Effects of biological and physical factors on seasonal oxygen dynamics in a stratified, eutrophic coastal ecosystem

Using a dual-budget approach based on oxygen concentrations and stable isotopes, we examined primary production, respiration, and production to respiration ratios (P : R) across the Louisiana continental shelf to determine the relative roles of biological and physical factors in controlling seasonal oxygen dynamics. Regression models for delta O-18 and P : R indicated that the concentration of algal biomass explained most of the variability of the seasonal oxygen dynamics in surface waters. Physical factors, such as salinity, temperature, water-column stability, and station depth were only of minor importance. Seasonal oxygen dynamics were more pronounced in bottom waters than in surface waters as hypoxia was common in bottom waters during summer months (May-October). During cooler winter months (November-April), oxygen concentrations throughout the water column were close to equilibrium with the atmosphere. The relative importance of benthic respiration to water-column respiration in bottom waters was more pronounced during summer months when the water column was stratified. A strong physical disturbance caused by Hurricane Claudette during the summer of 2003 resulted in lower surface productivity relative to the calm 2001 and 2002 summertime conditions. Even though estimates of surface primary production and P : R were lower in July 2003 compared to July 2002, concentration of algal biomass (particulate organic carbon, C: N) remained the most important factor for delta O-18 and P : R dynamics. The physical disturbance due to the hurricane, however, diminished the relative importance of benthic respiration, so that the system rather resembled wintertime conditions.