An open-top chamber was designed for measuring ambient community clearance rate on undisturbed bivalve populations in the field. The chamber was pressed 5-10 cm down in the sediment on the mussel bed. It holds approximately 30-40 cm water column equal to a volume of 43-77 L. It was provided with an air lift connected to a SCUBA diver pressure tank generating a continuous and gentle water circulation. This ensures a complete mixture of suspended particles, and thereby, a maximum filtration by the bivalves. An in situ fluorometer was mounted to record plant pigment reduction due to mussel clearance in real-time. To calibrate the in situ fluorometer triplicate water samples were obtained initially in each of the bivalve filtration measurements. The water samples were filtrated, extracted, and later analyzed for plant pigment concentration on a laboratory spectrophotometer. The main conclusion is that bivalve community clearance was an order of magnitude lower than predicted from laboratory extrapolations, which we explain by a natural variable activity level among the individuals in a given bivalve assemblage.