Dissolved organic matter (DOM) and bacterial growth in floodplains of the Danube River under varying hydrological connectivity

Inorganic nutrients, distribution of low and high molecular weight dissolved organic matter (DOM), bulk DOM constituents, carbohydrate contents and a fluorescence index of DOM, along with bacterial secondary production (BSP), were determined in a large river floodplain system. The aim was to elucidate the significance of DOM in relation to hydrological conditions. Two floodplain segments with varying degree of connection to the main channel (and for some parameters also the main channel) of the Danube River downstream of Vienna (Austria) were investigated during two hydrologically dynamic periods. In most cases, inorganic nutrients were significantly Coupled to hydrology, but bulk dissolved organic nutrient species in two DOM size fractions were poorly correlated with changing connectivity with the main river. A more specific investigation of DOM properties (fluorescence index based on fulvic acids, carbohydrate content of dissolved organic carbon) revealed good correlation with hydrology, thus mirroring more directly the effect of changing floodplain connectivity and emphasising the importance of local (autochthonous) carbon sources at lower connectivity. BSP, measured during one of the investigated periods, was highly variable, with low values at high discharge and a significant positive dependency on increasing autochthonous DOM-sources indicated by higher fluorescence indices. Hydrological connectivity between the main river and its floodplains appears to be a crucial driving force influencing the quantity but in particular the quality of DOM as well as related microbial processes.