Dataset: particle number depth profiles

A black and white CCD video camera (Sentec) was mounted on a CTD frame which also held Niskin sample bottles, conductivity, temperature and oxygen probes and an optical backscatter sensor (Seapoint turbidity meter). The camera and digital video recorder were housed in a stainless steel cylinder with a 2.54 cm thick sapphire window as the optical port. Two high intensity LED lights were mounted in separate housings at approximately 45 degree angles at each side of the camera. The camera gain was set so that gelatinous zooplankton and the gelatinous matrix of marine snow particles were detectable but more opaque particles such as hard-bodied copepods were slightly overexposed. At a shutter speed of 1/1000 s, the camera recorded sharp images of particles at descent speeds of 1- 1.5 m s-1.   The customized image analysis program (modified Avidemux on Ubuntu-Linux) was set to record particles larger than 6 contiguous pixels, which sets the lowest detectable particle sizes at approximately 500 microns. Absolute particle numbers per volume cannot be determined with this video setup. Because of interference from ambient light, particles could in most cases only be enumerated below the euphotic zone.