An assessment of historical change in two northern Minnesota lakes

Abstract

/ Two northern Minnesota lakes that had been studied in detail 22 years earlier (1958) were restudied to determine the extent of alteration in ecological conditions. Approximately one year after the original investigation, a coal-fired power plant, which incremented sulfate loading by about 6 kg/ha-yr, began operation nine miles away. These lakes lie within a region judged susceptible to acidic precipitation, though each lake, based on its buffering capacity, would be judged only moderately sensitive. In spite of the influence of this plant and other anthropogenic inputs, the change in lake ecology was apparently minimal. Water clarity decreased in both lakes and some alteration in zooplankton community structure was observed. The long-term utility of lake surveys depends upon how carefully and completely conditions can be reconstructed from records and reports. Past surveys generally omit measures of variability for the data, allowing only r Jalitative comparisons to be drawn. In order to juoge the graded responses of aquatic ecosystems, necessary to sound management, quantitative measures are needed.Recent studies of the apparent effects of acidic deposition on the invertebrate biota of low-alkalinity lakes have raised important questions about the stability of species composition in these systems. In effect, as studies identified areas of potential change in limnological systems, possibly attributable to changes in atmospheric inputs or other local interventions, a number of investigators have questioned whether the dominant plankton species of lake systems, and other characteristics, remain stable even in the absence of anthropogenic stresses.Thus the goal of this study was to examine whether historical lake survey information could be useful in detailing specific conditions of change, relatii~e to recent surveys. A wealth of historical survey data exists on the physical, chemical, and biological state of northern lakes , but unfortunately most of these records concern only single-event summer sampling programs. An opportunity to restudy two northern Minnesota lakes that had previously been studied on a two-week interval led to the comparisons cited here. We emphasize the obstacles encountered in drawing conclusions, based upon even this relatively detailed study from 22 years ago, in order to suggest a more rigorous basis for considering questions of chemical and biological change over time.The lakes we investigated~ Hare and Echo Lakes, Lake County, Minnesota, were selected because an unusually rich information base was available .relative to those on other lakes in the region. Each lake had been intensively sampled by the Minnesota Department of Natural Resources (MDNR) during" the spring, summer, and fall of 1958 (Micklus 1961) in