The sediments immediately under the cage once rearing Oreochromis niloticus showed significantly higher content s of organic matter ( OM) and phosphatase hydrolyzable phosphorus ( PHP) , and alkaline phosophatase act ivity ( APA) as well. In interstitial water, both APA and orthoposphate ( o-P) concentrations were signif icantly higher, while PHP was significantly lower, at vertical and horizontal scales. These results suggested that the enhancement of o-P release from sediment, resulted f rom the enrichment of OM, might be controlled by an enzymatical mechanism. The Langmuir isotherm approach revealed that, in interstitial water under the cage, sorption capacity was signif icantly higher, while the corresponding sorption strength was signif icant ly lower in the upper layers, which lends an additional explanation to the enhancement of o-P release in the sediments as affected by the cage culture. A remarkably seasonal variation in vertical patterns of kinetics of APA was observed in interstitial water.