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

Knowles, S. C., and J. T. Wells

Limnol. Oceanogr., 43(8), 1998, 1954-1962 | DOI: 10.4319/lo.1998.43.8.1954

ABSTRACT: The ISAAC (in situ aggregate analysis camera) system was developed to provide quantitative analysis of finegrained suspended material in waters of inner-coastal, estuarine, fluvial, and lacustrine environments with suspended sediment concentrations (SSC) up to 200 mg/liter. Light from strobes is collimated by physically treated Plexiglas prisms, producing a well-defined volume of illumination (VOI) in front of a 35mm camera mounted in an underwater camera housing. Particles within the VOI are in focus. Using a measured scale factor, researchers can measure suspended particle number, size, shape, and volume concentration. A submersible pump collects water from the depth of each photograph, allowing laboratory analysis of filtered suspensate. The present system, which is easily modified to meet specific field conditions, illuminates a 70.0 mm X 46.9 mm X 17.5 mm volume of water and documents particles ~ 10 µm in diameter and larger. We use a 2,700-dot-per-inch scanner to convert photo negatives to digital 256-graytone images. In-house software is used to classify particle morphology within l/4 φ size classes, from -20-4,000 µm. Conversion of the ISAAC system to digital or video camera technology would allow real-time data collection and analysis. In situ photographic data from the Elbe, Hudson, and Columbia River estuaries, as well as several coastal environments in North and South Carolina, illustrate the utility of the system for particulate concentrations ranging from ~2 to 200 mg/liter. A random composite sample from these data reveals that virtually all particles may form larger aggregates, characterized by an average equivalent spherical diameter (ESD) of ~300 µm, with <10% of the aggregate volume in suspension containedwithin aggregates <50 µm in diameter. Aggregates larger than 2 mm ESD have been observed in < 1% of the over 1,000 photographs taken, supporting suggestions in the literature that the Kolmogorov microscale of turbulent eddies limits the growth of suspended aggregates.

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