BACTERIAL ABUNDANCE, BIOMASS, AND PRODUCTION IN RELATION TO PHYTOPLANKTON BIOMASS IN THE LEVANTINE BASIN OF THE SOUTHEASTERN MEDITERRANEAN-SEA

Little is known about the relative significance of heterotrophic bacte ria in open-ocean oligotrophic environments. The pelagic waters of the Levantine Basin of the eastern Mediterranean Sea are among the most o ligotrophic on record. We surveyed the spatial distribution of bacteri al abundance, biomass and production along 2 transects of the pelagic waters of the southern Levantine Basin to assess which changes in thes e parameters may occur in association with varying physical structure and chlorophyll concentrations, to calculate the relative biomass cont ributions of bacteria and phytoplankton, and to estimate the magnitude of carbon flux from phytoplankton to bacteria. Chlorophyll had an ave rage concentration of 134 +/- 35.4 ng l(-1) and was relatively uniform throughout the upper 200 m. Bacterial numbers ranged from 0.40 to 3.9 0 x 10(8) cells l(-1) and were generally highest above 110 m. Cocci ce lls comprised 87% of the population with an average volume of 0.049 mu m(3). Bacterial numbers and biomass were notably high in the Ierapetr a Eddy and Mersa Matruh Gyre. Although bacterial numbers and chlorophy ll concentrations were not generally correlated, the mean bacterial nu mber was accurately predicted from a regression equation using chlorop hyll. Over the upper 200 m, bacterial biomass (<(x)over bar = 603 mgC m(-2)) was on average about 50% of phytoplankton biomass ((x) over bar = 1235 mgC m(-2), which is contrary to other published studies report ing bacterial biomass equalled or exceeded algal biomass in oligotroph ic marine waters. Bacterial production ranged from 0 to 3.91 pmol TdR l(-1) h(-1). Average bacterial carbon production varied from 1.76 ng l (-1) h(-1) at 150 m to 9.09 ng l(-1) h(-1) at 100 m. Specific growth r ates reached a maximum value of 0.54 d(-1) at 100 m while mean doublin g time was 70.8 +/- 180.9 d. Mean daily bacterial production for the u pper 200 m was 24.3 mgC m(-2) d(-1), indicating bacteria could consume on average 154% (69.3 mgC m(-2) d(-1)) of phytoplankton primary produ ction. Our calculations indicate that the bacterial population may be acquiring organic carbon not derived from phytoplankton. In order to c onfirm this imbalance of carbon flux from phytoplankton to heterotroph ic bacteria in the Levantine Basin, good estimates of water column pri mary production, which do not exist, must be obtained.