Progress in Oceanography 46 (2000) 279–310 Biochemical composition of pico-, nano- and micro-particulate organic matter and bacterioplankton biomass in the oligotrophic Cretan Sea (NE Mediterranean) Roberto Danovaro a,* , Antonio Dell’Anno b , Antonio Pusceddu b , Daniela Marrale b , Norberto Della Croce b , Mauro Fabiano b , Anastasios Tselepides c a Cattedra di Ecologia, Facolta ` di Scienze, Universita ` di Ancona, Via Brecce Bianche, Monte D’Ago, 60131 Ancona, Italy b Istituto di Scienze Ambientali Marine, Universita ` di Genova, Corso Rainusso 14, 16038 S. Margherita L., Genova, Italy c Institute of Marine Biology of Crete, P.O. Box 2214, 710 03 Heraklion, Crete, Greece Abstract The biochemical composition of different particle size classes (pico-, nano- and micro-par- ticulate matter) and the bacterioplankton biomass were studied over an annual cycle in the Cretan Sea (South Aegean Sea, NE Mediterranean; from 40 to 1540 m depth) to investigate the origin, composition and fate of the suspended particles and to quantify bacterioplankton contribution to organic carbon pools. The oligotrophy of this system was indicated by the extremely low particulate lipid, protein and carbohydrate concentrations (4–15 times lower than in more productive systems). The biopolymeric carbon (BPC as the sum of lipid, protein and carbohydrate carbon) accounted for 80–100% of POC, suggesting the autochthonous origin of the particles. The most evident characteristic of this oligotrophic environment was the domi- nance of the pico-particles through all seasons, accounting for 43–45% of total carbohydrates, proteins and lipids. The proximate composition of the organic particles revealed the dominance of carbohydrates in all size-classes and highest values of the protein to carbohydrate ratio in the pico-particulate fraction. The relative proportion of the pico-, nano- and micro-particulate carbohydrates, proteins and lipids varied seasonally. The increase in the average particle size from February to September 95, probably as a result of aggregation, appeared to be related to the ‘thermal stability’ of the water column. The analysis of the vertical distribution of the * Corresponding author. E-mail address: danovaro@popcsi.unian.it (R. Danovaro). 0079-6611/00/$ - see front matter  2000 Published by Elsevier Science Ltd. PII:S0079-6611(00)00023-9