MEROMIXIS IN AN URBAN KETTLE LAKE: EFFECTS OF ROAD DE-ICING SALTS

Recent studies suggest that seasonal application of road de-icing salts in densely populated areas may disrupt well-established seasonal mixing cycles in urban lakes through the gradual elevation of lake salinity. Elevated salinity in hypolimnetic waters may lead to persistent density and redox stratification (ectogenic meromixis), stable anoxia in bottom waters, and a decrease in biological diversity. The goal of this study is to examine the influence of long-term road salt loading on the geochemical and physical mixing cycles of Woods Lake, a small, kettle lake in urban Kalamazoo, MI (surface area ~ 9.3 Hectares Z­max ~13 m). Water samples were collected monthly between May and July 2010, and approximately twice a month between August 2010 and July 2011 at 1 m intervals using a van Dorn sampler. Temperature, pH, conductivity, and dissolved oxygen values were obtained in situ using a YSI 650MDS/600QS multiparameter sonde. Filtered samples were analyzed colorimetrically for dissolved Fe²⁺, total alkalinity, ∑NH₄⁺, ∑PO₄^3-, ∑H₂S, and Mn²⁺, by IC for Cl^- and SO₄^2-, and by ICP-OES for select major ions and trace metals (Mg, Ca, K, Na, Zn, Ni, Co, Cu, and Pb). Typical summertime thermal stratification is antecedent to inverse wintertime stratification, punctuated by transition periods of thermal homogeneity (fall and spring) within the water column. However, analyses of redox sensitive species (e.g., Fe²⁺, ­Mn²⁺, SPO₄^3-, SH₂S) show that Woods Lake displays persistent redox stratification and monimolimnetic anoxia throughout the sampling period. Conductivity profiles indicate the presence of a chemocline that varies from ~8 m in spring, summer, and fall, to ~10 m in winter. Conductivity and Cl- data correlate strongly throughout the sampling period, typically with an R² > 0.95. These data imply persistent density stratification, presumably due to nutrient and road salt inputs, and that both fall and spring turnover events never fully reach completion. A previous assessment conducted in 1996 reports minimum monimolimnetic Cl- concentrations of ~140 ppm (August), compared to a minimum concentration of ~250 ppm in this study, suggesting that salt levels in the lake continue to increase with time.