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<p>Hydrolysis of dissolved organic phosphorus by marine planktonic microorganisms is a key process in the <span class="inline-formula">P</span> cycle, particularly in <span class="inline-formula">P</span>-depleted, oligotrophic environments. The present study assessed spatiotemporal variations in phosphomonoesterase (PME) and phosphodiesterase (PDE) activities using concentration kinetics in the eastern Mediterranean Sea in two contrasting situations: the end of winter (including a small bloom period) and autumn. The distribution and regulation of the maximum hydrolysis rate (<span class="inline-formula">Vm</span>) and half-saturation constant (<span class="inline-formula">Km</span>) of both ectoenzymes were assessed in relation to the vertical structure of the epipelagic layers. PME reached its maximum activities (<span class="inline-formula">Vm</span>) after the addition of 1 <span class="inline-formula">µM</span> MUF-P (4-methylumbelliferyl phosphate), whereas, for PDE, it was necessary to add up to 50 <span class="inline-formula">µM</span> bis(4-methylumbelliferyl)phosphate (bis-MUF-P) to reach saturation state. On average, the <span class="inline-formula">Km</span> of PDE was 33 <span class="inline-formula">±</span> 25 times higher than that of PME. The <span class="inline-formula">Vm</span> of PME and <span class="inline-formula">Vm</span> of PDE were linearly correlated. Conversely to the <span class="inline-formula">Km</span> values, <span class="inline-formula">Vm</span> values were on the same order of magnitude for both ectoenzymes, with their ratio (<span class="inline-formula">Vm</span> PME : <span class="inline-formula">Vm</span> PDE) ranging between 0.2 and 6.3. Dissolved organic phosphorus (DOP) and the phosphomonoesterase hydrolysable fraction of DOP explained most of the lack of variability in <span class="inline-formula">Vm</span> PME and <span class="inline-formula">Vm</span> PDE. On the contrary, <span class="inline-formula">Vm</span> of both phosphohydrolase enzymes was inversely correlated to the concentration of dissolved inorganic phosphorus. The particular characteristics of concentration kinetics obtained for PDE (saturation at 50 <span class="inline-formula">µM</span>, high <span class="inline-formula">Km</span>, high turnover times) are discussed with respect to the possible unequal distribution of PDE and PME among the size continuum of organic material and accessibility of phosphodiesters.</p>