MICROBIAL CARBON UTILIZATION ON THE CONTINENTAL-SHELF AND SLOPE DURING THE SEEP-II EXPERIMENT

Microbial processes were measured as part of the Shelf Edge Exchange Processes II (SEEP-II) experiment, which was designed to re-examine the hypothesis that a substantial fraction of primary production on the continental shelf is exported to the slope and deeper ocean. Bacterial-mediated carbon remineralization was defined as the sum of measured bacterial production plus estimated bacterial respiration. Carbon remineralization, summed over the water column plus surficial sediment, was 90-180 mg C m(-2) day(-1) on the shelf and 270-540 mg C m(-2) day(-1) on the slope. These are equivalent to 12-24% and 33-66% of daily primary production on the shelf and slope, respectively. The majority of bacterial activity occurs in sediments on the shelf, but on the slope it is attributable to bacteria in the deep water column. Concurrent sediment trap data indicate that material sinking from surface waters overlying the slope can supply only a small fraction of carbon remineralized by bacteria in the intermediate water column. Bacterial production in slope water and sediments may be subsidized by export from the shelf. Because of the greater width of the shelf than dope, this additional demand on shelf carbon would represent only 7.5-15% of shelf primary production. Mean specific growth rates of sediment bacteria were very uniform at depths between 40 and 2000 m. This suggests bacterial growth may be limited by some common factor, although environmental factors known to affect growth rates generally vary with depth. DNA synthesis rates (measured by thymidine incorporation) were comparable to rates observed for marine bacteria in nutrient-replete culture. The implication is that sediment bacteria were not limited by nutrient availability. If true, water column and sediment bacterial production are likely to be controlled by very different processes: water-column bacteria by both nutrient limitation and grazing, and sediment bacteria by grazing alone.