Balance of dissolved organic matter in inland floodplain wetlands

Dissolved organic matter (DOM) are key drivers of microbial processes in aquatic ecosystems. We measured the concentrations of dissolved organic nitrogen (DON) and phosphorus (DOP) and examined their relationships in channel and non channel floodplain habitats of inland floodplain wetlands of NSW (Gwydir, Macquarie Marshes and Lower Murrumbidgee), following environmental flows. The mean(SE) concentrations of DON were 38.5(2.2) µmol/L (range: 11.1-90.5 µmol/L, n=77) in channel habitats and 114.4(10.4) (range: 23.3-301.6 µmol/L, n=55) in non-channel floodplain habitats. The mean concentrations of DOP were 0.49(0.04) µmol/L (range: 0.12-1.60 µmol/L, n=77) in channel habitats and 3.66(0.59) (range: 0.33-17.34 µmol/L, n=55) in non-channel floodplain habitats. There were significant differences in the mean DON and DOP concentrations between the habitats (F-test, P<0.0001). For each habitat, the measured DON and DOP concentrations were related by the equation log2DON = a + blog2DOP, using ordinary least squares, where a is the intercept and b is the slope of the regression. The intercepts (a=5.849 for channel and a=5.977 for non-channel floodplain) and the slopes (b=0.538 for channel and b=0.539 for non-channel floodplain) of the two regression lines were not significantly different from each other (Student’s t-tests: P>0.28). The estimated common slope (b=0.564) of the combined equation indicated that DON concentration increase by a factor of approximately 1.5 (=20.564) for every doubling concentration of DOP in inland floodplain wetlands. Our results suggest that there are additional sources of DOM available to overlaying floodwaters in non-channel floodplain habitats, and microbial processes may be affected differently between channel and non-channel floodplain habitats due to the concomitant stoichiometric change in DOM.