Role of DOP on the alkaline phosphatase activity of size fractionated plankton in coastal waters in the NW Mediterranean Sea (Toulon Bay, France)

The particulate material was fractionated into 5 size classes (> 90 μ, 50-90 μ, 6-50 μ, 1-6 μ, and < 1 μ). DOP was analysed as easily (DOPh, DOPpa) and less easily hydrolysable compounds (DOPox). Based on Vmax, 94% of the high affinity AP activity was due to < 50 μ cells and 77% to < 1 μ cells. 83% of the low affinity activity was due to > 90 μ cells. The high affinity activities were negatively correlated with DOP for the < 50 μ classes. These correlations came mostly from DOPox. They were more significant when NO3 + NO2 concentrations were high, when DIP concentrations were low and when N/P ratio was > 10. At lower N/P ratios, AP was more significantly correlated with DIP. The low affinity activities showed significant negative correlation with DIP and with DOP and DOPox for the > 90 μ class. The inhibition of AP activities by DOPox may originate from stable compounds interfering with DIP for the control of AP synthesis.