SUSPENDED AGGREGATE ANALYSIS USING ISAAC, ELSE RIVER, 9-10 JUNE 1993

A portable underwater camera system, the In Situ Aggregate Analysis Ca mera (ISAAC), was used for precise and accurate sizing of suspended pa rticulate material in the Elbe River estuary during an intercalibratio n experiment in 1993. ISAAC differs from other camera designs in that flow disruption is minimized by the small housing size and by maintain ing an orientation into the flow with the lens port facing down. Probl ems with depth-of-field have been reduced by using Plexiglas collimato rs which eliminate most of the out-of-focus particles. Profiles in the Elbe show that particles occur as irregular ellipsoids and elongate s tringers. Maximum diameters are on the order of 2 to 3 mm although ind ividual stringers bound by thin threads can exceed 10 mm. Whereas appa rent vertical changes in population size can result from a small numbe r of abnormally large aggregates, significant size changes in fact do occur on hourly time scales. Composite samples reveal that particle po pulations tend to be lognormally distributed.