Particulate organic matter dynamics in a river floodplain system: impact of hydrological connectivity

The retention efficiency of a specific reach is one key factor controlling the dynamics of particulate organic matter (POM) in running waters. Floodplains enhance the retention of riverine POM, thereby altering its structure and diagenetic state, and constitute a substantial autochthonous source. Hydrological connectivity between the river and its floodplains determines the impact of floodplains for the POM dynamics of the entire river. The elemental and isotopic (delta(13)C, delta(15)N) composition and microbial utilisation of POM was investigated in relation to hydrological connectivity in the Danube River and two floodplain segments, one of which was isolated, the other dynamically connected. The latter had been subjected to river restoration measures. An increased integration in the riverine network was the effect of the restoration. Within both floodplains, isolated, disconnected and connected conditions were distinguished depending on the location of inflow areas and the riverine water level. Hydrological connectivity, expressed as water age, significantly influenced the quantity and composition of POM in the dynamic floodplain. Carbon isotopic composition of POM clearly separated riverine and connected conditions from those disconnected and isolated, the latter representing autochthonous material mainly derived from plankton. At disconnection, the maximum contribution of phytoplankton to POC was determined (54.5% +/- 28.8 SE), which also supported the highest bacterial productivity (4.61 mug C l(-1) h(-1) +/- 0.55 SE). Connected conditions were characterised by relatively enriched, allochthonous POM (delta(13)C: -23.27parts per thousand +/- 0.98 SE). In the isolated floodplain, high standing stocks of aquatic macrophytes developed which act as 'sinks' of carbon for the river. Restoration efforts like the Danube restoration project, which increase hydrological connectivity, enhance the importance of autochthonous POM and its further transformation by re-establishing dynamically connected floodplains in regulated, temperate large rivers.