Vertical and longitudinal gradients in HNA-LNA cell abundances and cytometric characteristics in the Mediterranean Sea
1CNRS, Université de la Méditerranée, Laboratoire de Microbiologie, Géochimie et Ecologie Marines, UMR6117, Case 901, Campus de Luminy, 13 288 Marseille cedex 9, France
2UPMC Univ Paris 06, Laboratoire ARAGO – Observatoire Océanologique, 66651 Banyuls/mer, France
3CNRS, UMR7621, LOMIC, Observatoire Océanologique, 66651 Banyuls/mer, France
Abstract. Heterotrophic bacterioplankton abundance and production were investigated with depth (down to bathypelagic layers) and with longitude (from 4.9° E to 32.7° E) along a cruise track across the Mediterranean Sea in early summer 2008. Abundances and flow cytometric characteristics (green fluorescence and side scatter signals) of high nucleic acid (HNA) and low nucleic acid (LNA) bacterial cells were determined using flow cytometry. Contrary to what is generally observed, the relative importance of HNA cells, as a percent of total cells, (%HNA, range 30–69 %) was inversely related to bacterial production (range 0.15–44 ng C l−1 h−1) although the negative relation was weak (log–log regression r2=0.19). The %HNA as well as the mean side scatter of HNA group increased significantly with depth in the meso and bathypelagic layers. Vertical stratification played an important role in influencing the distribution and characteristics of bacterial cells especially with regard to layers located above, within or below the deep chlorophyll maximum. Within a given layer, the relationships between the flow cytometric characteristics and environmental variables such as chlorophyll-a, nutrients or bacterial production changed. Overall, the relationships between HNA and LNA cells and environmental parameters differed vertically more than longitudinally.