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Bacterioplankton growth is expected to depend on the availability of organic and inorganic nutrients. Still, no studies have investigated how the magnitude and type of nutrient limitation experienced by marine bacteria change on a temporal scale. We carried out a series of nutrient enrichment experiments to examine the variability in nutrient limitation of bacteria in the NW Mediterranean Sea, at monthly intervals, over an 18 mo period. Short-term enrichment bioassays (24 h incubation) showed that bacterial P limitation could occur throughout the year, but was most pronounced during spring and summer, coinciding with very low concentrations of dissolved inorganic phosphorus and chlorophyll a, and higher N: P ratios. During the non-stratified period in autumn and winter, bacteria were at times strongly C limited. Inorganic nitrogen limitation was not detected at any time. Long-term bioassays with and without …