Extracellular phosphatases in a Mediterranean reservoir: seasonal, spatial and kinetic heterogeneity

1. Epilimnetic alkaline phosphatase activity (APA) was measured in longitudinal profiles of the canyon‐shaped, eutrophic Sau Reservoir (Catalonia, Spain) during the autumn, winter and spring periods of 1997–2000. 2. The spatial pattern of APA depended on lake circulation. During periods of stable stratification, when the ratio of mixed to euphotic depth (z_mix/z_eu < 1.7) was low and the Chlorophyll a (Chl a) concentration high, APA was also comparatively high (0.5–3.4 μmol L^?1 h^?1) and located mostly in the >2‐μm size‐fraction. APA increased towards the dam at the same time as the concentration of soluble reactive phosphorus (SRP) decreased. In periods of unstable stratification, deep mixing (z_mix/z_eu > 2.4) and low Chl a concentration, APA was low (<0.1 μmol L^?1 h^?1) and without longitudinal changes, consistent with a high and stable SRP concentration. 3. A high input of mostly dissolved (in the <0.2‐μm size‐fraction) phosphatases from the river Ter was found in 1997–98. At the river inflow, independently of season and despite a continuously high SRP concentration, APA was approximately 0.7 μmol L^?1 h^?1 and decreased towards the dam within the inflowing, canyon‐like part of the reservoir. 4. Analysis of saturation kinetics revealed the kinetic heterogeneity of APA. Low‐affinity APA was localised in the >2‐μm (algal) size‐fraction while, in the <2‐μm (picoplankton, mainly bacteria + dissolved) size‐fraction, high‐affinity APA, or a mixture of both, was found. The presence of two kinetic components, whose substrate affinities (i.e. in Michaelis constants, K_m) differed, was confirmed statistically in 13 of 18 cases analysed. The range of K_mH values of the high‐affinity component was 0.15–2.4 μmol L^?1, K_mL values of the low‐affinity component ranged from 18 to 275 μmol L^?1.