Diel variations in bacterioplankton, phytoplankton, and related parameters in the Southern California Bight

The principal objectives of this study were (i) to determine the extent of coupling between phytoplankton and microheterotrophs on the shelf off Southern California. (ii) to compare different measures of primary and bacterial secondary production, and (iii) to assess whether sampling times should be as strictly controlled for microheterotroph as for autotroph studies. Two diel cycles (May and October) were studied by sampling an isotherm as the ship followed paired submerged drogues. We found significant die1 changes of chlorophyll, 14C bicarbonate incorporation, bacterial abundance and thymidine incorporation, frequency of dividing bacterial cells (FDC), abundance of non-pigmented flagellates, particulate organic carbon and nitrogen and their ratios, and dissolved oxygen. These parameters all had higher values dunng daylight hours than at night, showing close coupling between the phytoplankton (light-forced) and the microheterotrophs. The ratio of in vivo to extracted chlorophyll a fluorescence, however, displayed a maximum at midnight and minimum at midday, suggesting an endogenous rhythm. Primary production measured by the 14C method was similar to net production inferred from in situ oxygen changes. Short-lived peaks in FDC values suggested partly synchronized bacterial division.