Impact of the pycnocline layer on bacterioplankton: diel and spatial variations in microbial parameters in the stratified water column of the Gulf of Trieste (Northern Adriatic Sea)

Die1 variations in bacterial density, frequency of bvidlng cells (FDC) and bssolved organic carbon (DOC) in the stratified water column of the Gulf of Trieste were investigated at various depths.In the surface layers morning and afternoon DOC peaks (up to 10 mg I-') were observed in 3 out of 4 diel cycles.Bacterial abundance remained fairly constant over the diel cycles; however, bacterial activity as measured by FDC showed pronounced peaks in late afternoon and dawn coinciding with the DOC maximum concentrations.The pycnocline layer exhibited DOC concentrations similar to those of the overlying waters.The frequency of dividing cells (FDC), however, remained high throughout the entire diel cycle.The observed pronounced diel variations in bacterial production were therefore largely restricted to the layers well above the pycnocline; bacterial production contributed less than 20 % to the overall diel production (500 to 1000 pg C 1-' d-') in the uppermost 5 m water body.Highest diel production was found in the pycnocline layer at the end of the phytoplankton bloom (June and July) probably reflecting the formation of nutrient-enriched microzones around decaying phytoplankton cells due to reduced sinking velocities in the pycnocline layer.Additionally, FDC-based bacterial secondary production estimates were compared with in situ incubations of dlalysis bags filled with 2 pm filtered seawater for 24 h.Bacterial biomass product~on estimates based on FDC for the 5 and 10 m layer were 3 and 14 times higher, respectively.The observed variations of microbial parameters within different layers point to the importance of small-scale investigations in both time and space.