Suspended aggregate analysis using ISAAC, Elbe River, 9–10 June 1993

Abstract

A portable underwater camera system, the In Situ Aggregate Analysis Camera (ISAAC), was used for precise and accurate sizing of suspended particulate material in the Elbe River estuary during an intercalibration experiment in 1993. ISAAC differs from other camera designs in that flow disruption is minimized by the small housing size and by maintaining an orientation into the flow with the lens port facing down. Problems with depth-of-field have been reduced by using Plexiglas collimators which eliminate most of the out-of-focus particles. Profiles in the Elbe show that particles occur as irregular ellipsoids and elongate stringers. Maximum diameters are on the order of 2 to 3 mm although individual stringers bound by thin threads can exceed 10 mm. Whereas apparent vertical changes in population size can result from a small number of abnormally large aggregates, significant size changes in fact do occur on hourly time scales. Composite samples reveal that particle populations tend to be lognormally distributed.