Nitrogen budget for low and high freshwater inflows, Nueces Estuary, Texas

Nueces Estuary is a relatively shallow, microtidal estuary which receives inputs from a significant industrial and urban area and from a semi-arid drainage basin. Nitrogen (N) loadings were compiled using FLUX model and flow X concentration calculations for four annual periods spanning a range of inflow volumes. For each annual period, N budgets were developed, supported by water and total dissolved solids budgets. Budget compilations include materials transported in tidal exchange with the Gulf of Mexico and with neighboring bays. Wastewater discharge contributes twice as much N to the bay than inflows from the Nueces River. Net flows and tidal entrainment of materials from neighboring bays contribute 24-32% of total loadings except during flood flows of the Nueces River. Atmospheric deposition to the estuary surface provides 8% of N loading. Despite low tidal amplitude and a restricted Gulf pass, tidal entrainment losses are the main vehicle of export, except during flood flows, when net flows to the Gulf become important. The system exports much more of its total N load than would be expected from its water residence time, possibly facilitated by ship channel longitudinal transport. Denitrification accounts for 30-40% of all N lost. Uncertainties in components of the budget are estimated and included in compilations of confidence bounds for N budget components.