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To examine the magnitude of phytoplankton-bacterial coupling in low productivity areas, diel changes of bacterial abundance and production as well as primary production (PP) in the oligotrophic Kuroshio surface waters were investigated by on deck carboy incubation and consecutive hydrocast sampling. Both methods yielded similar results. Cell counts varied < l 3 % in 1 diel cycle, while thymidine (TdR) incorporation rates and TdR incorporation per cell (TdR cell-') varied 2to 5-fold with higher values appearing at night. Such opposing trends between the bacterial rate parameters and PP were consistent over different locations and months. When incubated under an artificial light source, the TdR cell-' in whole water samples were negatively correlated with light intensity and PP. In nutrient enrichment experiments, the addition of labile organic carbon (glucose) had no effect on bacterial growth in noon and midnight samples. Values of the TdR cell-' in midnight samples increased about 70% when the inorganic nutrient mixture (NH,' and was added; however, such addition did not enhance the TdR cell-' in noon samples. This study demonstrates that the diel pattern of bacterial growth in the ocean surface water is opposite to that of phytoplankton. UV radiation is not required in driving such a variation since UV was excluded in the experimental set-up. There seems to be no simple explanation for the phenomenon reported by this study. Other potential factors, such as inorganic nutrient, organic substrate supply rates and bacterivory, all might play roles in affecting the bacterial diel cycle.

Diel cycles of heterotrophic bacterioplankton abundance and production in the ocean surface waters