Seasonal Water Column NH^sub 4^ ^sup +^ Cycling Along a Semi-arid Sub-tropical River-Estuary Continuum: Responses to Episodic Events and Drought Conditions

River-estuary continuums represent a dynamic range of environmental conditions in aquatic ecosystems, providing an ideal gradient for understanding changes in nitrogen (N) cycling. We measured rates of ammonium (NH^sub 4^ ^sup +^) cycling, including uptake and regeneration, in the water column of upper river, lower river, and estuary sites. This 1-year study encompassed periods of flood and drought, in a coastal catchment of south Texas. Low NH^sub 4^ ^sup +^ concentrations and frequently balanced net NH^sub 4^ ^sup +^ fluxes suggest minimal N cycling, but these measurements alone did not reveal the patterns of water column NH^sub 4^ ^sup +^ uptake and regeneration in the river and estuarine systems. Rapid turnover of NH^sub 4^ ^sup +^ supported productive estuaries, particularly during periods of droughts when riverine sources of NH^sub 4^ ^sup +^ were minimal. However, NH^sub 4^ ^sup +^ demand declined during storms across the river-estuary continuum, and regeneration rates were high, especially in the rivers. Most research in rivers has focused on benthic or whole-system dynamics, but our data demonstrate that active NH^sub 4^ ^sup +^ cycling also occurs in isolated river water columns. Lower river sites were hotspots of NH^sub 4^ ^sup +^ cycling on the landscape. Continued studies across river-estuary continuums are needed to enhance our understanding of aquatic systems and improve our ability to manage nutrients in the face of increased anthropogenic pressures and a changing climate.