Response of bacteria to simulated upwelling phytoplankton blooms

Until recently, studies of the fate of primary production in coastal upwelling systems have focused mainly on export through sinking of particulate organic matter (POM). In week-long deck incubations conducted during the upwelling season off Oregon, a large accumulation of carbonrich (C:N ≥ 16) dissolved organic matter (DOM) occurred following nitrate depletion by diatom blooms. The response of bacterioplankton to the DOM release in the incubations was observed using flow cytometric analysis of abundances of bacteria with high nucleic acid (HNA) and low nucleic acid (LNA) content. Relatively small increases in the abundance of HNA bacteria were observed in nitrate-replete conditions (<1.0 × 106 cells ml–1). Coincident with nitrate depletion and accumulation of the DOM, abundances and growth rates of HNA bacteria increased rapidly while little response was observed from LNA bacteria. Although growth rates and abundances of HNA cells increased markedly, a net decrease in dissolved organic carbon (DOC) was observed in only 1 incubation. Within approximately 1 d of nitrate going to depletion, HNA bacterial abundances peaked and then decreased rapidly, possibly due to flagellate grazing or viral lysing. These results indicate that on the timescale of upwelling/relaxation events, which generally last 7 to 10 d, environmental controls on bacterial populations may prevent complete degradation of phytoplankton-derived DOM, thus allowing some of this material to be exported from the system through physical processes following termination of the upwelling event.