Hydrothermal activity within the caldera of Suiyo Seamount was investigated in detail by heat flow measurements using HOV/ROV through five research cruises in 2001-2003. The hydrothermal regime in the Suiyo-seamount is characterized by a geochemically uniform fluid, shallow reservoir depth, very permeable seafloor, and venting without creating big chimneys. Heat flow is highest (> 10 W/m(2)) within the active venting area. On the western and southern periphery, some heat flow values are lower than 0.3 W/m(2), only several tens of meters away from the venting area. To the east, heat flow is 2-4 W/m(2), indicating a more uniform reservoir overlain by impermeable layers. High temperature zones below seafloor, with non-linear temperature profiles, are identified within 5 m from isolated vents indicating secondary, probably transient circulations. We believe that we obtained several lines of direct evidence for discharge and recharge activity and their interface near the vents, constrained by the precipitation and permeability structure around the vent. Hydrothermal regime at the Suiyo site consists from a 100-m scale stable circulation to a 1-m scale local transient pattern. This multiple-scale circulation pattern, also identified at other hydrothermal sites, would be constrained by the distribution of heat source and hydrological properties near the seafloor. (c) 2006 Elsevier B.V. All rights reserved.