Plankton respiration and carbon flux through bacterioplankton on the Louisiana shelf

Carbon flow through bacterioplankton can be evaluated only if both growth and respiration are known. Measurements of community and bacterial respiration (oxygen consumption) and bacterial production (3Hleucine incorporation) were made in highly productive shelf and less productive slope waters in the northern Gulf of Mexico. Rates of bacterial production and community respiration, as well as bacterial abundance and dissolved organic C concentrations, declined with depth at both locations. Water-column bacterial production ranged from 0.1 to 3.1 µg C liter−1 h−1, and community respiration rates ranged from 0.05 to 0.45 µM O2 h−1. In comparison to the slope, the shelf was characterized by 2-fold higher bacterial abundance and bacterial production but similar community respiration rates. Estimated production per bacterium values decreased logarithmically with depth (1.4–0.15 fg C cell−1 h−1) and were similar at both locations. Estimated respiration per bacterium values for the surface water ranged from 0.10 to 0.36 fmol O2 cell−1 h−1 and were higher on the slope than the more densely populated shelf. A selective suppression of bacterial respiration occurred under both natural and experimentally (tangential-flow ultrafiltration) enhanced bacterial abundances. Measured growth efficiencies fell between 26 and 55%, with higher efficiencies occurring on the shelf (50%) than the slope (26%). Bacterioplankton at the less productive slope station processed a larger daily share of local primary production (69%) than bacteria at the highly productive shelf station (25%).