Heat flux from submarine vent fields can vary gradually on interannual scales, or nearly instantaneously in response to volcano-tectonic events in the underlying curst. Neither case is well documented because measurements of vent-field scale heat flux are scarce. We report here a new approach to hydrothermal plume monitoring sensitive to both progressive changes and hydrothermal events. From June 1991 to May 1992 we moored 35 self-contained temperature sensors and six current meters on seven moorings located in and around the plume from a vent field on the Cleft segment, Juan de Fuca Ridge. The hydrothermal plume was identified by a local temperature anomaly of 0.01¿ to 0.03 ¿C in the lowermost 200 m of the water column. Plume heat flux, defined as the net advection of this temperature anomaly, averages about 250 MW during the deployment. This flux is less than previous estimates and thus supports speculation that the heat flux is declining after a sudden reinvigoration triggered by a seafloor rifting event in 1986. ¿ American Geophysical Union 1993