Seasonal water column NH4+ cycling along a semi-arid sub-tropical river–estuary continuum: Responses to episodic events and drought conditions

Abstract

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 (NH4+) 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 NH4+ concentrations and frequently balanced net NH4+ fluxes suggest minimal N cycling, but these measurements alone did not reveal the patterns of water column NH4+ uptake and regeneration in the river and estuarine systems. Rapid turnover of NH4+ supported productive estuaries, particularly during periods of droughts when riverine sources of NH4+ were minimal. However, NH4+ 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 NH4+ cycling also occurs in isolated river water columns. Lower river sites were hotspots of NH4+ 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.