AME 25:21-30 (2001)  -  doi:10.3354/ame025021

ABSTRACT: Variation in bacterial abundance and activity was assessed by sampling the upper 35 to 80 m of the water column during 2 to 5 d periods at 3 sites: eutrophic-mesotrophic midshelf, mesotrophic-oligotrophic slope, and oligotrophic gyre edge, off the Oregon coast in late summer 1997 and 1998. Bacterial abundances varied 10-fold, from 0.2 to 2.3 x 10⁶ cells ml^-1, and leucine incorporation rates varied 160-fold, from 1.5 to 240 pM h^-1. During the strong El Niño event in 1997, bacterial abundances were similar at all 3 sites, but midshelf ³H-leucine incorporation rates were ~6-fold higher than rates at the slope and gyre sites. In 1998, after relaxation of the El Niño, bacterial abundances were lower, and average ³H-leucine incorporation rates were only 2.5 times higher at the midshelf site than at the slope and gyre sites. There was a close correlation between estimates of bacterial cell production rate based on ³H-leucine and on ³H-thymidine incorporation rates for the midshelf and slope sites, but no relation between the 2 estimates for the gyre site. During both years, bacterial abundance varied inversely with depth, salinity, and macronutrients and positively with temperature. Bacterial activity varied positively with chlorophyll concentration, temperature, and bacterial biomass. Rates of bacterial ³H-leucine incorporation were most strongly related to chlorophyll concentrations at the midshelf site and less related at sites farther offshore. There was no significant relation of bacterial parameters with the concentration of dissolved organic carbon. Our results showed dynamic mesoscale variability, on scales of 10s of meters to 10s of kilometers, and on scales of hours to days in rates of bacterial activity, which was positively related to phytoplankton concentration as a proxy for trophic state of the water mass. We also found inter-annual differences in distribution of bacterial abundance and activity in this region, which appeared to be related to climatic variability.

KEY WORDS: Marine bacteria · Bacterial activity · Mesoscale · El Niño · Northeast Pacific