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

Inland saline lakes in semiarid regions of the Canadian prairies contain so me of the highest known concentrations of dissolved organic carbon (DOC). T his dissolved organic matter (DOM) represents a potentially important carbo n and energy source for aquatic bacteria. Redberry Lake, an oligotrophic sa line lake in central Saskatchewan, is located in a hydrologically closed ba sin 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 allocht honous, and enters the lake via the major inflow, Oscar Creek. Despite its origin, this DOM is compositionally much different than its creek counterpa rt. 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 (similar to 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 lowe r C: N ratio than lake DOM. As a result of these changes in DOM, ultraviole t light penetrates much deeper into lake water as compared to the creek. Ph otolysis 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 explanatio n 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. Lit tle of the lake DOM appears available fur bacterial growth as a result of t hese photochemical changes. Creek DOM, having;a shorter residence time, doe s not appear to have been as photochemically changed and consequently is mo re available for bacterial growth.