Effect of dissolved organic matter on denitrification in lake sediments

Background/Question/Methods

Within freshwater lakes, denitrification helps to prevent buildup of nitrate within the hypolimnetic and benthic sediments. Because denitrifiers are typically heterotrophic, organic carbon availability can have a significant impact on their ability to perform denitrification. The aim of this study was to determine the potential impacts of different dissolved organic matter (DOM) sources on denitrification potentials within lakes. DOM in lakes can originate from allochthonous or autochthonous sources, and structure of organic compounds from each of these sources can differ. Bacteria can more readily access the simpler forms of DOM, which are primarily autochthonous, than the larger, more complex, recalcitrant forms of DOM, which are primarily allochthonous. In this study, the effect of DOM composition on denitrification was examined in three Ohio lakes, East Twin, West Twin, and Sandy. The DOM pool (concentration, colored DOM, % humics) and denitrification potentials of sediments (based on the acetylene block method, nitrate was added to prevent N limitation) were monitored throughout the summer of 2012. Correlations between allochthonous and autochthonous DOM fractions and denitrification potentials were assessed, to better understand the relationship between carbon sources and nitrogen cycling.

Results/Conclusions

The three lakes selected are similar in size and geographic location but differ in watershed characteristics and potentially DOM sources. Yet, denitrification potential did not differ among the three lakes, nor did it differ among dates. Denitrification potential was highly variable among sites within lakes and, in each lake, was significantly greater (on average 4 times higher) at the deeper stations (4m water depth, the approximate depth of the metalimnion) than those that were shallower (1m water depth). Colored DOM (expressed in relative fluorescence units and indicative of allochthonous DOM) tended to be higher in Sandy Lake than the other two lakes examined and differed among dates and depths. CDOM and denitrification potential were positively correlated (r=0.313, p=0.024). Chlorophyll a, dissolved organic carbon (DOC) concentration, and temperature all varied significantly among lakes and dates, but no correlation was found between these factors and denitrification potential. These data suggest that denitrification potential is significantly influence by CDOM concentration. Further research will allow us to understand the relative importance of other DOM pool features in comparison to CDOM.