Long-term trends of Great Lakes major ion chemistry

Abstract

Data from U.S. and Canadian federal monitoring programs are compiled to assess long-term trends of major ions in each of the Laurentian Great Lakes. Time series are developed for the primary cations and anions as well as for specific conductance and alkalinity. When combined with historical estimates, these modern datasets provide a 150-year overview of each lake's chemical makeup. Because of their long residence times, lakes Superior, Michigan and Huron exhibit persistent increases in most ions. For lakes Erie and Ontario, several ions (chloride, sodium, calcium and sulfate), as well as specific conductance, reached peak levels between 1965 and 1975, but then began to decline. The decreases are attributable to different mechanisms: industrial point discharge reductions (sodium, chloride), atmospheric loading declines (sulfate), and the introduction of exotic dreissenid mussels (calcium). Recent data indicate that these ions are now increasing again (chloride, sodium) or have leveled off (calcium, sulfate). The results establish how much the chemical makeup of the Great Lakes has changed due to anthropogenic influences, and underscore the importance of long-term, systematic, water-quality monitoring.