Dissolved organic matter composition and photoreactivity in prairie lakes of the U.S. Great Plains

Dissolved organic matter (DOM) of 27 prairie saline lake ecosystems was investigated in the Northern and Central Great Plains of the United States using absorbance, fluorescence, lignin concentration, and stable C isotope values. The majority of variation in DOM fluorescence among lakes was due to humic (peak C) and microbially formed (peak M) fluorescent components, which appear to be derived from autochthonous primary production. Strong correlations between peak M and nutrients allow us to model total phosphorus (TP) concentration using peak M fluorescence and chromophoric dissolved organic matter (CDOM) absorption. The rate of primary production (PP) was positively correlated with peak M fluorescence and negatively with lignin concentration. Lignin phenol yields in the DOM were generally smaller than those of most freshwater systems. delta C-13 values of dissolved organic carbon (DOC) ranged from -25.0% to -20.1% and were generally enriched relative to typical freshwaters (ca. -27%). Terrestrial DOM is degraded in prairie lakes, spanning a gradient from mixotrophic to eutrophic, as determined by a color-nutrient model. The photodegradation of autochthonous DOM was significant: CO2 fluxes from these prairie lakes, modeled from peak M fluorescence, ranged from 5 to 228 mmol C m(-2) d(-1) (median, 37 mmol C m(-2) d(-1)) and was similar to community respiration estimated from protein fluorescence (median, 50 mmol C m(-2) d(-1)). The combined estimates were about 50% of the global mean total C release previously reported for saline lake ecosystems. The implication of these new results is that the global C release from saline lake ecosystems is likely underestimated.