Changes in composition and reactivity of allochthonous DOM in a prairie saline lake

Abstract

Inland saline lakes in semiarid regions of the Canadian prairies contain some of the highest known concentrations of dissolved organic carbon (DOC). This dissolved organic matter (DOM) represents a potentially important carbon and energy source for aquatic bacteria. Redberry Lake, an oligotrophic saline lake in central Saskatchewan, is located in a hydrologically closed basin and has high levels of DOC (seasonal mean 35 mg L−1). Despite such high DOC concentrations, lake water is clear. Lake DOM is predominantly allochthonous, and enters the lake via the major inflow, Oscar Creek. Despite its origin, this DOM is compositionally much different than its creek counterpart. Approximately 73% of total lake DOM is low in molecular weight (<1000 D). XAD‐8 isolated hydrophobic organic acids (HPOA) are low in aromaticity, have high C:N ratios and a certain percentage are old (~700 years). In comparison, creek water, despite having lower DOC concentrations than the lake (14.9 mg L−1) is highly colored. Fifty‐five per cent of this DOC is low in molecular weight and isolated DOM has higher aromaticity and lower C:N ratio than lake DOM. As a result of these changes in DOM, ultraviolet light penetrates much deeper into lake water as compared to the creek. Photolysis experiments revealed that DOM in Oscar Creek and Redberry Lake is photoreactive. Changes in lake DOM are not only linked to location within a hydrologically closed basin and photochemistry, but provide the explanation for the optically different character of DOC in this geographical region. Changes in lake DOM have had an effect at the microbial level as well. Little of the lake DOM appears available for bacterial growth as a result of these photochemical changes. Creek DOM, having a shorter residence time, does not appear to have been as photochemically changed and consequently is more available for bacterial growth.