Alkaline phosphatase activity in the western English Channel

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Abstract
The spatial and vertical distributions of alkaline phosphatase activity (APA) in the western English Channel in July 1987 were determined using 2 μM 3-O-methylfluorescein phosphate as substrate. The response of APA to increases in substrate concentration followed Michaelis-Menten kinetics but differences in Ks values between stations suggested a species dependency of the half-saturation constant. Phosphate inhibition of APA was, however, similar at 3 locations. The chlorophyll specific APA (SAPA) in surface waters ranged from 0.004 to 0.57 nMPO43- μg chla-1 min-1 and was inversely exponentially related to the chlorophyll a concentration reflecting the fact that phytoplankton populations are largest where inorganic nutrient availability is greatest and synthesis of the enzyme is not derepressed. SAPA values were found to be low in the bloom of Gyrodinium aureolum in the Ushant front and in the sub-surface chlorophyll maximum in the stratified region. Vertical distributions of SAPA are discussed in terms of the hydrographic characteristics of the water column and an attempt has been made to distinguish between APA associated with phytoplankton and with bacteria.