PHOSPHATE TRANSFER BETWEEN MICROBIAL SIZE-FRACTIONS IN VILLEFRANCHE BAY (NW MEDITERRANEAN-SEA), FRANCE IN AUTUMN 1992

Turnover time of orthophosphate, uptake of phosphate into particulate size-fractions (0.2-1, 1-5, 5-10, > 10 mu m), and subsequent release from size-fractions were examined using P-32, in samples from surface waters of Villefranche Bay during the autumnal erosion of the thermocline. Turnover time of orthophosphate increased from 1.6 h in early October to 58 h in December. Throughout the study period, total uptake was dominated (50-68%) by the smallest size-fraction (0.2-1 mu m) presumably corresponding to auto- and heterotrophic bacteria as it contained little chlorophyll a. Among the size-fractions > 1 mu m, representing eucaryotic phytoplankton uptake, the erosion of the thermocline was accompanied by a shift from the total dominance of uptake by the 1-5 mu m fraction (98%) toward an increasing contribution from the > 10 mu m fraction (similar to 0 to 50%). In cold chase experiments, release of P-32 from the size-fractions which dominated uptake (0.2-1 mu m and 1-5 mu m), was relatively slow (less than or equal to 1% h(-1)) indicating long turnover times in particulate fractions; transfer to larger size-fractions was undetectable. When the concentration of oligotrich ciliates (predators of organisms 1-6 mu m in size) was artifically increased, labeled P-32 declined in particulate fractions and increased in the pool of inorganic suggesting a low P retention efficiency.