MEPS 270:1-13 (2004)  -  doi:10.3354/meps270001

ABSTRACT: Alkaline phosphatase (AP) kinetic experiments were performed in a broad range of trophic conditions in the transition zone between the North African upwelling and the open ocean, in order to investigate the effect of nutrient richness on kinetic parameters. Turnover times decreased from 224 h in the open ocean to 13 h in the upwelling region. K + S_n showed the lowest values at the most eutrophic station (27 nM), while at this station V_m reached its maximum value of 2.1 nM h^-1. Nevertheless, as multiphasic kinetics occurred, kinetic parameter values depended largely on the fitting technique used. Response of AP activity (APA) to phosphate enrichment was also studied along a trophic gradient at the surface and the chlorophyll maximum level. Inhibition of APA reached 50% with only 0.1 µM of phosphate added, and rose to 80 to 96% inhibition by additions of 1 µM at the oligotrophic stations, whereas in the most eutrophic station, phosphate amendments had no effect. Inhibition was higher at the surface and towards oligotrophy. The lack of inhibition in the upwelling station and the low K + S_n encountered suggest that the APA of eutrophic waters was mainly from bacterial origin. This supports the hypothesis of APA being mainly involved in supplying easily assimilable organic carbon to bacteria in eutrophic waters.

KEY WORDS: Alkaline phosphatase · Kinetic parameters · Phosphorus · Trophic gradient · Turnover