BACTERIA, AUTOTROPHIC AND HETEROTROPHIC NANOFLAGELLATES, AND THEIR RELATIONS IN MIXED, FRONTAL AND STRATIFIED WATERS OF THE NORTH-SEA

The horizontal and vertical distributions of bacteria and bacterial productivity were compared with nanoflagellate densities in the southern part of the central North Sea. Mixed, frontal and stratified waters were sampled in transects during summer 1988. High bacterial abundance, 2.7 to 4.5*106 cells·cm-3, distinguished coastal from offshore waters. Bacterial production and nanoflagellate densities were also high in the coastal zone but reached comparable or even higher values further offshore in frontal systems. We crossed two conspicuous fronts: the Frisian Frontal zone and a frontal zone along the northern slope of the Dogger Bank. These fronts were characterized by enhanced bacterial production and/or enhanced bacterial specific growth rates. In fronts as well as mixed waters, nanoflagellate densities covaried with bacterial specific growth rates and reached highest numbers in fronts, e.g. heterotrophic nanoflagellate densities peaked in the Frisian Front with 6000 to more than 10 000 cells·cm-3. These high densities were accompanied by low bacterial abundances (0.45*106 cells·cm-3) suggesting a regulation of bacterial numbers by heterotrophic nanoflagellates. A comparable pattern was found in the Dogger Bank front. Biomass of autotrophic nanoflagellates was significantly correlated with biomass of heterotrophic nanoflagellates. A maximum of nanoflagellates was present in the thermocline in stratified waters. The carbon distribution between bacteria and nanoflagellates over the water column was always dominated by flagellates, except in the coastal zone. Offshore, in mixed waters, bacterial biomass made up 30 to 51% of the bacterial plus auto/heterotrophic nanoflagellate biomass. In fronts and stratified waters the biomass of heterotrophic nanoflagellates alone exceeded bacterial biomass. Bacterial production amounted to a fraction of 3 to 31% of the primary production. © 1990.