Heterotrophic activities of bacterioneuston and bacterioplankton

Numbers of viable bacteria as indicated by colony-forming units (CFU), microbial adenosine triphosphate levels, dissolved organic carbon (DOC), particulate organic carbon and nitrogen (POC and PON), glucose concentration, heterotrophic potential (Vmax), and in situ heterotrophic activity (Vn) of near-surface microlayer samples (70-80 mum depth), the neuston, and bulk column water samples (0.2-1.5 m depth), the plankton, of several natural waters were compared. DOC and glucose concentration values of near-surface and bulk water were similar, whereas POC and PON values of the near-surface were normally one order of magnitude greater than those of bulk water. Viable counts were generally greater in the neuston, but ATP levels were greater in the plankton. Comparative values of total heterotrophic potentials and heterotrophic activities fluctuated greatly. Heterotrophic potential per CFU of the neuston was 33% of that of the plankton; neuston heterotrophic activity per CFU was 10% of that of the plankton. These data suggest that neustonic heterotrophic bacteria are not as metabolically active as their planktonic counterparts and that they are under greater stress at the near-surface than in the column water.