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 Ϫ1 ). 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 (Ͻ1000 D). XAD-8 isolated hydrophobic organic acids (HPOA) are low in aromaticity, have high C : N ratios and a certain percentage are old (ϳ700 years). In comparison, creek water, despite having lower DOC concentrations than the lake (14.9 mg L Ϫ1 ) 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.

Sources and composition of sediment pore‐water dissolved organic matter in prairie pothole lakes

“…Alternatively, our low e280 values may also reflect the intense photo-bleaching that occurs for DOM in PPL surface waters during the summer months. Waiser and Robarts (2000) reported similar low chromophoric and high DOC values for similar lakes in Canadian PPLs. They attribute this phenomenon to photo-bleaching effects because these water bodies also lack surface-water hydrologic inputs and outlets (aside from overland runoff) that prevent the introduction of more chromophoric allochthonous DOM.…”

“…3b). Like the temporal changes observed for DOM molecular weight distributions, we suspect that less-aromatic autochthonous organic material from surface water produced at the height of the growing season enters the pore fluids (Robarts et al 1995;Waiser and Robarts 2000). This DOM pool also undergoes photo-bleaching, which further lowers its e280.…”

“…They attribute this phenomenon to photo-bleaching effects because these water bodies also lack surface-water hydrologic inputs and outlets (aside from overland runoff) that prevent the introduction of more chromophoric allochthonous DOM. Thus, our low e280 values may reflect the nature of DOM precursor composition coupled with photo-bleaching (Opsahl and Benner 1998;Waiser and Robarts 2000).…”

Chemical characterization of dissolved organic matter (DOM): A prerequisite for understanding UV-induced changes of DOM absorption properties and bioavailability

“…Terrestrial ecosystems are significant sources of CDOM (Bracchini et al, 2006;Duan et al, 2007a;Duan et al, 2007b;Sobek et al, 2007), and rivers serve as important reactors as well as transporters of terrigenous CDOM to lakes, reservoirs, and oceans (Algesten et al, 2004;Blough et al, 1993;Cauwet, 2002;Chen et al, 2004;Conmy et al, 2004;Dittmar et al, 2006;Hansell et al, 2004;Hedges et al, 1997;Hopkinson et al, 1998;Kowalczuk et al, 2003;Kowalczuk et al, 2005;Kowalczuk et al, 2006;Opsahl and Benner, 1997;Opsahl et al, 1999;Waiser and Robarts, 2000;Zanardi-Lamardo et al, 2004). Terrigenous CDOM originates from the decomposition of nonliving plant material, and is introduced into aquatic systems via leaching and runoff from land.…”

Lakes and reservoirs as regulators of carbon cycling and climate

“…Prairie lakes in North America and Eurasia may become increasingly endorheic, resulting in evapoconcentration of dissolved substances, including DOC. Hence, the frequency of lakes with high conductivity and high concentrations of recalcitrant DOC with very low color because of extensive photobleaching (Waiser and Robarts 2000) will increase. Concomitant with projected alterations in lake abundance and size in temperate zones is a predicted increase in eutrophication and drought, resulting from growing requirements for biofuels and foods and their associated fertilizer and water demands.…”