PLANKTON RESPIRATION AND CARBON FLUX THROUGH BACTERIOPLANKTON ON THE LOUISIANA SHELF

Carbon flow through bacterioplankton can be evaluated only if both gro wth and respiration are known. Measurements of community and bacterial respiration (oxygen consumption) and bacterial production ([H-3]leuci ne incorporation) were made in highly productive shelf and less produc tive slope waters in the northern Gulf of Mexico. Rates of bacterial p roduction and community respiration, as well as bacterial abundance an d dissolved organic C concentrations, declined with depth at both loca tions. Water-column bacterial production ranged from 0.1 to 3.1 mug C liter-1 h-1, and community respiration rates ranged from 0.05 to 0.45 muM O2 h-1. In comparison to the slope, the shelf was characterized by 2-fold higher bacterial abundance and bacterial production but simila r community respiration rates. Estimated production per bacterium valu es decreased logarithmically with depth (1.4-0.15 fg C cell-1 h-1) and were similar at both locations. Estimated respiration per bacterium v alues 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 n atural and experimentally (tangential-flow ultrafiltration) enhanced b acterial abundances. Measured growth efficiencies fell between 26 and 55%, with higher efficiencies occurring on the shelf (50%) than the sl ope (26%). Bacterioplankton at the less productive slope station proce ssed a larger daily share of local primary production (69%) than bacte ria at the highly productive shelf station (25%).