Water quality of a coastal river receiving nutrient inputs: Ammonium nitrogen transformations

Abstract

A water quality study was conducted of the Calcasieu River which discharges into a major Louisiana Gulf Coast estuary (Calcasieu Lake). The river receives both point source and nonpoint source input of nutrients. Data were used to characterize the water quality and trophic state in the river. In addition nitrification, denitrification, and nitrogen assimilation rates were estimated. Transformations of added [¹⁵N]‐labelled nitrogen in water and sediment were measured seasonally at sites on the Calcasieu River. Nitrification rates were determined by adding ¹⁵NH₄ ⁺ and measuring the rate of ¹⁵NO₃ appearance. Denitrification was determined using ¹⁵N tracer and acetylene blockage techniques. Nitrification occurred in the oxidized surface of the bottom sediment from the Calcasieu River and minimally in the overlying water. Nitrogen and phosphorus concentrations in the river suggest that the Calcasieu River may become eutrophic unless means are taken for reducing nutrient loading into the system. Results show the importance of nitrification and denitrification reactions in minimizing the effect of nitrogen input.