Seasonal Patterns of Bacterial Abundance and Production in the Mississippi River Plume and Their Importance for the Fate of Enhanced Primary Production

Abstract

Bacterial abundance and leucine incorporation were measured in the Mississippi/Atchafalaya River plume area during May 1992 and July 1993. Both parameters were highest at intermediate salinities during both season. The peak of bacterial leucine incorporation (∼2.5 nM h⁻¹) and abundance was located at higher salinities in May 1992 than in July 1993. Leucine incorporation rates in surface waters decreased with increasing westward distance from the plume, whereas rates increased below the pycnocline with increasing westward distance. The high rates of bacterial activity in stratified bottom waters could potentially deplete oxygen within 3 to 50 days. The mean values for depth-integrated bacterial production were higher in May 1992 (787 mg C m⁻² d⁻¹) than in July 1993 (644 mg C m⁻² d⁻¹). The integrated values for bacterial production indicate that a variable proportion of the primary production is consumed by bacteria during spring and summer. Comparisons to the vertical export of POM and zooplankton grazing suggest that zooplankton grazing determines the amount of organic material available for bacterial mineralization in deeper layers. Depth-integrated gross bacterial production rates were highest in May 1992 when vertical export of particulate organic matter (POM) was also the greatest. The response of heterotrophic bacteria to increased organic matter input in the plume area during spring indicates that the microbes play a crucial role in the development of hypoxic conditions on the Louisiana shelf during early summer.