Journal of Plankton Research Vol.25 no.9 pp.1011-1019, 2003
© Oxford University Press 2003
Low bacterial growth efficiency in the oligotrophic eastern Mediterranean Sea: a modelling analysis
Southampton Oceanography Centre, Waterfront Campus, European Way, Southampton SO14 3ZH and 1 Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK
* Corresponding Author: tra{at}soc.soton.ac.uk
Bacterial growth efficiency (BGE) is generally related to the trophic status of marine systems, with values as low as 0.15 or less associated with the most nutrient-depleted areas. A simple steady-state model is used to examine whether the observed ratio of bacterial to primary production (BP:PP) of 0.22 in the eastern Mediterranean Sea, an ultra-oligotrophic system, can be sustained given a BGE of 
0.15. There is no a priori reason to believe that BGE should be higher in this area relative to other open ocean environments, although accurate measurements are required to investigate this possibility. The model includes primary production [both particulate and exudation of dissolved material expressed as a percentage of extracellular release (PER)] and terriginous dissolved organic carbon (DOC) as sources, cycling via zooplankton, viruses and bacteria, and bacterial and zooplanktonrespiration as sinks for carbon. Results indicate that a BP:PP of 0.22 cannot be maintained if bacterial carbon demand is met solely with DOC of autochthonous origin, given a BGE of 0.15. Sufficient carbon is, however, supplied to maintain this ratio if a high phytoplankton exudation of DOC (PER = 40%) is permitted, in conjunction with a BGE of 0.16. A BGE of 0.28 is required if PER takes on a more typical value of 10%. The possibility of BP being significantly enhanced by DOC of terrestrial origin is discounted, there being no major riverine sources near the Cretan Sea. Our analysis emphasizes the need for accurate estimates of BGE, and an improved understanding of sources and sinks of organic carbon, in marine systems.