Temporal trends in low alkalinity lakes of the Upper Midwest (1983-1989)
The Upper Midwest contains a large concentration of low alkalinity lakes located across a west to east gradient of increasing deposition acidity. We present temporal trends in the chemistry of 28 lakes (4 in Minnesota, 13 in Wisconsin, and 11 in Michigan) representative of the acid-sensitive resource of the region. Lakes were sampled three times per year between 1983 and 1989. Temporal trends in SO42- were all negative in direction, consistent with a regional decline in SO2 emissions and atmospheric SO42- deposition. However, these trends occurred predominantly in higher ANC (100 to 225 Μeq L-1), non-seepage lakes and were associated with increases in ANC and pH in only one of the 8 lakes. ANC decreased in a second group of lakes, usually in concert with decreased [Ca2++Mg2+], a response we associate with a severe drought. Disruptions in hydrologic flowpaths caused one lake to acidify rapidly after inputs of ANC-rich groundwater ceased and appeared to cause ANC and [Ca2++Mg2+] declines in a second lake by reducing stream-water inflow. Our analysis was thus complicated by hydrochemical effects of climatic variability, which confounded trends related to acidic deposition. Periods longer than 6 yr are needed to transcend climatic signals and verify subtle trends related to atmospheric pollutants.