Phosphorus dynamics in an ephemeral wetland ecosystem after re-flooding

To test whether large amounts of phosphorus (P) are accumulated in the floodplain during dry antecedent conditions, and whether they would be released as the predominant source of P in the overlaying floodwater, in accord with the Flood Pulse Concept, we calculated the mass balance of P in an intermittent floodplain wetland after environmental water application. The P mass balance was calculated by combining a wetland water balance model and P releasing dynamics that were estimated from glasshouse sediment inundation experiments. Upon receiving environmental water, which inundated 412 ha of swamps and river red gum (Eucalyptus camaldulensis) woodlands, our results showed that 394.5 kg of P was mobilised in the system (342.0 kg from 368.4 ha of woodland and 52.5 kg from 43.6 ha of swamp and channels). In addition, the mass of P in incoming water was 74.0 kg, giving a peak in situ mass of 468.5 kg P in the water column. The estimation was verified using the water column P calculated from field samples. Our results indicated that the majority of P (84.2%) was internal loading, and the floodplain may be a source of P enrichment for adjacent water bodies or groundwater if floodwater is discharged rapidly. However, our modelling results also suggested that the high concentration of P in the water column was not sustained most probably because soils re-adsorbed the dissolved P. Approximately 110 days for the woodland and 39 days for the swamp were needed to reduce the P levels to 20% of their peak values respectively. Environmental water managers must decide how to manage P-enriched floodwater; retaining floodwater in the floodplain for a long time could reduce the risk of P enrichment downstream receiving water bodies; or, keeping internal P loading in the wetland could cause eutrophication in the system that was initially targeted for restoration. Best management practices, such as staged flooding and flooding timing are discussed.