Floodplain plant productivity is better predicted by particulate nutrients than by dissolved nutrients in floodwater

When rivers flood, nutrients exchange between the river and the floodplain, enhancing floodplain vegetation productivity, as described by the flood pulse concept. However, river water may not reach the edge of the floodplain if there are other sources of inundation water. Also, particulate nutrients travel less far from the river than dissolved nutrients and are more likely to remain available to vegetation during the growing season. This would mean that high nutrient input by river water, and associated high floodplain plant productivity, likely occurs only in the vicinity of the river channel. This paper tests 3 hypotheses derived from these observations related to the validity of the Flood Pulse concept and the relation between floodplain plant productivity and dissolved resp. particulate nutrients. We mapped particulate nutrient and dissolved nutrient concentrations in floodplain inundation water across the Biebrza floodplain (Poland), and assessed their spatial relationships with topographical parameters, aboveground biomass and species richness. Particulate nitrogen (N) and phosphorus (P) deposition and dissolved N decreased significantly with distance from the river and with elevation, and increased significantly with water depth. Dissolved P was not significantly correlated with topographical parameters or water depth. Aboveground biomass correlated significantly with particulate N and P deposition, but not with dissolved N or P. We conclude the edge of inundation is a poor predictor of the zone of high productivity. Nutrient input is higher near the river channel due to deposition of particulate nutrients, which are thus more relevant for vegetation productivity than dissolved nutrients. Spatial particulate nutrient patterns should therefore be integrated in monitoring programmes for policy and research. We further recommend refining the flood pulse concept to take into account the spatial deposition of river derived particulate nutrients, as well as non river-derived inundation water sources.