TEMPORAL TRENDS IN LOW ALKALINITY LAKES OF THE UPPER MIDWEST (1983-1989)

The Upper Midwest contains a large concentration of low alkalinity lak es located across a west to east gradient of increasing deposition aci dity. We present temporal trends in the chemistry of 28 lakes (4 in Mi nnesota, 13 in Wisconsin, and 11 in Michigan) representative of the ac id-sensitive resource of the region. Lakes were sampled three times pe r year between 1983 and 1989. Temporal trends in SO42- were all negati ve in direction, consistent with a regional decline in SO2 emissions a nd atmospheric SO42- deposition. However, these trends occurred predom inantly in higher ANC (100 to 225 mueq L-1), non-seepage lakes and wer e 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 decr eased [Ca2+ + Mg2+], a response we associate with a severe drought. Di sruptions in hydrologic flowpaths caused one lake to acidify rapidly a fter inputs of ANC-rich groundwater ceased and appeared to cause ANC a nd [Ca2+ + Mg2+] declines in a second lake by reducing stream-water in flow. Our analysis was thus complicated by hydrochemical effects of cl imatic variability, which confounded trends related to acidic depositi on. Periods longer than 6 yr are needed to transcend climatic signals and verify subtle trends related to atmospheric pollutants.