Abundance and size distribution of marine snow aggregates from profiles conducted during R/V Polar Star cruises in the Ross Sea, Antarctica between 2001 and 2005 (NCEI Accession 0288064)

Acquisition Description:
Methodology:

A marine snow imaging system was lowered through the water column to acquire photographs of aggregates. The imaging system consisted of a collimated strobe system and either a 35-mm film camera (Lobsiger Deepslope 6000; Years 1 - 3) or a digital camera (Insite Pacific Scorpio; Year 4) mounted on an aluminum frame. The system was lowered at a rate of 10 m min-1 through the water column, acquiring ca. six images min-1 at an interval of ca. 1.7 m throughout the entire water column. Illumination was provided by a pair of strobe lights (Deep Sea Power and Light) positioned to produce an 8.4-cm deep beam of uniform, collimated light 66 or 79 cm from the camera lens (Fig. 2). Aggregates larger than 0.5 mm within this volume (3 - 15 L, depending on the camera, lens and resultant geometry) are quantified from images taken by a camera that is mounted perpendicular to the long axis of the light beam, which can subsequently be distinguished and quantified by digital analysis. Ambient light illuminates particles in front of and behind this light beam and invalidates the calculation of illuminated volume. Thus, profiles were obtained near local midnight whenever possible (24-h photoperiods occurred throughout all cruises) to minimize this interference; images with the frame visible, due to ambient light, were discarded. The depth of first aggregate counts ranged from 30 - 88 m (a function of water clarity and solar angle). A Sea-Bird SeaCat CTD and 25-cm SeaTech transmissometer mounted on the camera frame provided continuous measurements of temperature, salinity and optical transmission. These data were used to calculate the depth at which each image was acquired. All film (Tmax 400) and images were returned to the laboratory for processing except for short sections viewed at sea to assess camera operation. The films were developed and then digitized to JPG format either in house, using a Nikon camera and macro lens (3767 x 2368), or commercially (3544 x 2341).