Long-term dissolved oxygen responses to increasing dissolved organic matter in small temperate lakes
In recent decades, dissolved organic matter (DOM) concentrations have been increasing in many northeastern North American and European lakes. These changes have been attributed to, in whole or combination, long-term reductions in atmospheric sulfate deposition, increases in air temperature, or increases in precipitation. The ecological consequences of these increases are expected to be numerous, but are only beginning to be explored. Here we use a 27-year dataset for two small, thermally stratified lakes of differing transparency (clear-water Lake Giles and brown-water Lake Lacawac) in northeastern Pennsylvania. We use three dissolved oxygen metrics to better understand where vertically in the lake dissolved oxygen concentrations are changing the most (i.e., epilimnion, metalimnion, or hypolimnion): 1) the depth of the maximum dissolved oxygen concentration in the water column, 2) the maximum percent saturation in the water column minus the minimum percent saturation in the water column, and 3) the minimum dissolved oxygen concentration in the water column. Our goal is to examine whether changes in summer dissolved oxygen concentrations are related to observed long-term increases in DOM and corresponding decreases in water transparency.
In the clear-water Lake Giles, we found a shallowing of the depth of maximum dissolved concentration and decreased minimum dissolved oxygen concentrations. These changes were strongly related to corresponding increases in DOM and decreases in water transparency. However, we observed no temporal changes in dissolved oxygen metrics for the brown-water Lake Lacawac despite decreased water transparency over the past 27 years. Increasing terrestrial DOM has not been linked to anoxia in historically well-oxygenated oligotrophic lakes, therefore our study highlights an emerging concern related to browning, particularly for lakes that are often regarded as some of the most pristine. Our results also extend the findings of earlier studies that physical lake properties are more responsive to increasing DOM in clear-water lakes than in brown-water lakes.