Reversing impact of phytoplankton phosphorus limitation on coastal hypoxia due to interacting changes in surface production and shoreward bottom oxygen influx.

Phosphorus (P) limitation of phytoplankton growth is increasingly common in estuarine and coastal waters due to rising anthropogenic nitrogen input faster than that of phosphorus. However, the impact of P limitation on coastal hypoxia remains inconclusive and is challenging to observe. By combining observations with results from a three-dimensional physical-biogeochemical model off the Pearl River Estuary, we illustrate that during the summer upwelling period, the impact of P limitation reverses from suppressing hypoxia to amplifying hypoxia as P-limitation severity decreases. When P limitation is severe in the ecosystem (i.e., P limitation extensively covers the stratified waters where hypoxia tends to develop), the surface primary production and the coupled bottom oxygen consumption are diluted along the upstream-downstream axis because of the P limitation. In addition, the increased downstream bottom oxygen level enhances the shoreward bottom oxygen influx. These effects, together, reduce coastal hypoxia. In contrast, when P-limitation severity is low (i.e., P limitation is spatially constrained), the downstream relocated surface production reduces upstream hypoxia but increases downstream hypoxia, which subsequently weakens the shoreward bottom oxygen influx and hence lowers its capacity to relieve upstream hypoxia. The net effect can amplify the coastal hypoxic extent. Our results emphasize how different P-limitation severity can reverse its impact on coastal hypoxia due to the interacting changes in surface production and bottom oxygen influx. We propose the potential of using the spatial extent of P limitation as a proxy to predict its impact on coastal hypoxia and support ecosystem nutrient management.