In situ aggregate analysis camera (ISAAC): A quantitative tool for analyzing fine-grained suspended material

The ISAAC tin situ aggregate analysis camera) system was developed to provi de quantitative analysis of fine-grained suspended material in waters of in ner-coastal, estuarine, fluvial, and lacustrine environments with suspended sediment concentrations (SSC) up to 200 mg/liter. Light from strobes is co llimated by physically treated Plexiglas prisms, producing a well-defined v olume of illumination (VOI) in front of a 35-mm camera mounted in an underw ater camera housing. Particles within the VOI are in focus. Using a measure d scale factor, researchers can measure suspended particle number, size, sh ape, and volume concentration. A submersible pump collects water from the d epth of each photograph, allowing laboratory analysis of filtered suspensat e. The present system, which is easily modified to meet specific field cond itions, illuminates a 70.0 mm x 46.9 mm x 17.5 mm volume of water and docum ents particles similar to 10 mu m in diameter and larger. We use a 2,700-do t-per-inch scanner to convert photo negatives to digital 256-graytone image s. In-house software is used to classify particle morphology within 1/4 phi size classes, from similar to 20-4,000 mu m Conversion of the ISAAC system to digital or video camera technology would allow Teal-time data collectio n and analysis. In situ photographic data from the Elbe, Hudson, and Columbia River estuari es, as well as several coastal environments in North and South Carolina, il lustrate the utility of the system for particulate concentrations ranging f rom similar to 2 to 200 mg/liter. A random composite sample from these data reveals that virtually all particles may form larger aggregates, character ized by an average equivalent spherical diameter (ESD) of similar to 300 mu m, with <10% of the aggregate volume in suspension contained within aggreg ates <50 mu m in diameter. Aggregates larger than 2 mm ESD have been observ ed in <1% of the over 1,000 photographs taken, supporting suggestions in th e literature that the Kolmogorov microscale of turbulent eddies limits the growth of suspended aggregates.