The software package ImageJ 1.52 (IJ, National Institutes of Health) was used for particle size analysis. In this method, we manually inspected the 4 to 5 sequential attenuance images taken during a two hour long image cycle to determine the point of onset of particle overlap. The image at this point was subtracted from the image of the clean sample stage taken immediately preceeding the image set.

A threshold of 0.04 attenuance unts and above was used to define the presence of particles to facilitate particle identification. At this threshold setting, large aggregates were fully detected; however, touching particles – particularly 200-400 µm sized fecal pellets were not separable. Each IJ-identified “particle” with multiple units was counted and these counts assigned to its sequence number. Inspection of the imagery also identified touching aggregates which were similarly treated.

During secondary data processing (using written Fortran codes), the area of each multi-unit “particle” was divided by the number of subunits, and its particle number was changed from 1 to the determined count. Living organisms rarely appeared in images; when when they did appear, we were able to identify Pteropods, Amphipods, Copepods, Siphonophores, Acantharia, Radiolaria, and Foraminifera. These, identified living particles were removed from the secondary processed data. Total particle attenuance (average particle attenuance times particle area) and particle number were binned into 65 logarithmically spaced size catagories from (30 µm to 20000 µm).

Data from each image cycle were weighted by the total number of images in that cycle; data from the multiple imaging/cleaning cycles during a dive were binned and weighted by the duration of each imaging cycle. The particle attenuance and number size-binned data were scaled to convert results to flux units (mATN–cm2 cm–2 d–1; and number m–2 d–1). The partitioning of particle flux was further broken into 30-100,100-200,200-400,400-1000, and >1000 µm categories. The 200-400 µm bin primarily was populated by the numerous ovoid pellets. The >1000 µm bin was dominated by aggregates.