Impacts of a subtropical storm on nitrogen functional microbes and associated cycling processes in a river-estuary continuum

Storms, in subtropical regions such as S.E. China, cause major changes in the physical and biogeochemical fluxes of anthropogenic N species through the river-estuary continuum to the coast. Two weeks continuous observations at a sampling station (Station E) in the upper Jiulong River Estuary (S.E. China) were conducted to track the changes of physical and biogeochemical parameters together with genomic identification of nitrogen cycling microbes through a complete storm event in June 2019. In conjunction with previous N flux measurements, it was found that there was greatly increased flux of N to and through the upper estuary during the storm. During the storm, the freshwater/brackish water boundary moved downstream, and previously deposited organic rich sediment was resuspended. During baseflow, anthropogenically derived ammonium was oxidised dominantly by the marine nitrifying (AOA) microbe Nitrosopelagicus. However, during the storm, the dominant ammonia-oxidizing archaea (AOA) at Station E changed to the riverine genus (Nitrosotenuis) while the marine genus, Nitrosopumilus decreased. At the same time the dominant ammonia-oxidizing bacteria (AOB) was still the marine genus (Nitrosomanas). Estuarine nitrifiers had higher abundance, weighted entropy and diversity during the Flood, suggesting that the high NH₄-N and DO during the Rising period of the Flood resulted in a bloom of nitrifiers. The changing gene abundances of nitrifiers were reflected in changes in the concentration and isotopic composition of DIN confirming active nitrification in the oxygen-rich water column. During the storm the numbers of denitrifiers (narG, nirS and nod), DNRA (nrfA) and anammox (hzsB) were found in the water column increased, and the larger fraction was associated with the <22 μm free-living fraction. However it was not possible with the data obtained to estimate what fraction of these anaerobic bacteria were active in the dominantly oxic water column.