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 thein situ characteristics of the aggregates, or flocs. Floc sizes varied from the lower limit of detection (125 μm) to a maximum greater than 2 mm, with a mean diameter of 518 μm and a modal diameter of 646 μm 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 floc-size analyses (in terms of the maximum floc size,dMAX) were compared to salinity, particle concentration, and current velocity. Additionally,dMAX was also compared to shear (G) and the Kolmogorov microscale (λ). Results suggest that salinity has little effect ondMAX (r2 = 0.06), while particle volume concentration demonstrates a strong relationship withdMAX (r2 = 0.92). The maximum floc size increased with increased current velocity andG up to a critical value (floc growth stage) and then decreased as the current velocity andG continued to increase (floc breakup stage). The Kolmogorov microscale, which describes the size of the smallest turbulent eddies (and, thus, should limit thedMAX observed) also demonstrates a relationship with the observed maximum floc size. The results suggest thatdMAX is approximately one-half the magnitude of the Kolmogorov microscale (λ).