Effects of Natural Light on Nitrogen Cycling Rates in the Mississippi River Plume

• Published in: Limnology and oceanography 1997-03, Vol.42 (2), p.273-281
• Main Authors: Gardner, Wayne S., Cavaletto, Joann F., Cotner, James B., Johnson, Jeffrey R.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Bacteria; Biological and medical sciences; Bottles; Fresh water ecosystems; Freshwater; Fundamental and applied biological sciences. Psychology; Marine; Natural light; Nitrogen; Nitrogen cycle; Phytoplankton; Plumes; Quaternary ammonium compounds; Salinity; Surface water; Synecology
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.1997.42.2.0273

Isotope-dilution experiments with15NH4 +were conducted of near-surface water samples in the Mississippi River plume during May 1992 and July 1993 to quantify community cycling rates for ammonium and determine whether regeneration rates are enhanced by light. Experiments done under natural light in May showed ranges of potential uptake and regeneration rates of 0-0.4 μ M h-1 and 0-0.18 μ M h-1. Samples collected offshore from the Atchafalaya River and the Southwest Pass of the Mississippi River in July Hielded potential uptake rates of 0.4-1.8 μ M h-1 under natural light vs. 0-0.45 μ M h-1 in the dark. Ammonium regeneration rates ranged from 0.08 to 0.75 μ M h-1 in the light and from 0.02 to 0.3 μ M h-1 in the dark. The observed light/dark regeneration-rate differences imply a close coupling between phytoplankton production and ammonium regeneration. The ratio of bacterial cell-specific regeneration to uptake rates increased in the outer regions of the plume, indicating a changing contribution of bacterial-sized organisms to nitrogen cycling processes in different regions of the plume.