BACTERIAL NUMBERS, BACTERIAL PRODUCTION, AND HETEROTROPHIC NANOPLANKTON ABUNDANCE IN A WARM CORE EDDY IN THE EASTERN MEDITERRANEAN

Bacterial numbers and production, and abundance of heterotropic nanoplankton [HNAN] were recorded over 24 h at the core and boundary of the Cyprus Eddy, a warm-core eddy in the Levantine Basin of the Eastern Mediterranean Sea. The basin is known as one of the most oligotrophic on record. Bacterial numbers in the euphotic zone were close to the lower threshold value for bacterial abundance in the oceans, with means of 2.7 x 10(5) bacteria ml-1 at the boundary and 4.9 x 10(5) ml-1 at the core. Bacterial production, measured as the rate of DNA labelling by thymidine (TdR), ranged from 0.01 to 0.70 pmol TdR l-1 h-1, values which fall at the lower end of ranges reported from other freshwater and marine ecosystems. No diel cycles of bacterial numbers or rate of TdR incorporation were observed, but the TdR rate normalized to cell numbers showed marked diel variation with the highest rates usually found in early morning hours. HNAN were characteristically small (mostly < 3-mu-m in diameter) and more abundant in the core (37 to 850 cells ml-1) than in the boundary station (19 to 414 cells ml-1). The long bacterial doubling times (ca 50 d), and the parallel distribution of bacteria and HNAN, suggested low grazing pressure. A possible alternative food source for HNAN is prochlorophytes. Higher microbial standing crop and activity are anticipated in the core of the eddy in winter, when deep mixing occurs.