Hydrological connectivity, and the exchange of organic matter and nutrients in a dynamic river–floodplain system (Danube, Austria)

Abstract

Summary 1. The relationship between hydrological connectivity, and the exchange processes of suspended sediments, organic matter and nutrients (NO3‐N) was investigated in a dynamically connected river–floodplain segment of the Danube over a 15‐month period in 1995 and 1996 in the Alluvial Zone National Park, Austria. 2. Based on water level dynamics and water retention times, three phases of river–floodplain connectivity were identified: disconnection (phase I), seepage inflow (phase II) and upstream surface connection (phase III). The frequency of occurrence of these phases was 67.5%, 29.3% and 3.2%, respectively, during the study period. 3. A conceptual model is presented linking hydrological connectivity with ecological processes. Generally, the floodplain shifts from a closed and mainly biologically controlled ecosystem during phase I to an increasingly open and more hydrologically controlled system during phases II and III. Phase I, with internal processes dominating, is designated the ‘biotic interaction phase’. 4. Phase II, with massive nutrient inputs to the floodplain yet relatively high residence times, and therefore, high algal biomass, is classified as the ‘primary production phase’. This demonstrates that water level fluctuations well below bankfull may considerably enhance floodplain productivity. 5. Finally, since transport of particulate matter is mainly restricted to short flood pulses above bankfull level, phase III has been defined as the ‘transport phase’. 6. The floodplain served as a major sink for suspended sediments (250 mt ha−−1 year−−1), FPOM (96 mt ha−−1 year−−1), particulate organic carbon (POC; 2.9 mt ha−−1 year−−1) and nitrate‐nitrogen (0.96 mt ha−−1 year−−1), but was a source for dissolved organic carbon (DOC; 240 kg ha−−1 year−−1), algal biomass (chlorophyll‐a; 0.5 kg ha−−1 year−−1) and CPOM (21 kg ha−−1 year−−1). Considerable quantities of DOC and algal biomass were exported to the river channel during phase II, whereas particulate matter transport was largely restricted to the short floods of phase III. 7. The Danube Restoration Project will create a more gradual change between the individual phases by increasing hydrological connectivity between the river channel and the floodplain, and is predicted to enhance productivity by maintaining a balance between retention and export of nutrients and organic matter.

Lateral and longitudinal patterns of water physico-chemistry and trace metal distribution and partitioning in a large river floodplain

“…Ca 2+ concentrations could thus be considered as indicator of groundwater inputs, however the local concentrations in the main channel have to be measured to draw valid conclusions. Similar differences between connected and disconnected sites were described in the Danube floodplain (Tockner et al, 1999;Weigelhofer et al, 2015). As in a former study on the Rhône River (Golterman and Meyer, 1985), pH decreased in downstream sections.…”

“…The location of the interface between the oxic and the anoxic layer is known to have a major impact on trace metal burial or remobilisation (Merz et al, 2009). Another possible mechanism is the scavenging of metals by accumulation on autochthonous organic matter, the availability of which increases with increasing of water residence time (Ahearn et al, 2006;Hein et al, 2004;Noe and Hupp, 2009;Tockner et al, 1999). Reservoirs along a river could have similar effects as they increase the residence time of water.…”

“…Secondary channels with a permanent connection to the mainstem are characterised by coarse mineral sediment, mineral suspended particulate matter (SPM), relatively low conductivity and dissolved organic carbon (DOC) and higher nutrient levels. Cut-off channels with no permanent connection are characterised by fine organic sediment, mostly organic SPM, higher conductivity due to groundwater inputs, higher DOC due to phytoplankton, and lower nutrients (Amoros and Bornette, 2002;Carrel and Juget, 1987;Hein et al, 2004;Tockner et al, 1999). While connected sites are strongly influenced by the mainstem and show comparable characteristics (Obolewski et al, 2015), disconnected sites are also influenced by local factors such as surrounding land-use or the hillslope aquifer.…”

Organic matter governs N and P balance in Danube Delta lakes

“…Other studies also observed that particulate material (N and P) was to a great extent retained in sediments during high flood and vegetation periods and then released in the fall season (SandJensen 1998;Schulz and Gucker 2005). Tockner et al (1999) found that the Danube floodplain close to Vienna behaved as a trap for POM, whereas it exported DOM and very coarse particles. These findings were widely applicable to N, P, and C dynamics in the Uzlina-Isac complex.…”

A floodplain-scale lake classification based on characteristics of macroinvertebrate assemblages and corresponding environmental properties

“…The TP-Chl a relationship was generally a linear function (Nicholls and Dillon, 1978;Schindler, 1978;Brown et al, 2000). However, in the lotic waters associated with river, Chl a was usually found to be influenced more strongly by water flow than nutrients (Pace et al, 1992;Tockner et al, 1999a;Lewis et al, 2000). The distinct relationships of log 10 TP-log 10 Chl a in five types of Yangtze lakes are compared (Fig.…”