Biochemical composition of pico-, nano- and micro-particulate organic matter and bacterioplankton biomass in the oligotrophic Cretan Sea (NE Mediterranean)

The biochemical composition of different particle size classes (pico-, nano - and micro-particulate matter) and the bacterioplankton biomass were studi ed over an annual cycle in the Cretan Sea (South Aegean Sea, NE Mediterrane an; from 40 to 1540 m depth) to investigate the origin, composition and fat e of the suspended particles and to quantify bacterioplankton contribution to organic carbon pools. The oligotrophy of this system was indicated by th e 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 8 0-100% of POC, suggesting the autochthonous origin of the particles. The mo st evident characteristic of this oligotrophic environment was the dominanc e of the pico-particles through all seasons, accounting for 43-45% of total carbohydrates, proteins and lipids. The proximate composition of the organ ic particles revealed the dominance of carbohydrates in all size-classes an d highest values of the protein to carbohydrate ratio in the pico-particula te fraction. The relative proportion of the pico-, nano- and micro-particul ate carbohydrates, proteins and lipids varied seasonally. The increase in t he average particle size from February to September 95, probably as a resul t of aggregation, appeared to be related to the 'thermal stability' of the water column. The analysis of the vertical distribution of the three size c lasses revealed an increase in the pico fraction and a decrease in the larg er components with increasing depth suggesting that nano- and micro-particl es were being degraded and fragmented in the deeper water layers. Bacterial densities ranged from 1.1 to 8.8 x 10(8) cells 1(-1). Bacterial biomass ac counted on average for more than 56% (up to 74%) of BPC and was by far, the most important living component. Bacterial-N accounted for a large proport ion (>90%) of the protein nitrogen pool, indicating that almost no particul ate detrital N was available for heterotrophic metabolism. Therefore, it is likely that bacteria utilise other sources, such as DOM and inorganic nutr ients, to support their growth. The lack in particle variability appeared t o be responsible for the rather consistent size structure and biochemical c omposition of the suspended particulate organic matter in this system. (C) 2000 Published by Elsevier Science Ltd.