Significance of coupling of nitrification and nitrate reduction on water quality of a coastal lake that receives nitrate in diverted Mississippi River water

Abstract

Nitrification and denitrification rates were estimated simultaneously using a ¹⁵N dilution technique in sediment-water columns of Lake Cataouatche that receives diverted Mississippi River water. Labeled and unlabeled NO₃ were added to surface water of replicated sediment-water columns and changes in the concentrations of labeled and non-labeled nitrate in water were determined over time. The average rate of total nitrite plus nitrate NO₂ + NO₃ decrease from the water columns was 16.2 mg N m⁻³ d⁻¹, whereas ¹⁵N-labeled (NO₂ + NO₃)-N decreased at the rates of 5.8 mg ¹⁵N m⁻³ d⁻¹ over the 57 d incubation period. The averaged rates of nitrate reduction and nitrification were 51.8 μmol N m⁻² h⁻¹ (43.5 mg N m⁻³ d⁻¹) and 30.8 μ mol N m⁻² h⁻¹ (25.9 mg N m⁻³ d⁻¹), respectively. Results indicate the lake sediment has capacity to process nitrate entering the system via denitrification. Nitrification occurring simultaneously at the sediment-water interface was also a significant process representing 59.4 percent of the denitrification rate. Water quality issues associated with nitrate levels in diverted Mississippi water entering Lake Cataouatche should consider both the coupling of nitrate reduction of river water nitrate and nitrification of nitrogen in nitrogen-enriched lake sediment. Nitrification in bottom sediment is a significant nitrate source and should be regarded as important factor in the determination of the maximum daily load of nitrate that the lake can effectively assimilate without adversely affecting water quality.