The variability of suspended aggregates on the Amazon Continental Shelf

Underwater photographs taken as part of the AmasSeds project from 1989 to 1992 reveal the abundance of suspended aggregates on the Amazon Continental Shelf. Computerized image analysis of photographs from specific layers within the water column at inner shelf, midshelf, outer shelf, and rivermouth anchor stations allowed description of the in situ characteristics of the aggregates, or flocs. Flee sizes varied from the lower limit of detection (125 mu m) to a maximum greater than 2 mm, with a mean diameter of 518 mu m and a modal diameter of 646 mu m for the complete data set. Additionally, large, elongate, comet-shaped aggregates (stringers) as large as 2.5 cm were observed in individual photographs. Results of hoc-size analyses (in terms of the maximum hoc size, d(MAX)) were compared to salinity, particle concentration, and current velocity. Additionally, d(MAX) was also compared to shear (G) and the Kolmogorov microscale (lambda). Results suggest that salinity has little effect on d(MAX) (r(2) = 0.06), while particle volume concentration demonstrates a strong relationship with d(MAX) (r(2) = 0.92). The maximum flee size increased with increased current velocity and G up to a critical value (flee growth stage) and then decreased as the current velocity and G continued to increase (flee breakup stage). The Kolmogorov microscale, which describes the size of the smallest turbulent eddies (and, thus, should limit the d(MAX) observed) also demonstrates a relationship with the observed maximum flee size. The results suggest that d(MAX) is approximately one-half the magnitude of the Kolmogorov microscale (lambda). Copyright (C) 1996 Elsevier Science Ltd