THE ELEMENTAL CONSEQUENCES OF LAKE STRATIFICATION AND IMPLICATIONS FOR URBAN LAKES IN THE UPPER MIDWEST

Cultural meromixis, or the conversion of lakes that mix seasonally to permanent density and temperature stratification, is a well-documented phenomenon of urban lakes. A major contributor to increasing density is road salt use, but a lake’s physiography is a first-order control. Here we calculate the relative depth (Z_r) from surface area and depth data of Minnesota lakes. We will present some small lakes with Z_r > 4, which may be at risk of becoming permanently stratified due to urban road salt use.

As density stratification results from an enhancement of dissolved ions, redox-active elements may become important constituents of the dissolved ion load of anoxic bottom waters, with secondary biogeochemical consequences. Groundwater in much of MN contains dissolved iron, which is in the reduced (Fe²⁺) form. Sulfate (SO₄^2-) is routinely added to lakes through alum [KAl(SO₄)₂·12H₂O] treatments to remove phosphate, and through mining activities. Iron and sulfur speciation controls the availability of other elements in the water column. For instance, Fe³⁺-oxides adsorb the nutrient phosphorus, but in anoxic bottom waters, these minerals dissolve and release Fe²⁺ and sequestered phosphorus. Sulfate is reduced to hydrogen sulfide (H₂S) by microbial activity in anoxic conditions, which is toxic to aquatic life. Sulfate-reducing conditions also promote the methylation of mercury, which then becomes available for biological uptake. We use groundwater data from MN wells to highlight the spatial occurrence of iron-rich groundwater, and suggest lakes that may be or will become stratified and could contain dissolved iron if there is a strong connection to groundwater.

We will present a case study of Brownie Lake, in Minneapolis, which became permanently stratified due to changes in its water level, surface area, and dissolved ion load in the last 100 years. Brownie contains up to 45 mg/L iron in its bottom waters, and its bottom waters also contain the highest concentration of dissolved phosphorus (often exceeding 2.5 mg/L) of any lake monitored in the city of Minneapolis. Our recent fieldwork documents its extreme and seasonally variable methane (CH₄) fluxes. We will discuss the biogeochemical factors that control the cycling of C, P, and other elements in stratified lakes, and suggest how our region’s lakes may behave in the future.