Mesoscale variability in bacterial activity in the Northeast Pacific Oceanoff Oregon, USA

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: eu trophic-mesotrophic midshelf, mesotrophic-oligotrophic slope, and oligotrop hic gyre edge, off the Oregon coast in late summer 1997 and 1998. Bacterial abundances varied 10-fold, from 0.2 to 2.3 x 10(6) cells ml(-1), and leuci ne incorporation rates varied 160-fold, from 1.5 to 240 muM h(-1). During t he strong El Nino event in 1997, bacterial abundances were similar at all 3 sites, but midshelf H-3-leucine incorporation rates were similar to6-fold higher than rates at the slope and gyre sites. In 1998, after relaxation of the El Nino, bacterial abundances were lower, and average 3H-leucine incor poration 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 ba cterial cell production rate based on 3 H-leucine and on 3 H-thymidine inco rporation rates for the midshelf and slope sites, but no relation between t he 2 estimates for the gyre site. During both years, bacterial abundance va ried inversely with depth, salinity, and macronutrients and positively with temperature. Bacterial activity varied positively with chlorophyll concent ration, temperature, and bacterial biomass. Rates of bacterial H-3-leucine incorporation were most strongly related to chlorophyll concentrations at t he midshelf site and less related at sites farther offshore. There was no s ignificant relation of bacterial parameters with the concentration of disso lved organic carbon. Our results showed dynamic mesoscale variability, on s cales 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 phytoplank ton concentration as a proxy for trophic state of the water mass. We also f ound inter-annual differences in distribution of bacterial abundance and ac tivity in this region, which appeared to be related to climatic variability .