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 bacteria in open-ocean oligotrophic environments. The pelagic waters of the Levantine Basin of the eastern Mediterranean Sea are among the most oligotrophic on record. We surveyed the spatial distribution of bacterial abundance, biomass and production along 2 transects of the pelagic waters of the southern Levantine Basin to assess which changes in these parameters may occur in association with varying physical structure and chlorophyll concentrations, to calculate the relative biomass contributions of bacteria and phytoplankton, and to estimate the magnitude of carbon flux from phytoplankton to bacteria. Chlorophyll had an average concentration of 134 ± 85.4 ng l−1 and was relatively uniform throughout the upper 200 m. Bacterial numbers ranged from 0.40 to 3.90 × 108 cells l−1 and were generally highest above 110 m. Cocci cells comprised 87% of the population with an average volume of 0.049 μm3. Bacterial numbers and biomass were notably high in the Ierapetra Eddy and Mersa Matruh Gyre. Although bacterial numbers and chlorophyll concentrations were not generally correlated, the mean bacterial number was accurately predicted from a regression equation using chlorophyll. Over the upper 200 m, bacterial biomass (x̄ = 603 mgC m−2) was on average about 50% of phytoplankton biomass (x̄ = 1235 mgC m−2), which is contrary to other published studies reporting bacterial biomass equalled or exceeded algal biomass in oligotrophic 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 rates reached a maximum value of 0.54 d−1 at 100 m while mean doubling time was 70.8 ± 180.9 d. Mean daily bacterial production for the upper 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 production. Our calculations indicate that the bacterial population may be acquiring organic carbon not derived from phytoplankton. In order to confirm this imbalance of carbon flux from phytoplankton to heterotrophic bacteria in the Levantine Basin, good estimates of water column primary production, which do not exist, must be obtained.