has largely been confined to areas near large point sources of air pollutants ( 4 , 5 ) , to small numbers of study lakes (6,7),or to experimentally acidified lakes (8). Demonstrations of widespread biological effects of acidification have been greatly hampered by the general absence of observations of the occurrence or abundance of important, ubiquitous species in large numbers of lakes ranging widely in acidity, coupled with laboratory determinations of lethal acid thresholds for these species (9,lO). In consequence, it has been necessary to estimate rather than to document the regional extent of biological damage in North America (11). In this report we couple determination of the lethal acid threshold of Daphnia galeata mendotae Birge, a large, ubiquitous, planktonic crustacean (12),with results of extensive lake surveys, to examine if the acidification of lakes in Ontario has resulted in widespread losses of this important member of the zooplankton. Methods D. galeata mendotae were collected in zooplankton surveys from four areas of Ontario (termed South-central, Northeastem, Algoma, and Northwestem) spanning a wide range in sulfur deposition regimes (Figure 1). Animals were collected from the South-central lakes on 2-13 occasions per annum, from 1983 to 1985 in a series of metered, 76-pm mesh net hauls through selected depths that were combined to form a composite sample that corrected for the diminution of stratum volume with depth. Animals were collected from the Northeastern (1981), Algoma (1986), and Northwestern (1981) lakes in single, vertical net hauls from 1 m above bottom to the lake surface at a central location in each lake, except in some shallow (6.0 in each data set, and it was the first to third most important contributor (11-14%) to average herbivorous zooplankton biomass in these lakes. The biomass of D. galeata mendotae declined dramatically near pH 6.0 in all the regions with low pH lakes, and it was absent from virtually all lakes with pH 6.0, D. galeata mendotae were observed in every sample collected at monthly intervals over the ice-free seasons of 1985 and 1986 (Keller and Yan, unpublished data). Third, because D. galeata mendotae is one of the most commonly observed zooplankton species in nonacidified lakes in all regions of Ontario (12,26),many colonization sources are available. Lake acidification in Ontario is attributable to atmospheric sulfur deposition, which is caused by elevated sulfur emissions in North America (1). Therefore, our data strongly suggest that widespread damage to D. galeata mendotae populations has occurred as a result of anthropogenic influences. This damage commences at comparatively low levels of acidity, near or even above pH 6.0. It is estimated that there are at least 190o0 lakes that now have pH levels
