Phosphorus export during storm events from a human perturbed watershed, southeast China: Implications for coastal ecology

• Effects of storms on P export from a Chinese human perturbed river were examined.
• Extreme storms exhibited broad fluctuation in P concentration, flux and composition.
• River water DRP and DOP had a different supply source and transport mechanism.
• Storms resulted in a 3.4–16-fold increase in P loads (with more DOP) to estuary.
• P concentration in storm runoff is elevated in high human perturbed watersheds.

Understanding how major storms impact riverine nutrient export to estuaries and the coastal region is crucial in the context of increasing anthropogenic climate and environmental perturbation. In this study, the effects of major storms on river phosphorus (P) were investigated in an agricultural river (SE China), through continuous sampling of dissolved and particulate P during the three largest storm events (A–C) in 2013. There was a major increase in the total P load (3.4–16 fold compared with baseflow). The event mean concentration of storm A was the highest likely due to the first flush effect mobilizing accumulated waste. The flux of DOP and DRP was controlled by discharge as DOP in storm B and DRP in storm C with a relatively simple hysteresis effect with higher fluxes on the rising limb being diluted by rainfall on the falling limb. DOP in storm B remained relatively constant due to delay in DOP flushed from upstream areas balancing dilution by rainfall down stream. DRP in storm C also remained relatively constant caused by successive release of soil DRP to the river from previous unsaturated surface layers. TPP export was greatest towards the early to high stages of the storm events suggesting that most of the eroded sediment and resuspended sediment-bound P are exported during the early stages of the storm. The total flux of P is elevated in watersheds with high levels of human perturbation while climate change is predicted to increase the frequency of major storms. The results of this study are important in predicting the ecosystem response of estuarine and coastal regions to major storms in the riverine catchment area.

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