MEPS 193:11-18 (2000)  -  doi:10.3354/meps193011

Relationship between primary producers and bacteria in an oligotrophic sea--the Mediterranean and biogeochemical implications

C. M. Turley1,*, M. Bianchi2, U. Christaki3, P. Conan4, J. R. W. Harris1, S. Psarra5, G. Ruddy1, E. D. Stutt1, A. Tselepides5, F. Van Wambeke2

1Centre for Coastal & Marine Sciences, Plymouth Marine Laboratory, Citadel Hill, Plymouth PL1 2PB, England
2Microbiologie Marine, C. N. R. S., UPR. 223, Campus de Luminy, Case 907, 13288 Marseille Cedex 9, France
3NCMR, National Centre for Marine Research, 16604 Aghios, Kosmas, Hellinikon, Greece
4Centre d'oceanologie de Marseille, LOB, Campus de Luminy, Case 901, 13288 Marseille Cedex 9, France
5Institute of Marine Biology of Crete, PO Box 2214, 71003 Heraklion, Crete, Greece

ABSTRACT: The proverbial blue colour of the Mediterranean reflects some of the most extreme oligotrophic waters in the world. Sea-surface Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite data show the relatively clear, pigment poor, surface waters of the Mediterranean with a generally increasing oligotrophy eastward, apparent even from space. Integrated over depth, however, the east and west Mediterranean show similar amounts of phytoplankton and bacterial biomass. By contrast, primary production and bacterial production are 2 to 3 times lower in the eastern Mediterranean than in the west. However, the relationship between bacterial production and primary production in the east and west are significantly different. While bacterial production is directly proportional to primary production in the east, in the west it increases as approximately the square root of primary production. This suggests that the bacteria in the west are relatively decoupled from local contemporaneous primary production. In contrast, the gradient of close to 1 in the log bacterial production versus log primary production relationship in the east suggests less temporal decoupling and, therefore, less seasonal accumulation of DOC. In addition, the constant proportionality between bacterial and primary production of 0.22, which, if all primary products are respired, gives an estimated geometric mean bacteria growth efficiency of 22% (95% confidence limits of 17 and 29%) for data in the eastern Mediterranean. Our data suggest that the degree of bacteria-phytoplankton coupling has an important effect on apparent trends between bacterial and phytoplankton production in high frequency data. The combination of low primary production and bacterial dominance of secondary production in the east is also of significance as it could account for the low fisheries production, the low vertical flux of material and low biomass of benthic organisms in the region.

KEY WORDS: Bacteria · Phytoplankton · Bacterial growth efficiency · Ocean productivity · Oligotrophy · Mediterranean

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