REDOX STRATIFICATION IN THE WATER COLUMN OF EUTROPHIC RURAL AND URBAN LAKES IN SOUTHWEST MI, USA

Agricultural and residential runoff supplies lakes with excess nutrients, especially phosphorous and nitrogen, causing algal blooms, loss of dissolved oxygen, excess turbidity and other symptoms of eutrophication. In this study, eutrophication of three lakes in southwest Michigan was examined during the summer months. Asylum Lake is a natural kettle lake located in an urban area of Kalamazoo impacted by nutrient loading from residential and commercial runoff. Brewster Lake, a natural kettle lake, and Aurohn Lake, a man-made lake, are located in rural Hastings, MI where past and current agricultural runoff has likely led to input of excess nutrients. To assess eutrophication of these three lakes, a YSI 650 MDS Probe was used to measure in situ pH, dissolved oxygen, temperature and conductivity at intervals of 0.5 m. Water column samples were obtained as a function of depth, at ~1 m intervals, using a van Dorn sampler. Four replicate samples were immediately filtered into 20 mL bottles. Two bottles were acidified with two drops of nitric acid and two bottles were unacidified. The acidified samples were used to analyze Fe⁺² colorimetrically and for analysis of Pb, Fe, Co, Cu, Na, Mg, K, and Ca using via ICP-OES. Unacidified samples were used for to analyze total alkalinity, Mn⁺², and ΣNH₃ using colorimetric methods. Anions (F^-, Cl^-, Br^-, NO₃^-, NO₂^-, PO₄^-3, SO₄^-2) were measured with an IC. The three lakes differ significantly with respect to maximum depth. The depth at which dissolved oxygen dropped below ~5% saturation was shallower in the shallower lakes. The shallowest lake, Aurohn Lake, has a maximum depth of 3.5 m; DO was <5% at just 3 m depth. Below 3 m in Aurohn Lake pH decreases to ~7. Fe⁺² and Mn⁺² exceed concentrations of ~300 and ~200 µM, respectively, in the deepest sample. Brewster Lake has a maximum depth of ~8 meters, and DO drops to <5% by 6 m depth. In the hypolimnion of Brewster Lake, pH decreases to ~7, and Mn⁺² increases to ~66 µM. The deepest lake, Asylum Lake, has a maximum depth of ~16 m. DO drops to <5% saturation by ~8 m depth. Similar to the other two lakes, pH in the bottom of Asylum lake drops to ~7, while Mn⁺² concentration reaches ~25 µM in the hypolimnion. Sulfide is detected at 11 m and is >100 µM at the bottom of the lake. Data collected in this study clearly demonstrate that all three lakes are eutrophic and anoxic in the bottom waters.