LIMITS TO GROWTH AND RESPIRATION OF BACTERIOPLANKTON IN THE GULF-OF-MEXICO

We compared microbial community respiration and related parameters in the Gulf of Mexico in January and dune 1993. Microbial community respiratory rates in the upper mixed layer varied from <0.03 mu M O-2 h(-1) in the central Gulf in January to 1.4 mu M O-2 h(-1). in the Mississippi River plume in June. Although higher respiratory rates were found in June than in January, no significant differences were found in bacterial numbers or mean cell volume. Dissolved free amino acid concentrations were an order of magnitude higher in June, but there was Little difference in concentrations of phosphate or monosaccharides between January and June. Enrichment experiments in June showed phosphate to be the primary limiting factor for bacterial production and microbial community respiration and organic carbon substrates to be a secondary Limiting factor. Respiratory rate and bacterial secondary production increased when phosphate was added to water samples. Ammonium, iron and other trace metals, vitamins and chelators had no effect. Glucose was utilized only when supplemented with phosphate. Turnover time of bacterial biomass in June, based on counts, sizes, and production data, was 7 to 30 h, with the shortest times at oligotrophic stations. The observed rates of bacterial respiration and production imply the utilization of multiple sources of organic and recycled inorganic nutrients in a complex and inefficient food web.