A new look at the Flood Pulse Concept : The (ir)relevance of the moving littoral in temperate zone rivers

The Flood Pulse Concept links the hydraulic river flood pulse to floodplain nutrient status. The edge of inundation, referred to as the moving littoral, causes wetting and drying and input of river derived nutrients, resulting in high floodplain productivity. If during the floods other water sources affect the floodplain, questions arise on the relative contribution of river imported nutrients. Thus, identifying the contribution of other water sources to floodplain inundation water is important to understand ecological implications of flood pulses. We hypothesize that a spatial distribution of different inundation water types exists that can be explained by interaction of river flooding with other flow processes operating in the floodplain. To test this, we used end member mixing analysis, to relate spatial and temporal inundation water quality to principal water sources and hydrological flow processes on the near natural Biebrza River floodplain in Poland, based on 1154 water sample analyses over the period 2001-2012. Cluster analysis allowed us to classify inundation water samples into water types that could be related to the principal water sources. The spatial pattern of inundation water types was linked to distinct flow processes in different parts of the floodplain. Spatial patterns appeared relatively stable over time with temporal fluctuations principally related to changes in Biebrza River discharge. The extent of river water flooding determined by our hydrochemical method corresponded well to that determined by independent hydraulic modelling. It appeared that the most productive vegetation was strongly related to the river water flood zone, but not to other inundated floodplain parts. We conclude that the moving littoral is not necessarily a good indicator of the zone of high vegetation productivity. As the river water zone is significantly smaller than the zone of inundation and not at all related to the moving littoral, we question the relevance of the moving littoral and its significance for input and turnover of nutrients