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

Abstract

Inorganic nutrients, distribution of low and high molecular weight dissolved organic matter (DOM), bulk DOM constituents, carbohydrate contents and a fl uorescence index of DOM, along with bacterial secondary production (BSP), were determined in a large river fl oodplain system. The aim was to elucidate the signifi cance of DOM in relation to hydrological conditions. Two fl oodplain 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 signifi cantly 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 specifi c investigation of DOM properties (fl uorescence in- dex based on fulvic acids, carbohydrate content of dissolved organic carbon) revealed good correlation with hydrol- ogy, thus mirroring more directly the effect of changing fl oodplain 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 signifi cant positive dependency on increasing autoch- thonous DOM-sources indicated by higher fl uorescence indices. Hydrological connectivity between the main river and its fl oodplains appears to be a crucial driving force infl uencing the quantity but in particular the quality of DOM as well as related microbial processes.